Because C# (as well as some other languages) does not allow optional parameters, Missing allows those languages to pass this value to indicate that a value will not be specified for those optional parameters. The only way to access an instance of this class—there can only be one—is by the return value of the static field, called Value.

public sealed class Missing {
// Public Static Fields
   public static readonly Missing Value; 
// =System.Reflection.Missing
}