thank you for reaching out to us. Please have a look at our Error System documentation. You are missing an iferr_return and an error scope in your code.

int c4d_main(int action, void* p1, void* p2, void* p3) 
{
  // Errors imply being propagated upwards in a call chain. When we want to terminate such chain,
  // i.e., have this function return `int` (you should use Int32 as int is ambiguous) instead of
  // `maxon::Result<maxon::Int32>` we can use a custom error handler to handle errors being thrown.
  // When the return type is `maxon::Result<T>` we can just use an `iferr_scope;` instead.
  iferr_scope_handler
  {
      DebugOutput("c4d_main failed with '@'", err);
      return 0;
  };

  // Create methods return a `maxon::Result<T>` object, i.e., either the value or the error. The
  // `iferr_return` keyword will unpack that error for us.
  MyIfcRef ifc = MyIfcDecl().Create() iferr_return;

  // Technically possible, but bad code:
  maxon::Result<MyIfcRef> result = MyIfcDecl().Create();

  // Can make sense in some cases, but should be generally avoided:
  MyIfcRef ifc = MyIfcDecl().Create() iferr_ignore("We don't care about the error here."_s);

  // Finally, there is a special form of branching that allows to handle errors in a more detailed way:
  ifnoerr (MyIfcRef ifc = MyIfcDecl().Create())
  {
    // Runs when the creation was successful.
  }
  
  iferr (MyIfcRef ifc = MyIfcDecl().Create())
  {
    // Would run when the creation failed.
  }

  return 1;
}

Cheers,
Ferdinand