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Compiler Warning (level 1) CS0197

Passing 'argument' as ref or out or taking its address may cause a runtime exception because it is a field of a marshal-by-reference class

Any class that derives, directly or indirectly, from MarshalByRefObject is a marshal-by-reference class. Such a class can be marshaled by reference across process and machine boundaries. Thus, instances of this class could be proxies to remote objects. You cannot pass a field of a proxy object as ref or out. So, you cannot pass fields of such a class as ref or out, unless the instance is this, which can not be a proxy object.

The following sample generates CS0197.

// CS0197.cs
// compile with: /W:1
class X : System.MarshalByRefObject
{
   public int i;
}

class M
{
   public int i;
   static void AddSeventeen(ref int i)
   {
      i += 17;
   }

   static void Main()
   {
      X x = new X();
      x.i = 12;
      AddSeventeen(ref x.i);   // CS0197

      // OK
      M m = new M();
      m.i = 12;
      AddSeventeen(ref m.i);
   }
}

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