C# Cast Extension: System.Linq

You want to cast all the elements in a sequence to another type in your C# program. For example, if you have a collection of objects, you can cast these to a base class. The Cast extension method, found in System.Linq, is ideal for this purpose.

Example

This program introduces the A class and the B class. The B class derives from the A class. In the Main entry point, we create an array of B object instances. Next, we use the Cast extension method with type parameter A. This means that each B object will be cast to its base class A.

This C# example demonstrates the Cast extension method from System.Linq.

Program that uses Cast<A> [C#]

using System;
using System.Linq;

class A
{
    public void Y()
    {
	Console.WriteLine("A.Y");
    }
}

class B : A
{
}

class Program
{
    static void Main()
    {
	B[] values = new B[3];
	values[0] = new B();
	values[1] = new B();
	values[2] = new B();

	// Cast all objects to a base type.
	var result = values.Cast<A>();
	foreach (A value in result)
	{
	    value.Y();
	}
    }
}

Output

A.Y
A.Y
A.Y

Result. The final result of the program is that you have an IEnumerable collection of A instances. These are actually still B instances as well, but are now referenced through their base type. We call the Y() method to demonstrate that they are real A objects.

Numeric casts

The Cast extension method is not terribly useful for casting numeric types. For example, you cannot cast a double to an int without an exception. Implicit conversions are allowed, however. For example, you can cast an int to a uint. You can cast anything to the base class object.

Summary

The Cast extension method is of limited utility in most programs. However, if you need to cast a collection in a single statement, this is the best way to do it. Check out the other LINQ articles on this site for more neat declarative programming techniques.