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Compiler Error CS0172

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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 implicitly convert the types on either side of the : token. Also, there cannot be mutual implicit conversions; you only need one conversion.

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;  
   }  
}

See also