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Compiler Error CS0217

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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 about short-circuit operators, see && operator and || operator. For more information about user-defined short-circuit, or conditional, operators, see the User-defined conditional logical operators section of the C# language specification.

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