Extension Method การสร้างเมธอดเสริมให้กับคลาส Class (.NET Framework 3.0)

protected void Page_Load( object sender, EventArgs e )
{
         string email = Request.QueryString["email"];
         if ( EmailValidator.IsValidEmail( email ) )
         {
         }
}

protected void Page_Load( object sender, EventArgs e )
{
         string email = Request.QueryString["email"];
         if (email.IsValidEmail() )
         {
         }
}

public static class StringExtension
{
   public static bool IsValidEmail( this string s )
   {
        Regex regex = new Regex( @"^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$" );
        return regex.IsMatch( s );
   }
}

using System.Collections;

public static class MyExtension
{
    public static bool IsIn( this object o, IEnumerable e )
    {
        foreach ( object i in e )
        {
             if ( o.Equals( i ) )
             {
                return true;
             }
         }
         return false;
     }
}

if ( TextBox1.IsIn( Form.Controls ) )
{
}

    int[] values = { 1, 2, 3, 4, 5, 6 };
    bool hasValue = 40.IsIn( values );
}