Compiler Error CS0217

In order to be applicable as a short circuit operator a user-defined logical operator ('operator') must have the same return type as the type of its 2 parameters.

If you define an operator for a user-defined type, and then try to use the operator as a short-circuit operator, the user-defined operator must have parameters and return values of the same type. For more information on short-circuit operators, see && Operator and || Operator.

The following sample generates CS0217:

// CS0217.cs
using System;

public class MyClass
{
   public static bool operator true (MyClass f)
   {
      return false;
   }

   public static bool operator false (MyClass f)
   {
      return false;
   }

   public static implicit operator int(MyClass x)
   {
      return 0;
   }

   public static int operator & (MyClass f1, MyClass f2)   // CS0217
   // try the following line instead
   // public static MyClass operator & (MyClass f1, MyClass f2)
   {
      return new MyClass();
   }

   public static void Main()
   {
      MyClass f = new MyClass();
      int i = f && f;
   }
}

See Also

Reference

Overloadable Operators (C# Programming Guide)