Overloading struct members (or cast?)

[Wsdes via Digitalmars-d-learn]

Hello again and welcome to another tale of mystery :)
I, again, am facing problems connecting my D program to the C API 
as seen in the callback thread before.

I want to write a callback function that is assigned to a struct 
member. The function should write data from members of another 
struct to the command line. Please consider my code:


   void MyCB(void* pVoid) {
     DataInfo_t* userData; //DataInfo_t is a struct
     userData = cast(DataInfo_t*) pVoid;

     writeln("Value: ", pMsgUserData.Values[i].value.string);
   }

Here the DataInfo_t struct has a member Values which itself is a 
struct. The programmer of the API called the members of the union 
inside the Values struct after simple data types or pointers to 
them, specifically

     union value {
          const char       *string;
     }

When I try to compile I get the following error:

           Error: need 'this' for 'string' of type 'const(char*)'

I'm not quite sure what the error wants to tell me. The way I see 
it is that there is some kind of problem with the instance of the 
struct and hence the reference to that specific member value does 
not work.

Any ideas?