using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        B b = null;

        var array = new A[] { new C(), new A(),  b };

        var count = array.OfType<B>().Count();

        Console.WriteLine(count);

    }

}

​

public class A {}

public class B : A {}

public class C : A {}