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Compiler Error CS0552

'conversion routine' : user defined conversion to/from interface

You cannot create a user-defined conversion to or from an interface. If you need the conversion routine, resolve this error by making the interface a class or derive a class from the interface.

The following sample generates CS0552:

// CS0552.cs
public interface ii

public class a
   // delete the routine to resolve CS0552
   public static implicit operator ii(a aa) // CS0552
      return new ii();

   public static void Main()
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