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Compiler Error CS0559 

Error Message

The parameter type for ++ or -- operator must be the containing type

The method declaration for an operator overload must follow certain guidelines. For the ++ and -- operators, it is required that the parameter be of the same type as the type in which the operator is being overloaded.

Example

The following sample generates CS0559:

// CS0559.cs
// compile with: /target:library
public class iii
{
   public static implicit operator int(iii x)
   {
      return 0;
   }

   public static implicit operator iii(int x)
   {
      return null;
   }

   public static int operator ++(int aa)   // CS0559
   // try the following line instead
   // public static iii operator ++(iii aa)
   {
      return (iii)0;
   }
}

The following sample generates CS0559.

// CS0559_b.cs
// compile with: /target:library
public struct S
{
   public static S operator ++(S? s) { return new S(); }   // CS0559
   public static S operator --(S? s) { return new S(); }   // CS0559
}

public struct T
{
// OK
   public static T operator --(T t) { return new T(); }
   public static T operator ++(T t) { return new T(); }

   public static T? operator --(T? t) { return new T(); }
   public static T? operator ++(T? t) { return new T(); }
}

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