C# Error CS0662 – Cannot specify the Out attribute on a ref parameter without also specifying the In attribute.

C# Error

CS0662 – Cannot specify the Out attribute on a ref parameter without also specifying the In attribute.

Reason for the Error & Solution

‘method’ cannot specify only Out attribute on a ref parameter. Use both In and Out attributes, or neither.

An interface method has a parameter that uses with just the attribute. A ref parameter that uses the Out attribute must also use the attribute.

The following sample generates CS0662:

// CS0662.cs  
using System.Runtime.InteropServices;  
  
interface I  
{  
   void method([Out] ref int i);   // CS0662  
   // try one of the following lines instead  
   // void method(ref int i);  
   // void method([Out, In]ref int i);  
}  
  
class test  
{  
   public static void Main()  
   {  
   }  
}