Dynamic type

Remarks#

The dynamic keyword declares a variable whose type is not known at compile time. A dynamic variable can contain any value, and the type of the value can change during runtime.

As noted in the book “Metaprogramming in .NET”, C# does not have a backing type for the dynamic keyword:

The functionality enabled by the dynamic keyword is a clever set of compiler actions that emit and use CallSite objects in the site container of the local execution scope. The compiler manages what programmers perceive as dynamic object

references through those CallSite instances. The parameters, return types, fields, and properties that get dynamic treatment at compile time may be marked with some metadata to indicate that they were generated for dynamic use, but the underlying data type for them will always be System.Object.

Creating a dynamic variable

dynamic foo = 123;
Console.WriteLine(foo + 234);
// 357    Console.WriteLine(foo.ToUpper())
// RuntimeBinderException, since int doesn't have a ToUpper method

foo = "123";
Console.WriteLine(foo + 234);
// 123234
Console.WriteLine(foo.ToUpper()):
// NOW A STRING

Returning dynamic

using System;

public static void Main()
{
    var value = GetValue();
    Console.WriteLine(value);
    // dynamics are useful!
}

private static dynamic GetValue()
{
    return "dynamics are useful!";
}

Creating a dynamic object with properties

using System;
using System.Dynamic;

dynamic info = new ExpandoObject();
info.Id = 123;
info.Another = 456;

Console.WriteLine(info.Another);
// 456

Console.WriteLine(info.DoesntExist);
// Throws RuntimeBinderException

Handling Specific Types Unknown at Compile Time