Direct casting vs as operator

Question

Consider the following code   void Handler object o  EventArgs e          I swear o is a string    string s    string o     1      -OR-    string s   o as string     2       -OR-    string s   o ToString       3     What is the difference between the three types of casting  okay  the 3rd one is not a casting  but you get the intent   Which one should be preferred

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When trying to get the string representation of anything  of any type  that could potentially be null  I prefer the below line of code  It s compact  it invokes ToString    and it correctly handles nulls  If o is null  s will contain String Empty   String s   String Concat o

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It really depends on whether you know if o is a string and what you want to do with it  If your comment means that o really really is a string  I d prefer the straight  string o cast - it s unlikely to fail   The biggest advantage of using the straight cast is that when it fails  you get an InvalidCastException  which tells you pretty much what went wrong   With the as operator  if o isn t a string  s is set to null  which is handy if you re unsure and want to test s   string s   o as string  if   s    null            well that s not good      gotoPlanB        However  if you don t perform that test  you ll use s later and have a NullReferenceException thrown  These tend to be more common and a lot harder to track down once they happens out in the wild  as nearly every line dereferences a variable and may throw one  On the other hand  if you re trying to cast to a value type  any primitive  or structs such as DateTime   you have to use the straight cast - the as won t work   In the special case of converting to a string  every object has a ToString  so your third method may be okay if o isn t null and you think the ToString method might do what you want

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I would like to attract attention to the following specifics of the as operator   https   docs microsoft com en-us dotnet csharp language-reference keywords as     Note that the as operator performs only reference conversions    nullable conversions  and boxing conversions  The as operator can t   perform other conversions  such as user-defined conversions  which   should instead be performed by using cast expressions

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string s   o as string     2   Is prefered  as it avoids the performance penalty of double casting

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Since nobody mentioned it  the closest to instanceOf to Java by keyword is this   obj GetType   IsInstanceOfType otherObj

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2 is useful for casting to a derived type   Suppose a is an Animal   b   a as Badger  c   a as Cow   if  b    null     b EatSnails    else if  c    null     c EatGrass      will get a fed with a minimum of casts

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The as keyword is good in asp net when you use the FindControl method   Hyperlink link   this FindControl  linkid   as Hyperlink  if  link    null                 This means you can operate on the typed variable rather then having to then cast it from object like you would with a direct cast   object linkObj   this FindControl  linkid    if  link    null         Hyperlink link    Hyperlink linkObj      It s not a huge thing  but it saves lines of code and variable assignment  plus it s more readable

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It seems the two of them are conceptually different   Direct Casting  Types don t have to be strictly related  It comes in all types of flavors     Custom implicit explicit casting  Usually a new object is created  Value Type Implicit  Copy without losing information   Value Type Explicit  Copy and information might be lost  IS-A relationship  Change reference type  otherwise throws exception  Same type   Casting is redundant     It feels like the object is going to be converted into something else   AS operator   Types have a direct relationship  As in     Reference Types  IS-A relationship Objects are always the same  just the reference changes  Value Types  Copy boxing and nullable types    It feels like the you are going to handle the object in a different way   Samples and IL      class TypeA               public int value             class TypeB               public int number           public static explicit operator TypeB TypeA v                        return new TypeB     number   v value                         class TypeC   TypeB         interface IFoo         class TypeD   TypeA  IFoo          void Run                 TypeA customTypeA   new TypeD     value   10            long longValue   long MaxValue          int intValue   int MaxValue              Casting          TypeB typeB    TypeB customTypeA     custom explicit casting -- IL   call class ConsoleApp1 Program TypeB ConsoleApp1 Program TypeB  op Explicit class ConsoleApp1 Program TypeA          IFoo foo    IFoo customTypeA     is-a reference -- IL  castclass  ConsoleApp1 Program IFoo          int loseValue    int longValue     explicit -- IL  conv i4         long dontLose   intValue     implict -- IL  conv i8             AS          int  wraps   intValue as int      nullable wrapper -- IL   call instance void valuetype  System Runtime System Nullable 1 lt int32 gt    ctor  0          object o1   intValue as object     box -- IL  box  System Runtime System Int32         TypeD d1   customTypeA as TypeD     reference conversion -- IL  isinst ConsoleApp1 Program TypeD         IFoo f1   customTypeA as IFoo     reference conversion -- IL  isinst ConsoleApp1 Program IFoo            TypeC d   customTypeA as TypeC     wouldn t compile

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as  is based on  is   which is a keyword that checks at runtime if the object is polimorphycally compatible  basically if a cast can be made  and returns null if the check fails   These two are equivalent   Using  as    string s   o as string    Using  is    if o is string       s   o  else     s   null    On the contrary  the c-style cast is made also at runtime  but throws an exception if the cast cannot be made   Just to add an important fact   The  as  keyword only works with reference types  You cannot do      I swear i is an int int number   i as int    In those cases you have to use casting

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string o  will result in an InvalidCastException as there s no direct cast    o as string  will result in s being a null reference  rather than an exception being thrown    o ToString    isn t a cast of any sort per-se  it s a method that s implemented by object  and thus in one way or another  by every class in  net that  does something  with the instance of the class it s called on and returns a string   Don t forget that for converting to string  there s also Convert ToString someType instanceOfThatType  where someType is one of a set of types  essentially the frameworks base types

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If you already know what type it can cast to  use a C-style cast   var o    string  iKnowThisIsAString     Note that only with a C-style cast can you perform explicit type coercion   If you don t know whether it s the desired type and you re going to use it if it is  use as keyword   var s   o as string  if  s    null  return s Replace      -       or for early return  if  s  null  return    Note that as will not call any type conversion operators  It will only be non-null if the object is not null and natively of the specified type   Use ToString   to get a human-readable string representation of any object  even if it can t cast to string

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string s    string o     1   Throws InvalidCastException if o is not a string  Otherwise  assigns o to s  even if o is null   string s   o as string     2   Assigns null to s if o is not a string or if o is null  For this reason  you cannot use it with value types  the operator could never return null in that case   Otherwise  assigns o to s   string s   o ToString       3   Causes a NullReferenceException if o is null  Assigns whatever o ToString   returns to s  no matter what type o is     Use 1 for most conversions - it s simple and straightforward  I tend to almost never use 2 since if something is not the right type  I usually expect an exception to occur  I have only seen a need for this return-null type of functionality with badly designed libraries which use error codes  e g  return null   error  instead of using exceptions    3 is not a cast and is just a method invocation  Use it for when you need the string representation of a non-string object

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All given answers are good  if i might add something  To directly use string s methods and properties  e g  ToLower  you can t write    string o ToLower       won t compile   you can only write     string o  ToLower      but you could write instead    o as string  ToLower      The as option is more readable  at least to my opinion

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Use direct cast string s    string  o  if in the logical context of your app string is the only valid type  With this approach  you will get InvalidCastException and implement the principle of Fail-fast  Your logic will be protected from passing the invalid type further or get NullReferenceException if used as operator   If the logic expects several different types cast string s   o as string  and check it on null or use is operator   New cool feature have appeared in C  7 0 to simplify cast and check is a Pattern matching   if o is string s         Use string variable s    or  switch  o      case int i          Use int variable i      break    case string s          Use string variable s      break

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string s    string o  Use when something should definitely be the other thing   string s   o as string  Use when something might be the other thing   string s   o ToString    Use when you don t care what it is but you just want to use the available string representation

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According to experiments run on this page  http   www dotnetguru2 org sebastienros index php 2006 02 24 cast vs as   this page is having some  illegal referrer  errors show up sometimes  so just refresh if it does   Conclusion is  the  as  operator is normally faster than a cast  Sometimes by many times faster  sometimes just barely faster    I peronsonally thing  as  is also more readable    So  since it is both faster and  safer   wont throw exception   and possibly easier to read  I recommend using  as  all the time

[c#] Direct casting vs 'as' operator?

Examples related to c#

Examples related to casting