Use of var keyword in C

Question

After discussion with colleagues regarding the use of the  var  keyword in C  3 I wondered what people s opinions were on the appropriate uses of type inference via var   For example I rather lazily used var in questionable circumstances  e g  -  foreach var item in someList                Type of  item  not clear  var something   someObject SomeProperty     Type of  something  not clear  var something   someMethod       Type of  something  not clear    More legitimate uses of var are as follows -  var l   new List lt string gt        Obvious what l will be  var s   new SomeClass       Obvious what s will be    Interestingly LINQ seems to be a bit of a grey area  e g  -  var results   from r in dataContext SomeTable               select r     Not  entirely clear  what results will be here    It s clear what results will be in that it will be a type which implements IEnumerable  however it isn t entirely obvious in the same way a var declaring a new object is   It s even worse when it comes to LINQ to objects  e g  -  var results   from item in someList               where item    3               select item    This is no better than the equivilent foreach var item in someList             equivilent   There is a real concern about type safety here - for example if we were to place the results of that query into an overloaded method that accepted IEnumerable lt int gt  and IEnumerable lt double gt  the caller might inadvertently pass in the wrong type   var does maintain strong typing but the question is really whether it s dangerous for the type to not be immediately apparent on definition  something which is magnified when overloads mean compiler errors might not be issued when you unintentionally pass the wrong type to a method

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I think people do not understand the var keyword. They confuse it with the Visual Basic / JavaScript keyword, which is a different beast all toghether.

Many people think the var keyword implies weak typing (or dynamic typing), while in fact c# is and remains strongly typed.

If you consider this in javascript:

var something = 5;

you are allowed to:

something = "hello";

In the case of c#, the compiler would infer the type from the first statement, causing something to be of type "int", so the second statement would result in an exception.

People simply need to understand that using the var keyword does not imply dynamic typing and then decide how far they want to take the use of the var keyword, knowing it will have absolutely no difference as to what will be compiled.

Sure the var keyword was introduced to support anonymous types, but if you look at this:

LedDeviceController controller = new LedDeviceController("172.17.0.1");

It's very very verbose, and I'm sure this is just as readable, if not more:

var controller = new LedDeviceController("172.17.0.1");

The result is exactly the same, so yes I use it throughout my code

UPDATE:

Maybe, just maybe... they should have used another keyword, then we would not be having this discussion... perhaps the "infered" keyword instead of "var"

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I don t understand why people start debates like this  It really serves no purpose than to start flame wars at then end of which nothing is gained  Now if the C  team was trying to phase out one style in favor of the other  I can see the reason to argue over the merits of each style  But since both are going to remain in the language  why not use the one you prefer and let everybody do the same  It s like the use of everybody s favorite ternary operator  some like it and some don t  At the end of the day  it makes no difference to the compiler   This is like arguing with your siblings over which is your favorite parent  it doesn t matter unless they are divorcing

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var is good as it follows the classic DRY rule  and it is especially elegant when you indicate the type in the same line as declaring the variable   e g  var city   new City

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Using var instead of explicit type makes refactorings much easier  therefore I must contradict the previous posters who meant it made no difference or it was purely  syntactic sugar     You can change the return type of your methods without changing every file where this method is called  Imagine      List lt MyClass gt  SomeMethod                 which is used like      IList lt MyClass gt  list   obj SomeMethod    foreach  MyClass c in list    System Console WriteLine c ToString           If you wanted to refactor SomeMethod   to return an IEnumerable lt MySecondClass gt   you would have to change the variable declaration  also inside the foreach  in every place you used the method   If you write      var list   obj SomeMethod    foreach  var element in list    System Console WriteLine element ToString           instead  you don t have to change it

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erlando  out of curiosity  why do you need to know the variable s type looking at the source code    In my practice I found that variable type is matter for me only at the time I m using it in the code     If I m trying to do some inappropriate operation on someVar compiler gladly gives me an error warning   I really don t care what type someVar has if I understand why it s being used it the given context

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Neither of those is absolutely true  var can have both positive and negative effects on readability  In my opinion  var should be used when either of the following is true    The type is anonymous  well  you don t have any choice here  as it must be var in this case  The type is obvious based upon the assigned expression  i e  var foo   new TypeWithAReallyLongNameTheresNoSenseRepeating      var has no performance impacts  as it s syntactic sugar  the compiler infers the type and defines it once it s compiled into IL  there s nothing actually dynamic about it

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Amazed this hasn t been noted so far  but it is common sense to use var for foreach loop variables   If you specify a specific type instead  you risk having a runtime cast silently inserted into your program by the compiler   foreach  Derived d in listOfBase      The above will compile  But the compiler inserts a downcast from Base to Derived  So if anything on the list is not a Derived at runtime  there is an invalid cast exception  Type safety is compromised  Invisible casts are horrible   The only way to rule this out is to use var  so the compiler determines the type of the loop variable from the static type of the list

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Well this one is pretty much gonna be opinionated all the way through  but I will try to give my view on it - albeit I think my view is so mixed up that you are probably not gonna get much out of it anyways   First of all - there s anonymous types  for which you need to use the  var  keyword in order to assign an object with an anonymous type as its class - there s not much discussion here   var  is needed   For simpler types however  ints  longs  strings - so forth - I tend to put in the proper types  Mainly because it is a bit of a  lazy man s tool  and I don t see much gain here  very few keystrokes and the possible confusion it could provide later down the road just ain t worth it  Especially the various types for floating point numbers  float  double  decimal  confuse me as I am not firm with the postfixes in the literals - I like to see the type in the source code   With that said  I tend to use var alot if the type is more complex and or it is explicitly repeated on the righthand-side of the assignment  This could be a List lt string gt  or etc  such as   var list   new List lt string gt       In a such case  I see no use to repeat the type twice - and especially as you start changing your code and the types change - the generic types might get more and more complicated and as such having to change them twice is just a pain  However of course if you wish to code against an IList lt string gt  then you have to name the type explicitly   So in short I do the following    Name the type explicitly when the type is short or cannot be read out of context Use var when it has to be used  duh  Use var to be lazy when it  in my mind  does not hurt readability

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Sure  int is easy  but when the variable s type is IEnumerable lt MyStupidLongNamedGenericClass lt int  string gt  gt   var makes things much easier

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I had the same concern when I started to use var keyword  However I got used to it over time and not going to go back to explicit variable types   Visual Studio s compiler intellisense are doing a very good job on making work with implicitly typed variables much easier    I think that following proper naming conventions can help you to understand code much better then explicit typing   It seems to be same sort of questions like  shoud I use prefixes in variable names    Stick with good variable names and let the compiler think on variable types

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I don t see what the big deal is    var something   someMethod       Type of  something  not clear  lt -- not to the compiler    You still have full intellisense on  something   and for any ambiguous case you have your unit tests  right    do you     It s not varchar  it s not dim  and it s certainly not dynamic or weak typing  It is stopping maddnes like this   List lt somethinglongtypename gt  v   new List lt somethinglongtypename gt       and reducing that total mindclutter to   var v   new List lt somethinglongtypename gt       Nice  not quite as nice as   v   List lt somethinglongtypename gt       But then that s what Boo is for

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You don t have to write out the type name and no this is not less performant as the type is resolved at compile time

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var is like the dotted spaces in kids  books where kids have to fill it  Except in this case the Compiler will fill it with the right type which is usually written after the   sign

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erlando   Talking about refactoring it seems to be much easier to change variable type by assigning instance of new type to one variable rather then changing it in multiple places  isn t it    As for code review I see no big issues with var keyword  During code review I prefer to check code logic rather variable types  Of course there might be scenarios where developer can use inappropriate type but I think that number of such cases is so small it wouldn t be a reason for my to stop using var keyword   So I repeat my question  Why does variable type matter to you

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I find that using the var keyword actually makes the code more readable because you just get used to skipping the  var  keyword   You don t need to keep scrolling right to figure out what the code is doing when you really don t care about what the specific type is   If I really need to know what type  item  is below  I just hover my mouse over it and Visual Studio will tell me   In other words  I would much rather read  foreach  var item in list     DoWork  item        over and over than  foreach  KeyValuePair lt string  double gt  entry in list     DoWork  Item        when I am trying to digest the code   I think it boils down to personal preference to some extent   I would rely on common sense on this one -- save enforcing standards for the important stuff  security  database use  logging  etc    -Alan

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Use it for anonymous types - that s what it s there for  Anything else is a use too far  Like many people who grew up on C  I m used to looking at the left of the declaration for the type  I don t look at the right side unless I have to  Using var for any old declaration makes me do that all the time  which I personally find uncomfortable   Those saying  it doesn t matter  use what you re happy with  are not seeing the whole picture  Everyone will pick up other people s code at one point or another and have to deal with whatever decisions they made at the time they wrote it  It s bad enough having to deal with radically different naming conventions  or - the classic gripe - bracing styles  without adding the whole  var or not  thing into the mix  The worst case will be where one programmer didn t use var and then along comes a maintainer who loves it  and extends the code using it  So now you have an unholy mess    Standards are a good thing precisely because they mean you re that much more likely to be able to pick up random code and be able to grok it quickly  The more things that are different  the harder that gets  And moving to the  var everywhere  style makes a big difference   I don t mind dynamic typing  and I don t mind implict typing - in languages that are designed for them  I quite like Python  But C  was designed as a statically explicitly-typed language and that s how it should stay  Breaking the rules for anonymous types was bad enough  letting people take that still further and break the idioms of the language even more is something I m not happy with  Now that the genie is out of the bottle  it ll never go back in  C  will become balkanised into camps  Not good

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In most cases  it s just simpler to type it - imagine  var sb   new StringBuilder      instead of   StringBuilder sb   new StringBuilder      Sometimes it s required  for example  anonymous types  like   var stuff   new   Name    Me   Age   20      I personally like using it  in spite of the fact that it makes the code less readable and maintainable

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I use var in the following situations    When I have to  result is anonymous  When the type is on the same line as the code  e g   var emp   new Employee      Its obvious we want an Employee  because we re creating a new Employee object   so how is   Employee emp   new Employee   any more obvious    I do NOT use var when the type cannot be inferred  e g   var emp   GetEmployee      Because the return type is not immediately obvious  is at an Employee  an IEmployee  something that has nothing to do with an Employee object at all  etc

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I use var whenever possible    The actual type of the local variable shouldn t matter if your code is well written  i e   good variable names  comments  clear structure etc

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The only thing you can really say about my taste is that it is old fashioned  and in time yours will be too   -Tolkien

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I split var all over the places  the only questionable places for me are internal short types  e g  I prefer int i   3  over var i   3

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var is essential for anonymous types  as pointed out in one of the previous responses to this question    I would categorise all other discussion of its pros and cons as  religious war   By that I mean that a comparison and discussion of the relative merits of     var i   5  int j   5   SomeType someType   new SomeType    var someType   new SomeType         is entirely subjective   Implicit typing means that there is no runtime penalty for any variable being declared using the var keyword  so it comes down to being a debate about what makes the developers happy

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Sometimes the compiler can also infer what is required  better  than the developer - at least a developer who does not understand what the api he s using requires   For example - when using linq   Example 1  Func lt Person  bool gt  predicate    i    gt  i Id  lt  10  IEnumerable lt Person gt  result   table Where predicate     Example 2  var predicate    Person i    gt  i Id  lt  10  var result   table Where predicate     In the above code - assuming one is using Linq to Nhibernate or Linq to SQL  Example 1 will bring the entire resultset for Person objects back and then do filter on the client end  Example 2 however will do the query on the server  such as on Sql Server with SQL  as the compiler is smart enough to work out that the Where function should take a Expression  rather than a Func    The result in Example 1 will also not be further queryable on the server as an IEnumerable is returned  while in Example 2 the compiler can work out if the result should rather be a IQueryable instead of IEnumerable

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First   var is not a type and not some special feature  like c  4 0 dynamic   It is just a syntax sugar  You ask compiler to infer the type by the right hand side expression  The only necessary place is anonymous types   I don t think that using var is neither good or evil  it is coding style  Personally i don t use it  but i don t mind using it by other team members

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I think the use of var should be coupled with wisely-chosen variable names   I have no problem using var in a foreach statement  provided it s not like this   foreach  var c in list            If it were more like this   foreach  var customer in list                then someone reading the code would be much more likely to understand what  list  is  If you have control over the name of the list variable itself  that s even better   The same can apply to other situations  This is pretty useless   var x   SaveFoo foo         but this makes sense   var saveSucceeded   SaveFoo foo     Each to his own  I guess  I ve found myself doing this  which is just insane   var f    float 3    I need some sort of 12-step var program  My name is Matt  and I  ab use var

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From Eric Lippert  a Senior Software Design Engineer on the C  team   Why was the var keyword introduced      There are two reasons  one which   exists today  one which will crop up   in 3 0       The first reason is that this code is   incredibly ugly because of all the   redundancy       Dictionary lt string  List lt int gt  gt  mylists   new Dictionary lt string  List lt int gt  gt          And that s a simple example     I ve   written worse  Any time you re forced   to type exactly the same thing twice    that s a redundancy that we can   remove  Much nicer to write      var mylists   new Dictionary lt string List lt int gt  gt          and let the compiler figure out what   the type is based on the assignment       Second  C  3 0 introduces anonymous   types  Since anonymous types by   definition have no names  you need to   be able to infer the type of the   variable from the initializing   expression if its type is anonymous    Emphasis mine  The whole article  C  3 0 is still statically typed  honest   and the ensuing series are pretty good   This is what var is for  Other uses probably will not work so well  Any comparison to JScript  VBScript  or dynamic typing is total bunk  Note again  var is required in order to have certain other features work in  NET

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It s a matter of taste  All this fussing about the type of a variable disappears when you get used to dynamically typed languages  That is  if you ever start to like them  I m not sure if everybody can  but I do    C  s var is pretty cool in that it looks like dynamic typing  but actually is static typing - the compiler enforces correct usage   The type of your variable is not really that important  this has been said before   It should be relatively clear from the context  its interactions with other variables and methods  and its name - don t expect customerList to contain an int     I am still waiting to see what my boss thinks of this matter - I got a blanket  go ahead  to use any new constructs in 3 5  but what will we do about maintenance

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None  except that you don t have to write the type name twice  http   msdn microsoft com en-us library bb383973 aspx

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Var is not like variant at all  The variable is still strongly typed  it s just that you don t press keys to get it that way  You can hover over it in Visual Studio to see the type  If you re reading printed code  it s possible you might have to think a little to work out what the type is  But there is only one line that declares it and many lines that use it  so giving things decent names is still the best way to make your code easier to follow   Is using Intellisense lazy  It s less typing than the whole name  Or are there things that are less work but don t deserve criticism  I think there are  and var is one of them

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You can let the compiler  and the fellow who maintains the code next  infer the type from the right hand side of the initializer assignment    If this inference is possible  the compiler can do it so it saves some typing on your part    If the inference is easy for that poor fellow  then you haven t hurt anything   If the inference is hard  you ve made the code harder to maintain and so as a general rule I wouldn t do it   Lastly  if you intended the type to be something particular  and your initializer expression actually has a different type  using var means it will be harder for you to find the induced bug    By explicitly telling the compiler what you intend the type to be  when the type isn t that  you would get an immediate diagnostic   By sluffing on the type declaration and using  var   you won t get an error on the initialization  instead  you ll get a type error in some expression that uses the identifier assigned by the var expression  and it will be harder to understand why   So the moral is  use var sparingly  you generally aren t doing yourself or your downstream fellow maintainer a lot of good   And hope he reasons the same way  so you aren t stuck guessing his intentions because he thought using var was easy   Optimizing on how much you type is a mistake when coding a system with a long life

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I have to agree with Matt Hamilton   Var can make your code much more readable and understandable when used with good variable names   But var can also make your code as impossible to read and understand as Perl when used badly     A list of good and bad uses of var isn t really going to help much either   This is a case for common sense   The larger question is one of readability vs  write-ability   Lots of devs don t care if their code is readable   They just don t want to type as much   Personally I m a read   write guy

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We ve adopted the ethos  Code for people  not machines   based on the assumption that you spend multiple times longer in maintenance mode than on new development   For me  that rules out the argument that the compiler  knows  what type the variable is - sure  you can t write invalid code the first time because the compiler stops your code from compiling  but when the next developer is reading the code in 6 months time they need to be able to deduce what the variable is doing correctly or incorrectly and quickly identify the cause of issues   Thus   var something   SomeMethod      is outlawed by our coding standards  but the following is encouraged in our team because it increases readability   var list   new List lt KeyValuePair lt string  double gt  gt     FillList  list    foreach  var item in list        DoWork  item

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Static typing is about contracts  not source code   The idea there is a need to have the static information on a single line of what  should  be a small method   Common guidelines recommend rarely exceeding 25 lines per method   If a method is large enough that you can t keep track of a single variable within that method  you are doing something else wrong that would make any criticism of var pale in comparison   Actually  one of the great arguments for var is that it can make refactoring simpler because you no longer have to worry that you made your declaration overly restrictive  i e  you used List lt   when you should have used IList lt    or IEnumerable lt      You still want to think about the new methods signature  but at least you won t have to go back and change your declarations to match

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Var  in my opinion  in C  is a good thingtm   Any variable so typed is still strongly typed  but it gets its type from the right-hand side of the assignment where it is defined   Because the type information is available on the right-hand side  in most cases  it s unnecessary and overly verbose to also have to enter it on the left-hand side   I think this significantly increases readability without decreasing type safety   From my perspective  using good naming conventions for variables and methods is more important from a readability perspective than explicit type information   If I need the type information  I can always hover over the variable  in VS  and get it   Generally  though  explicit type information shouldn t be necessary to the reader   For the developer  in VS you still get Intellisense  regardless of how the variable is declared   Having said all of that  there may still be cases where it does make sense to explicitly declare the type -- perhaps you have a method that returns a List lt T gt   but you want to treat it as an IEnumerable lt T gt  in your method  To ensure that you are using the interface  declaring the variable of the interface type can make this explicit  Or  perhaps  you want to declare a variable without an initial value -- because it immediately gets a value based on some condition   In that case you need the type  If the type information is useful or necessary  go ahead and use it  I feel  though  that typically it isn t necessary and the code is easier to read without it in most cases

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I think you point out the main problem with var in your question    I don t have to figure out the type    As other have pointed out  there is a place for var  but if you don t know the type you re dealing with  there s a pretty good chance that you re going to have problems down the road - not in all cases  but there s enough of a smell there so that you should be suspicious

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In our office  our CTO has categorically banned the use of the var keyword  for the same reasons that you have stated   Personally I find the use of var only valid in new object declarations  since the type of the object is obvious in the statement itself   For LINQ queries  you can resolve results to   IEnumerable lt TypeReturnedBySelectObject gt

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var is not bad  Remember this  var is not bad  Repeat it  var is not bad  Remember this  var is not bad  Repeat it    If the compiler is smart enough to figure out the type from the context  then so are you  You don t have to have it written down right there at declaration  And intellisense makes that even less necessary    var is not bad  Remember this  var is not bad  Repeat it  var is not bad  Remember this  var is not bad  Repeat it

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aku  One example is code reviews  Another example is refactoring scenarios   Basically I don t want to go type-hunting with my mouse  It might not be available

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I think you may be misunderstanding the usage of var in C   It is still strong typing  unlike the VB varient type so there is no performance hit from using it or not   Since there is no effect on the final compiled code it really is a stylist choice  Personally I don t use it since I find the code easier to read with the full types defined  but I can imagine a couple of years down the line that full type declaration will be looked at in the same way as Hungarian notation is now - extra typing for no real benefit over the information that intellisense gives us by default

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It s not wrong  but it can be inappropriate  See all the other responses for examples   var x   5   bad   var x   new SuperDooperClass     good   var x   from t in db Something select new   Property1   t Field12     better

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var is a placeholder introduced for the anonymous types in C  3 0 and LINQ   As such  it allows writing LINQ queries for a fewer amount of columns within  let s say  a collection  No need to duplicate the information in memory  only load what s necessary to accomplish what you need to be done   The use of var is not bad at all  as it is actually not a type  but as mentioned elsewhere  a placeholder for the type which is and has to be defined on the right-hand side of the equation  Then  the compiler will replace the keyword with the type itself   It is particularly useful when  even with IntelliSense  the name of a type is long to type  Just write var  and instantiate it  The other programmers who will read your code afterward will easily understand what you re doing   It s like using  public object SomeObject   get  set      instead of   public object SomeObject       get           return  someObject             set            someObject   value          private object  someObject    Everyone knows what s the property s doing  as everyone knows what the var keyword is doing  and either examples tend to ease readability by making it lighter  and make it more pleasant for the programmer to write effective code

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I don t think var per say is a terrible language feature  as I use it daily with code like what Jeff Yates described   Actually  almost everytime I use var is because generics can make for some extremely wordy code   I live verbose code but generics take it a step too far   That said  I  obviously      think var is ripe for abuse   If code gets to 20  lines in a method with vars littered through out  you will quickly make maintenance a nightmare   Additionally  var in a tutorial is incredibly counter intuitive and generally is a giant no-no in my books   On the flipside  var is an  easy  feature that new programmers are going to latch onto and love   Then  within a few minutes hours days hit a massive roadblock when they start hitting the limits    Why can t I return var from functions    That kind of question   Also  adding a pseudo dynamic type to a strongly typed language is something that can easily trip up a new developer   In the long run  I think the var keyword will actually make c  harder to learn for new programmers   That said  as an experienced programmer I do use var  mostly when dealing with generics   and obviously anonymous types    I do hold by my quote  I do believe var will be one of the worst abused c  features

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One good argument why vars should not be used as a mere  typing shortcut   but should instead be used for scenarios they were primarily designed for  Resharper  at least v4 5  cannot find usages of a type if it is represented as a var  This can be a real problem when refactoring or analyzing the source code

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It s purely a convinience  The compiler will inferre the type  based on the type of the expression on the right hand side

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Someone doesn t like criticism of var   All answers downmodded   oh well     Jon Limjap  I know     What I meant was that the readability is degraded like it is in VB6  I don t like to rely on Intellisense to figure out what type a given variable is  I want to be able to figure it out using the source alone   Naming conventions doesn t help either - I already use good names  Are we going back to prefixing

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I guess it depends on your perspective   I personally have never had any difficulty understanding a piece of code because of var  misuse   and my coworkers and I use it quite a lot all over    I agree that Intellisense is a huge aid in this regard    I welcome it as a way to remove repetitive cruft   After all  if statements like  var index   5     this is supposed to be bad  var firstEligibleObject   FetchSomething       oh no what type is it                                                i am going to die if i don t know   were really that impossible to deal with  nobody would use dynamically typed languages

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I use var extensively  There has been criticism that this diminishes the readability of the code  but no argument to support that claim   Admittedly  it may mean that it s not clear what type we are dealing with  So what  This is actually the point of a decoupled design  When dealing with interfaces  you are emphatically not interested in the type a variable has  var takes this much further  true  but I think that the argument remains the same from a readability point of view  The programmer shouldn t actually be interested in the type of the variable but rather in what a variable does  This is why Microsoft also calls type inference    duck typing      So  what does a variable do when I declare it using var  Easy  it does whatever IntelliSense tells me it does  Any reasoning about C  that ignores the IDE falls short of reality  In practice  every C  code is programmed in an IDE that supports IntelliSense   If I am using a var declared variable and get confused what the variable is there for  there s something fundamentally wrong with my code  var is not the cause  it only makes the symptoms visible  Don t blame the messenger   Now  the C  team has released a coding guideline stating that var should only be used to capture the result of a LINQ statement that creates an anonymous type  because here  we have no real alternative to var   Well  screw that  As long as the C  team doesn t give me a sound argument for this guideline  I am going to ignore it because in my professional and personal opinion  it s pure baloney   Sorry  I ve got no link to the guideline in question    Actually  there are some  superficially  good explanations on why you shouldn t use var but I still believe they are largely wrong  Take the example of    searchabililty     the author claims that var makes it hard to search for places where MyType is used  Right  So do interfaces  Actually  why would I want to know where the class is used  I might be more interested in where it is instantiated and this will still be searchable because somewhere its constructor has to be invoked  even if this is done indirectly  the type name has to be mentioned somewhere

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I m fairly new in the C  world  after a decade as a Java professional  My initial thought was along the lines of  Oh no  There goes type safety down the drain   However  the more I read about var  the more I like it   1  Var is every bit as type safe as an explicitly declared type would be  It s all about compile time syntactic sugar   2  It follows the principle of DRY  don t repeat yourself   DRY is all about avoiding redundancies  and naming the type on both sides is certainly redundant  Avoinding redundancy is all about making your code easier to change   3  As for knowing the exact type    well    I would argue that you always have a general idea is you have an integer  a socket  some UI control  or whatever  Intellisense will guide you from here  Knowing the exact type often does not matter  E g  I would argue that 99  of the time you don t care if a given variable is a long or an int  a float or a double  For the last 1  of the cases  where it really matters  just hover the mouse pointer above the var keyword   4  I ve seen the ridiculous argument that now we would need to go back to 1980-style Hungarian warts in order to distinguish variable types  After all  this was the only way to tell the types of variables back in the days of Timothy Dalton playing James Bond  But this is 2010  We have learned to name our variables based upon their usage and their contents and let the IDE guide us as to their type  Just keep doing this and var will not hurt you   To sum it up  var is not a big thing  but it is a really nice thing  and it is a thing that Java better copy soon  All arguments against seem to be based upon pre-IDE fallacies  I would not hesitate to use it  and I m happy the R  helps me do so

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If you are lazy and use var for anything other than anonymous types  you should be required to use Hungarian notation in the naming of such variables   var iCounter   0   lives   Boy  do I miss VB

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Keith -      In your comparison between   IEnumerable lt int gt  and   IEnumerable lt double gt  you don t need to   worry - if you pass the wrong type   your code won t compile anyway    That isn t quite true - if a method is overloaded to both IEnumerable lt int gt  and IEnumerable lt double gt  then it may silently pass the unexpected inferred type  due to some other change in the program  to the wrong overload hence causing incorrect behaviour   I suppose the question is how likely it is that this sort of situation will come up   I guess part of the problem is how much confusion var adds to a given declaration - if it s not clear what type something is  despite being strongly typed and the compiler understanding entirely what type it is  someone might gloss over a type safety error  or at least take longer to understand a piece of code

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One specific case where var is difficult  offline code reviews  especially the ones done on paper   You can t rely on mouse-overs for that

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In my opinion  there is no problem in using var heavily  It is not a type of its own  you are still using static typing   Instead it s just a time saver  letting the compiler figure out what to do   Like any other time saver  such as auto properties for example   it is a good idea to understand what it is and how it works before using it everywhere

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Given how powerful Intellisense is now  I am not sure var is any harder to read than having member variables in a class  or local variables in a method which are defined off the visible screen area    If you have a line of code such as   IDictionary lt BigClassName  SomeOtherBigClassName gt  nameDictionary   new Dictionary lt BigClassName  SomeOtherBigClassName gt       Is is much easier or harder to read than   var nameDictionary   new Dictionary lt BigClassName  SomeOtherBigClassName gt

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Deleted for reasons of redundancy   vars are still initialized as the correct variable type - the compiler just infers it from the context  As you alluded to  var enables us to store references to anonymous class instances - but it also makes it easier to change your code  For example      If you change ItemLibrary to use int  you need to update this call byte totalItemCount   ItemLibrary GetItemCount        If GetItemCount changes  I don t have to update this statement  var totalItemCount   ItemLibrary GetItemCount      Yes  if it s hard to determine a variable s type from its name and usage  by all means explicitly declare its type

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Depends  somehow it makes the code look  cleaner   but agree it makes it more unreadable to

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I used to think that the var keyword was a great invention but I put a a limit on it this was   Only use var where it is obvious what the type is immediately  no scrolling or looking at return types    I came to realise this then gave me no benefit whatsoever and removed all var keywords from my code  unless they were specifically required   for now I think that they make the code less readable  especially to others reading your code   It hides intent and in at least one instance lead to a runtime bug in some code because of assumption of type

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It can make code simpler and shorter  especially with complicated generic types and delegates   Also  it makes variable types easier to change

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I still think var can make code more readable in some cases  If I have a Customer class with an Orders property  and I want to assign that to a variable  I will just do this   var orders   cust Orders    I don t care if Customer Orders is IEnumerable lt Order gt   ObservableCollection lt Order gt  or BindingList lt Order gt  - all I want is to keep that list in memory to iterate over it or get its count or something later on   Contrast the above declaration with   ObservableCollection lt Order gt  orders   cust Orders    To me  the type name is just noise  And if I go back and decide to change the type of the Customer Orders down the track  say from ObservableCollection lt Order gt  to IList lt Order gt   then I need to change that declaration too - something I wouldn t have to do if I d used var in the first place

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what most are ignoring   var something   new StringBuilder       isn t normally typed as fast as   StringBuilder something   KEY TAB

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Local variables can be given an inferred  type  of var instead of an explicit type  The var keyword instructs the compiler to infer the type of the variable from the expression on the right side of the initialization statement       z is compiled as an int  var z   100       s is compiled as a string below  var s    Hello        a is compiled as int    var a   new     0  1  2         expr is compiled as IEnumerable    or perhaps IQueryable  var expr       from c in customers     where c City     London      select c       anon is compiled as an anonymous type  var anon   new   Name    Terry   Age   34         list is compiled as List                               var list   new List lt int gt      var can only be used when a local variable is declared and initialized in the same statement  the variable cannot be initialized to null  or to a method group or an anonymous function    var cannot be used on fields at class scope   Variables declared by using var cannot be used in the initialization expression  In other words  this expression is legal  int i    i   20   but this expression produces a compile-time error  var i    i   20    Multiple implicitly-typed variables cannot be initialized in the same statement   If a type named var is in scope  then the var keyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration

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For the afficionados that think var saves time  it takes less keystrokes to type   StringBuilder sb   new StringBuilder      than  var sb   new StringBuilder      Count em if you don t believe me     19 versus 21  I ll explain if I have to  but just try it     depending on the current state of your intellisense you may have to type a couple more for each one   And it s true for every type you can think of    My personal feeling is that var should never be used except where the type is not known because it reduces recognition readabiltiy in code  It takes the brain longer to recognize the type than a full line  Old timers who understand machine code and bits know exactly what I am talking about  The brain processes in parallel and when you use var you force it to serialize its input  Why would anyone want to make their brain work harder  That s what computers are for

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VS2008 w resharper 4 1 has correct typing in the tooltip when you hover over  var  so I think it should be able to find this when you look for all usages of a class   Haven t yet tested that it does that yet though

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If you know the type  use the type  If you don t know the type  why not  If you can t know the type  that s okay -- you ve found the only valid use   And I m sorry  but if the best you can do is  it makes the code all line up   that s not a good answer   Find a different way to format your code

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It can certainly make things simpler  from code I wrote yesterday   var content    new Queue lt Pair lt Regex  Func lt string  bool gt  gt  gt         foreach  var entry in content            This would have be extremely verbose without var   Addendum  A little time spent with a language with real type inference  e g  F   will show just how good compilers are at getting the type of expressions right  It certainly has meant I tend to use var as much as I can  and using an explicit type now indicates that the variable is not of the initialising expression s type

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Don t use that  makes your code unreadable   ALWAYS use as strict typing as possible  crutches only makes your life hell

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The most likely time you ll need this is for anonymous types  where it is 100  required   but it also avoids repetition for the trivial cases  and IMO makes the line clearer  I don t need to see the type twice for a simple initialization   For example   Dictionary lt string  List lt SomeComplexType lt int gt  gt  gt  data   new Dictionary lt string  List lt SomeComplexType lt int gt  gt  gt        please don t edit the hscroll in the above - it kinda proves the point      vs   var data   new Dictionary lt string  List lt SomeComplexType lt int gt  gt  gt       There are  however  occasions when this is misleading  and can potentially cause bugs  Be careful using var if the original variable and initialized type weren t identical  For example   static void DoSomething IFoo foo   Console WriteLine  working happily     static void DoSomething Foo foo   Console WriteLine  formatting hard disk            this working code    IFoo oldCode   new Foo    DoSomething oldCode         is   very   different to this code var newCode   new Foo    DoSomething newCode

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From Essential LINQ   It is best not to explicitly declare the type of a range variable unless absolutely necessary  For instance  the following code compiles cleanly  but the type could have been inferred by the compiler without a formal declaration   List lt string gt  list   new List lt string gt     LINQ    query    adventure     var query   from string word in list       where word Contains  r         orderby word ascending       select word    Explicitly declaring the type of a range variable forces a behind-the-scenes call to the LINQ Cast operator  This call may have unintended consequences and may hurt performance  If you encounter performance problems with a LINQ query  a cast like the one shown here is one possible place to begin looking for the culprit   The one exception to this rule is when you are working with a nongeneric Enumerable  in which case you should use the cast

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Apart from readability concerns  there is one real issue with the use of  var   When used to define variables that are assigned to later in the code it can lead to broken code if the type of the expression used to initialize the variable changes to a narrower type  Normally it would be safe to refactor a method to return a narrower type than it did before  e g  to replace a return type of  Object  with some class  Foo   But if there is a variable whose type is inferred based on the method  then changing the return type will mean that this variable can longer be assigned a non-Foo object   var x   getFoo       Originally declared to return Object x   getNonFoo      So in this example  changing the return type of getFoo would make the assignment from getNonFoo illegal   This is not such a big deal if getFoo and all of its uses are in the same project  but if getFoo is in a library for use by external projects you can no longer be sure that narrowing the return type will not break some users code if they use  var  like this   It was for exactly this reason that when we added a similar type inferencing feature to the Curl programming language  called  def  in Curl  that we prevent assignments to variables defined using this syntax

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In your comparison between IEnumerable lt int gt  and IEnumerable lt double gt  you don t need to worry - if you pass the wrong type your code won t compile anyway   There s no concern about type-safety  as var is not dynamic  It s just compiler magic and any type unsafe calls you make will get caught    Var is absolutely needed for Linq   var anonEnumeration       from post in AllPosts       where post Date  gt  oldDate     let author   GetAuthor  post AuthorId       select new            PostName   post Name           post Date           AuthorName   author Name          Now look at anonEnumeration in intellisense and it will appear something like IEnumerable lt  a gt   foreach  var item in anonEnumeration            VS knows the type     item PostName    you ll get intellisense here        you still have type safety     item ItemId      will throw a compiler exception     The C  compiler is pretty clever - anon types generated separately will have the same generated type if their properties match   Outside of that  as long as you have intellisense it makes good sense to use var anywhere the context is clear     less typing  this is good var myList   new List lt UnreasonablyLongClassName gt        also good - I can t be mistaken on type var anotherList   GetAllOfSomeItem       but not here - probably best to leave single value types declared var decimalNum   123 456m

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Eric s answer here     Namespace scoped aliases for generic types in C   is related     Part of the issue is that there is no strongly typed aliasing in C    So many developers use var as a partial surrogate

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To me  the antipathy towards var illustrates why bilingualism in  NET is important   To those C  programmers who have also done VB  NET  the advantages of var are intuitively obvious   The standard C  declaration of   List lt string gt  whatever   new List lt string gt       is the equivalent  in VB  NET  of typing this   Dim whatever As List Of String    New List Of String    Nobody does that in VB  NET  though   It would be silly to  because since the first version of  NET you ve been able to do this     Dim whatever As New List Of String       which creates the variable and initializes it all in one reasonably compact line   Ah  but what if you want an IList lt string gt   not a List lt string gt    Well  in VB  NET that means you have to do this   Dim whatever As IList Of String    New List Of String    Just like you d have to do in C   and obviously couldn t use var for   IList lt string gt  whatever   new List lt string gt       If you need the type to be something different  it can be   But one of the basic principles of good programming is reducing redundancy  and that s exactly what var does

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I don t use var as it goes against the roots of C  - C C   Java  Even though it s a compiler trick it makes the language feel like it s less strongly typed  Maybe 20  years of C have engrained it all into our  the anti-var people s  heads that we should have the type on both the left and right side of the equals   Having said that I can see its merits for long generic collection definitions and long class names like the codinghorror com example  but elsewhere such as string int bool I really can t see the point  Particularly  foreach  var s in stringArray         a saving of 3 characters   The main annoyance for me is not being able to see the type that the var represents for method calls  unless you hover over the method or F12 it

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Arriving a bit late at this discussion  but I d just like to add a thought   To all those who are against type inference  because that s what we re really talking about here   what about lambda expressions  If you insist on always declaring types explicitly  except for anonymous types   what do you do with lambdas  How does the  Don t make me use mouseover  argument apply to var but not to lambdas   UPDATE  I ve just thought of one argument against  var  which I don t think anyone has mentioned yet  which is that it  breaks   Find all references   which could mean  for example  that if you were checking out usage of a class before refactoring  you would miss all the place where the class was used via var

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If someone is using the var keyword because they don t want to  figure out the type   that is definitely the wrong reason  The var keyword doesn t create a variable with a dynamic type  the compiler still has to know the type  As the variable always has a specific type  the type should also be evident in the code if possible   Good reasons to use the var keyword are for example    Where it s needed   i e  to declare a reference for an anonymous type  Where it makes the code more readable  i e  removing repetetive declarations    Writing out the data type often makes the code easier to follow  It shows what data types you are using  so that you don t have to figure out the data type by first figuring out what the code does

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var is great when you don t want to repeat yourself  For example  I needed a data structure yesterday that was similar to this  Which representation do you prefer   Dictionary lt string  Dictionary lt string  List lt MyNewType gt  gt  gt  collection   new Dictionary lt string  Dictionary lt string  List lt MyNewType gt  gt  gt       or  var collection   new Dictionary lt string  Dictionary lt string  List lt MyNewType gt  gt  gt       Note that there is little ambiguity introduced by using var in this example  However  there are times when it wouldn t be such a good idea  For example  if I used var as in the following   var value  5    when I could just write the real type and remove any ambiguity in how 5 should be represented   double value   5

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kronoz - in that case  overloads for both  would it matter  If you have two overloads that took the different types you would essentially be saying that either can be passed and do the same thing   You shouldn t have two overloads that do completely different actions depending on the types passed   While you might get some confusion in that instance it would still be entirely type safe  you d just have someone calling the wrong method

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There is bound to be disagreement near the edge cases  but I can tell you my personal guidelines   I look at these the criteria when I decide to use var    The type of the variable is obvious  to a human  from the context The exact type of the variable is not particularly relevant  to a human   e g  you can figure out what the algorithm is doing without caring about what kind of container you are using  The type name is very long and interrupts the readability of the code  hint  usually a generic    Conversely  these situations would push me to not use var    The type name is relatively short and easy to read  hint  usually not a generic  The type is not obvious from the initializer s name The exact type is very important to understand the code algorithm On class hierarchies  when a human can t easily tell which level of the hierarchy is being used   Finally  I would never use var for native value types or corresponding nullable lt  gt  types  int  decimal  string  decimal         There is an implicit assumption that if you use var  there must be  a reason    These are all guidelines  You should also think also about the experience and skills of your coworkers  the complexity of the algorithm  the longevity scope of the variable  etc  etc   Most of the time  there is no perfect right answer  Or  it doesn t really matter    Edit  removed a duplicate bullet

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Many time during testing  I find myself having code like this   var something   myObject SomeProperty SomeOtherThing CallMethod    Console WriteLine something     Now  sometimes  I ll want to see what the SomeOtherThing itself contains  SomeOtherThing is not the same type that CallMethod   returns  Since I m using var though  I just change this   var something   myObject SomeProperty SomeOtherThing CallMethod      to this   var something   myObject SomeProperty SomeOtherThing    Without var  I d have to keep changing the declared type on the left hand side as well  I know it s minor  but it s extremely convenient

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After just converting over to the 3 0 and 3 5 frameworks I learned about this keyword and decided to give it a whirl  Before committing any code I had the realization that it seemed backwards  as in going back toward an ASP syntax  So I decided to poke the higher ups to see what they thought    They said go ahead so I use it   With that said I avoid using it where the type requires some investigation  like this   var a   company GetRecords     Now it could just be a personal thing but I immediately cant look at that and determine if its a collection of Record objects or a string array representing the name of records  Whichever the case I believe explicit declaration is useful in such an instance

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It s not bad  it s more a stylistic thing  which tends to be subjective  It can add inconsistencies  when you do use var and when you don t    Another case of concern  in the following call you can t tell just by looking at the code the type returned by CallMe   var variable   CallMe      That s my main complain against var   I use var when I declare anonymous delegates in methods  somehow var looks cleaner than if I d use Func  Consider this code   var callback   new Func lt IntPtr  bool gt  delegate IntPtr hWnd                 EDIT  Updated the last code sample based on Julian s input

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From the discussion on this topic  the outcome appears to be   Good  var customers   new List lt Customer gt      Controversial  var customers   dataAccess GetCustomers     Ignoring the misguided opinion that  var  magically helps with refactoring  the biggest issue for me is people s insistence that they don t care what the return type is   so long as they can enumerate the collection      Consider     IList lt Customer gt  customers   dataAccess GetCustomers     var dummyCustomer   new Customer    customers Add dummyCustomer     Now consider   var customers   dataAccess GetCustomers     var dummyCustomer   new Customer    customers Add dummyCustomer     Now  go and refactor the data access class  so that GetCustomers returns IEnumerable lt Customer gt   and see what happens     The problem here is that in the first example you re making your expectations of the GetCustomers method explicit - you re saying that you expect it to return something that behaves like a list  In the second example  this expectation is implicit  and not immediately obvious from the code   It s interesting  to me  that a lot of pro-var arguments say  i don t care what type it returns   but go on to say  i just need to iterate over it        so it needs to implement the IEnumerable interface  implying the type does matter

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Stolen from the post on this issue at CodingHorror     Unfortunately  you and everyone else pretty much got it wrong  While I agree with you that redundancy is not a good thing  the better way to solve this issue would have been to do something like the following   MyObject m   new     Or if you are passing parameters   Person p   new  FirstName    LastName    Where in the creation of a new object  the compiler infers the type from the left-hand side  and not the right  This has other advantages over  var   in that it could be used in field declarations as well  there are also some other areas that it could be useful as well  but I won t get into it here    In the end  it just wasn t intended to reduce redundancy  Don t get me wrong   var  is VERY important in C  for anonymous types projections  but the use here is just WAY off  and I ve been saying this for a long  long time  as you obfuscate the type that is being used  Having to type it twice is too often  but declaring it zero times is too few   Nicholas Paldino  NET C  MVP on June 20  2008 08 00 AM    I guess if your main concern is to have to type less -- then there isn t any argument that s going to sway you from using it   If you are only going to ever be the person who looks at your code  then who cares   Otherwise  in a case like this   var people   Managers People   it s fine  but in a case like this   var fc   Factory Run      it short circuits any immediate type deductions my brain could begin forming from the  English  of the code   Otherwise  just use your best judgment and programming  courtesy  towards others who might have to work on your project

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With LINQ another very good reason to use var is that the compiler can optimize the query much more   If you use a static list to store the result it will execute where you assign it to the list but with var it can potential merge the original query with later queries in the code to make more optimized queries to the database   I had an example where I pulled some data in a first query and then looped over and requested more data to print out a table   LINQ merges these so that the first only pulled the id   Then in the loop it added an extra join I had not done there to fetch the data I had included in the original   When tested this proved much more efficient   Had we not used var it had made the queries exactly as we had written them

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I only use var when it s clear to see what type is used   For example  I would use var in this case  because you can see immediately that x will be of the type  MyClass    var x   new MyClass      I would NOT use var in cases like this  because you have to drag the mouse over the code and look at the tooltip to see what type MyFunction returns   var x   MyClass MyFunction      Especially  I never use var in cases where the right side is not even a method  but only a value   var x   5     because the compiler can t know if I want a byte  short  int or whatever

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var is the way to deal with anonymous types  whether from LINQ statements or not  Any other use is heavily dependent on who will read your code and what guidelines are in place    If you are the only audience or your audience is comfortable with using var or is very familiar with your code then I guess it doesn t matter  If you use it like  var s   new SqlConnection   then it largely doesnt matter and probably improves code readability  If people aren t too picky and its okay for them to do a little work to know the type when its not apparent  which is not needed in most cases  how you use it in the following statements would usually explain everything  then its alright   But if you have picky  close-minded teammates who love to whine or if your company s design guidelines specifically forbid using var when the type is not obvious then you will most likely meet heavy opposition    If using var makes your code insanely difficult to read  you will probably get shot by using var even if its probably your app design that is to blame   If var introduces ambiguity  sort of like your IEnumerable IEnumerable example   just don t use it and be explicit  But var does have its conveniences and in some cases  IMHO  even improves readabilty by reducing clutter

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I think the key thing with VAR is to only use it where appropriate i e  when doing things in Linq that it facilitates  and probably in other cases     If you ve got a type for something in the then you should use it - not to do so is simple laziness  as opposed to creative laziness which is generally to be encouraged - good programmers oft work very hard to be lazy and could be considered the source of the thing in the first place    A blanket ban is as bad as abusing the construct in the first place but there does need to be a sensible coding standard   The other thing to remember is that its not a VB type var in that it can t change types - it is a strongly typed variable its just that the type is inferred  which is why there are people that will argue that its not unreasonable to use it in  say  a foreach but I d disagree for reasons of both readability and maintainability    I suspect this one is going to run and run  -   Murph

[c#] Use of var keyword in C#

Examples related to c#

Examples related to type-inference

Examples related to var