C# - Dynamic Types

C# 4.0 (.NET 4.5) introduced a new type called dynamic that avoids compile-time type checking. A dynamic type escapes type checking at compile-time; instead, it resolves type at run time.

A dynamic type variables are defined using the dynamic keyword.

Example: dynamic Variable

dynamic MyDynamicVar = 1;

The compiler compiles dynamic types into object types in most cases. However, the actual type of a dynamic type variable would be resolved at run-time.

Example: dynamic Type at run-time

dynamic MyDynamicVar = 1;

Console.WriteLine(MyDynamicVar.GetType());

Output:

System.Int32

Dynamic types change types at run-time based on the assigned value. The following example shows how a dynamic variable changes type based on assigned value.

Example: dynamic

static void Main(string[] args)
{
    dynamic MyDynamicVar = 100;
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());

    MyDynamicVar = "Hello World!!";
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());

    MyDynamicVar = true;
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());

    MyDynamicVar = DateTime.Now;
    Console.WriteLine("Value: {0}, Type: {1}", MyDynamicVar, MyDynamicVar.GetType());
}

Output:

Value: 100, Type: System.Int32
Value: Hello World!!, Type: System.String
Value: True, Type: System.Boolean
Value: 05-02-2026, Type: System.DateTime

The dynamic type variables is converted to other types implicitly.

Example: dynamic Type Conversion

dynamic d1 = 100;
int i = d1;
		
d1 = "Hello";
string greet = d1;
		
d1 = DateTime.Now;
DateTime dt = d1;

---

Methods and Parameters

If you assign a class object to the dynamic type, then the compiler would not check for correct methods and properties name of a dynamic type that holds the custom class object. Consider the following example.

Example: Calling Methods

class Program
{
    static void Main(string[] args)
    {
        dynamic stud = new Student();

        stud.DisplayStudentInfo(1, "Bill");// run-time error, no compile-time error
        stud.DisplayStudentInfo("1");// run-time error, no compile-time error
        stud.FakeMethod();// run-time error, no compile-time error
    }
}

public class Student
{
    public void DisplayStudentInfo(int id)
    {
    }
}

In the above example, the C# compiler does not check for the number of parameters, parameters type, or non-existent. It validates these things at run-time, and if it is not valid, then throws a run-time exception. Note that Visual Studio IntelliSense is not supported for the dynamic types.

The dynamic language runtime (DLR) API provides the infrastructure that supports the dynamic type in C#. For more information about the DLR, visit Dynamic Language Runtime Overview.