Compiler Error CS0172

Type of conditional expression cannot be determined because 'type1' and 'type2' implicitly convert to one another

In a conditional statement, you must be able to convert the types on either side of the : operator. Also, there cannot be mutual conversion routines; you only need one conversion. For more information, see Conversion Operators (C# Programming Guide).

The following sample generates CS0172:

// CS0172.cs
public class Square
{
   public class Circle
   {
      public static implicit operator Circle(Square aa)
      {
         return null;
      }

      public static implicit operator Square(Circle aa)
      // using explicit resolves this error
      // public static explicit operator Square(Circle aa)
      {
         return null;
      }
   }

   public static void Main()
   {
      Circle aa = new Circle();
      Square ii = new Square();
      object o = (1 == 1) ? aa : ii;   // CS0172
      // the following cast would resolve this error
      // (1 == 1) ? aa : (Circle)ii;
   }
}