What is the dynamic type in C 4 0 used for

Question

C  4 0 introduced a new type called  dynamic   It all sounds good  but what would a programmer use it for   Is there a situation where it can save the day

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You can call into dynamic languages such as CPython using pythonnet:

dynamic np = Py.Import("numpy")

You can cast generics to dynamic when applying numerical operators on them. This provides type safety and avoids limitations of generics. This is in essence *duck typing:

T y = x * (dynamic)x, where typeof(x) is T

User · Answer

An example of use    You consume many classes that have a commun property  CreationDate     public class Contact          some properties      public DateTime CreationDate   get  set               public class Company          some properties      public DateTime CreationDate   get  set        public class Opportunity          some properties      public DateTime CreationDate   get  set         If you write a commun method that retrieves the value of the  CreationDate   Property  you d have to use reflection       static DateTime RetrieveValueOfCreationDate Object item                return  DateTime item GetType   GetProperty  CreationDate   GetValue item           With the  dynamic  concept  your code is much more elegant        static DateTime RetrieveValueOfCreationDate dynamic item                return item CreationDate

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Another use case for dynamic typing is for virtual methods that experience a problem with covariance or contravariance  One such example is the infamous Clone method that returns an object of the same type as the object it is called on  This problem is not completely solved with a dynamic return because it bypasses static type checking  but at least you don t need to use ugly casts all the time as per when using plain object  Otherwise to say  the casts become implicit   public class A          attributes and constructor here     public virtual dynamic Clone                 var clone   new A               Do more cloning stuff here         return clone           public class B   A          more attributes and constructor here     public override dynamic Clone                 var clone   new B                   Do more cloning stuff here         return clone               public class Program       public static void Main                 A a   new A   Clone        No cast needed here         B b   new B   Clone        and here            do more stuff with a and b

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The dynamic keyword is new to C  4 0  and is used to tell the compiler that a variable s type can change or that it is not known until runtime  Think of it as being able to interact with an Object without having to cast it   dynamic cust   GetCustomer    cust FirstName    foo      works as expected cust Process       works as expected cust MissingMethod       No method found    Notice we did not need to cast nor declare cust as type Customer  Because we declared it dynamic  the runtime takes over and then searches and sets the FirstName property for us  Now  of course  when you are using a dynamic variable  you are giving up compiler type checking  This means the call cust MissingMethod   will compile and not fail until runtime  The result of this operation is a RuntimeBinderException because MissingMethod is not defined on the Customer class   The example above shows how dynamic works when calling methods and properties  Another powerful  and potentially dangerous  feature is being able to reuse variables for different types of data  I m sure the Python  Ruby  and Perl programmers out there can think of a million ways to take advantage of this  but I ve been using C  so long that it just feels  wrong  to me   dynamic foo   123  foo    bar     OK  so you most likely will not be writing code like the above very often  There may be times  however  when variable reuse can come in handy or clean up a dirty piece of legacy code  One simple case I run into often is constantly having to cast between decimal and double   decimal foo   GetDecimalValue    foo   foo   2 5     Does not compile foo   Math Sqrt foo      Does not compile string bar   foo ToString  c      The second line does not compile because 2 5 is typed as a double and line 3 does not compile because Math Sqrt expects a double  Obviously  all you have to do is cast and or change your variable type  but there may be situations where dynamic makes sense to use   dynamic foo   GetDecimalValue       still returns a decimal foo   foo   2 5     The runtime takes care of this for us foo   Math Sqrt foo      Again  the DLR works its magic string bar   foo ToString  c      Read more feature   http   www codeproject com KB cs CSharp4Features aspx

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COM interop  Especially IUnknown  It was designed specially for it

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I am surprised that nobody mentioned multiple dispatch  The usual way to work around this is via Visitor pattern and that is not always possible so you end up with stacked is checks   So here is a real life example of an application of my own  Instead of doing   public static MapDtoBase CreateDto ChartItem item        if  item is ElevationPoint  return CreateDtoImpl  ElevationPoint item       if  item is MapPoint  return CreateDtoImpl  MapPoint item       if  item is MapPolyline  return CreateDtoImpl  MapPolyline item         other subtypes follow     throw new ObjectNotFoundException  Counld not find suitable DTO for     item GetType         You do   public static MapDtoBase CreateDto ChartItem item        return CreateDtoImpl item as dynamic      private static MapDtoBase CreateDtoImpl ChartItem item        throw new ObjectNotFoundException  Counld not find suitable DTO for     item GetType        private static MapDtoBase CreateDtoImpl MapPoint item        return new MapPointDto item      private static MapDtoBase CreateDtoImpl ElevationPoint item        return new ElevationDto item       Note that in first case ElevationPoint is subclass of MapPoint and if it s not placed before MapPointit will never be reached  This is not the case with dynamic  as the closest matching method will be called   As you might guess from the code  that feature came handy while I was performing translation from ChartItem objects to their serializable versions  I didn t want to pollute my code with visitors and I didn t want also to pollute my ChartItem objects with useless serialization specific attributes

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It evaluates at runtime  so you can switch the type like you can in JavaScript to whatever you want   This is legit   dynamic i   12  i    text     And so you can change the type as you need   Use it as a last resort  it i s beneficial  but I heard a lot goes on under the scenes in terms of generated IL and that can come at a performance price

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The best use case of  dynamic  type variables for me was when  recently  I was writing a data access layer in ADO NET  using SQLDataReader  and the code was invoking the already written legacy stored procedures  There are hundreds of those legacy stored procedures containing bulk of the business logic  My data access layer needed to return some sort of structured data to the business logic layer  C  based  to do some manipulations  although there are almost none   Every stored procedures returns different set of data  table columns   So instead of creating dozens of classes or structs to hold the returned data and pass it to the BLL  I wrote the below code which looks quite elegant and neat   public static dynamic GetSomeData ParameterDTO dto                        dynamic result   null              string SPName    a legacy stored procedure               using  SqlConnection connection   new SqlConnection DataConnection ConnectionString                                 SqlCommand command   new SqlCommand SPName  connection                   command CommandType   System Data CommandType StoredProcedure                                  command Parameters Add new SqlParameter   empid   dto EmpID                    command Parameters Add new SqlParameter   deptid   dto DeptID                    connection Open                    using  SqlDataReader reader   command ExecuteReader                                          while  reader Read                                                  dynamic row   new ExpandoObject                            row EmpName   reader  EmpFullName   ToString                            row DeptName   reader  DeptName   ToString                            row AnotherColumn   reader  AnotherColumn   ToString                                                    result   row                                                                    return result

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It makes it easier for static typed languages  CLR  to interoperate with dynamic ones  python  ruby      running on the DLR  dynamic language runtime   see MSDN      For example  you might use the following code to increment a counter   in XML in C    Scriptobj SetProperty  Count     int GetProperty  Count      1         By using the DLR  you could use the following code instead for the   same operation   scriptobj Count    1     MSDN lists these advantages         Simplifies Porting Dynamic Languages to the  NET Framework   Enables Dynamic Features in Statically Typed Languages   Provides Future Benefits of the DLR and  NET Framework   Enables Sharing of Libraries and Objects   Provides Fast Dynamic Dispatch and Invocation      See MSDN for more details

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The dynamic keyword was added  together with many other new features of C  4 0  to make it simpler to talk to code that lives in or comes from other runtimes  that has different APIs   Take an example   If you have a COM object  like the Word Application object  and want to open a document  the method to do that comes with no less than 15 parameters  most of which are optional   To call this method  you would need something like this  I m simplifying  this is not actual code    object missing   System Reflection Missing Value  object fileName    C   test docx   object readOnly   true  wordApplication Documents Open ref fileName  ref missing  ref readOnly      ref missing  ref missing  ref missing  ref missing  ref missing      ref missing  ref missing  ref missing  ref missing  ref missing      ref missing  ref missing     Note all those arguments  You need to pass those since C  before version 4 0 did not have a notion of optional arguments  In C  4 0  COM APIs have been made easier to work with by introducing    Optional arguments Making ref optional for COM APIs Named arguments   The new syntax for the above call would be   wordApplication Documents Open   C  Test docx   ReadOnly  true     See how much easier it looks  how much more readable it becomes   Let s break that apart                                       named argument  can skip the rest                                                                                                         v wordApplication Documents Open   C  Test docx   ReadOnly  true                                                                                                                                                            notice no ref keyword  can pass                                actual parameter values instead   The magic is that the C  compiler will now inject the necessary code  and work with new classes in the runtime  to do almost the exact same thing that you did before  but the syntax has been hidden from you  now you can focus on the what  and not so much on the how  Anders Hejlsberg is fond of saying that you have to invoke different  incantations   which is a sort of pun on the magic of the whole thing  where you typically have to wave your hand s  and say some magic words in the right order to get a certain type of spell going  The old API way of talking to COM objects was a lot of that  you needed to jump through a lot of hoops in order to coax the compiler to compile the code for you   Things break down in C  before version 4 0 even more if you try to talk to a COM object that you don t have an interface or class for  all you have is an IDispatch reference   If you don t know what it is  IDispatch is basically reflection for COM objects  With an IDispatch interface you can ask the object  what is the id number for the method known as Save   and build up arrays of a certain type containing the argument values  and finally call an Invoke method on the IDispatch interface to call the method  passing all the information you ve managed to scrounge together   The above Save method could look like this  this is definitely not the right code    string   methodNames   new      Open     Guid IID       int methodId   wordApplication GetIDsOfNames IID  methodNames  methodNames Length  lcid  dispid   SafeArray args   new SafeArray new     fileName  missing  missing           wordApplication Invoke methodId      args          All this for just opening a document   VB had optional arguments and support for most of this out of the box a long time ago  so this C  code   wordApplication Documents Open   C  Test docx   ReadOnly  true     is basically just C  catching up to VB in terms of expressiveness  but doing it the right way  by making it extendable  and not just for COM  Of course this is also available for VB NET or any other language built on top of the  NET runtime   You can find more information about the IDispatch interface on Wikipedia  IDispatch if you want to read more about it  It s really gory stuff   However  what if you wanted to talk to a Python object  There s a different API for that than the one used for COM objects  and since Python objects are dynamic in nature as well  you need to resort to reflection magic to find the right methods to call  their parameters  etc  but not the  NET reflection  something written for Python  pretty much like the IDispatch code above  just altogether different   And for Ruby  A different API still   JavaScript  Same deal  different API for that as well   The dynamic keyword consists of two things    The new keyword in C   dynamic A set of runtime classes that knows how to deal with the different types of objects  that implement a specific API that the dynamic keyword requires  and maps the calls to the right way of doing things  The API is even documented  so if you have objects that comes from a runtime not covered  you can add it    The dynamic keyword is not  however  meant to replace any existing  NET-only code  Sure  you can do it  but it was not added for that reason  and the authors of the C  programming language with Anders Hejlsberg in the front  has been most adamant that they still regard C  as a strongly typed language  and will not sacrifice that principle   This means that although you can write code like this   dynamic x   10  dynamic y   3 14  dynamic z    test   dynamic k   true  dynamic l   x   y   z - k    and have it compile  it was not meant as a sort of magic-lets-figure-out-what-you-meant-at-runtime type of system   The whole purpose was to make it easier to talk to other types of objects   There s plenty of material on the internet about the keyword  proponents  opponents  discussions  rants  praise  etc   I suggest you start with the following links and then google for more    DevDays 2010  Anders Hejlsberg - C  4 0 and beyond Channel 9  Mads Torgersen - Inside C  4 0  dynamic typing     DevX  COM Interop Gets Much Better in C  4 0 Scott Hanselman - C  4 and the dynamic keyword - Whirlwind Tour around  NET 4  and Visual Studio 2010  Beta 1

[c#] What is the 'dynamic' type in C# 4.0 used for?

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