型比較からのキャストして情報を扱う速度。 (1) o.GetType() == typeof( Obj ) ＋ var obj = ( Obj )o (2) o is Obj + var obj = ( Obj )o (3) var obj = o as Obj + obj != null ※ .NET Framework 4.5.1 (Windows 8.1, Core i7-2620M, RAM 8 GB (+ 4 GB on SSD)) で実行。(1)<(2)<(3) と時間がかかる。

CheckingPatternTest.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;

namespace CheckingPatternTest
{
	class Program
	{
		static void Main( string[] args )
		{
			const long c = 1000000000;

			var watch = Stopwatch.StartNew();
			watch.Stop();
			watch.Reset();

			var b = new Test();
			var a = ( Base )b;

			watch.Start();
			for( int i = 0; i < c; i++ )
			{
				if( a.GetType() == typeof( Test ) )
				{
					var testobj = ( Test )a;
					testobj.A = "ok";
				}
			}
			watch.Stop();
			Console.WriteLine( "a.GetType() == typeof( Test ), var testobj = ( Test )a: {0}", watch.Elapsed );

			watch.Start();
			for( int i = 0; i < c; i++ )
			{
				if( a is Test )
				{
					var testobj = ( Test )a;
					testobj.A = "ok";
				}
			}
			watch.Stop();
			Console.WriteLine( "a is Test, var testobj = ( Test )a: {0}", watch.Elapsed );

			watch.Start();
			for( int i = 0; i < c; i++ )
			{
				var testobj = a as Test;
				if( testobj != null )
				{
					testobj.A = "ok";
				}
			}
			watch.Stop();
			Console.WriteLine( "testobj = a as Test, testobj != null: {0}", watch.Elapsed );

			Console.ReadLine();
		}

		class Base
		{
			public string A { get; set; }
		}

		class Test: Base
		{
			public string B { get; set; }
		}
	}
}