Dynamic

In the C# language, the dynamic keyword influences compilation. A dynamic variable can have any type. Its type can change during runtime.

The downside is that performance suffers and you lose compile-time checking. Dynamic is advanced functionality—it can be useful, but usually it should be avoided.

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.

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.

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.

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.

The dynamic keyword provides access to a layer of functionality on top of the Common Language Runtime. Dynamic is implemented with a lot of inserted code.