C# - dynamic Keyword - Dot Net Perls

Example. Dynamic is used to replace other types such as int, bool or string. We assign the value 1 to the dynamic variable a. This means "a" is of type System.Int32.

int

Next We assign it to a string array. The type again changes. You can also use dynamic as a return type (or formal parameter type).

Finally We use a dynamic static field, and call a nonexistent property on it. This would not be possible without dynamic.

Array

using System; class Program { static dynamic _y; static void Main() { // Use dynamic local. dynamic a = 1; Console.WriteLine(a); // Dynamic now has a different type. a = new string[0]; Console.WriteLine(a); // Assign to dynamic method result. a = Test(); Console.WriteLine(a); // Use dynamic field. _y = "carrot"; // We can call anything on a dynamic variable. // ... It may result in a runtime error. Console.WriteLine(_y.Error); } static dynamic Test() { return 1; } }

1 System.String[] 1 Unhandled Exception: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'string' does not contain a definition for 'Error'...

Internals. You won't find any "dynamic" types in the virtual execution engine. Instead, the C# compile builds up an alternate representation of your dynamic code.

Detail Looking at the IL, we see that the dynamic keyword results in a custom class being generated by the compiler.

Next We can look at the call sites themselves. At each call site, the site field from the above static class is instantiated.

[CompilerGenerated] private static class <Main>o__SiteContainer0 { // Fields public static CallSite<Action<CallSite, Type, object>> <>p__Site1; public static CallSite<Action<CallSite, Type, object>> <>p__Site2; public static CallSite<Action<CallSite, Type, object>> <>p__Site3; }

if (<Main>o__SiteContainer0.<>p__Site1 == null) { <Main>o__SiteContainer0.<>p__Site1 = CallSite<Action<CallSite, Type, object>>.Create( Binder.InvokeMember(CSharpBinderFlags.ResultDiscarded, "WriteLine", null, typeof(Program), new CSharpArgumentInfo[] { CSharpArgumentInfo.Create( CSharpArgumentInfoFlags.IsStaticType | CSharpArgumentInfoFlags.UseCompileTimeType, null), CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null) })); } <Main>o__SiteContainer0.<>p__Site1.Target(<Main>o__SiteContainer0.<>p__Site1, typeof(Console), a);

Type. The dynamic keyword is used as a type in the C# language, but it is not a type in the underlying intermediate language. Thus, it can be considered a high-level type but not a low-level type.

Note Most constructs in the C# language correspond fairly closely to their underlying representation.

Tip Other exceptions to this rule include the yield return construct and the params keyword.

yield

params

Uses. It is definitely a bad idea to use dynamic in all cases where it can be used. This is because your programs will lose the benefits of compile-time checking and they will also be much slower.

Note All the code generated by the C# compiler would yield a program that is many times slower than a regular C# program.

Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.

Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.

This page was last updated on Nov 19, 2023 (simplify).

Home

Changes