Question

Consider the following DllImport:

[DllImport("lulz.so")]
public static extern int DoSomething(IntPtr SomeParam);

This is actually referencing a C style function like this:

int DoSomething(void* SomeParam); 

Consider that SomeParam is an “out” param, but can also be NULL. The C function behaves differently if the param is NULL. So I would probably want:

[DllImport("lulz.so")]
public static extern int DoSomething(out IntPtr SomeParam);

But, if I make it an out param in my import, I cannot pass it NULL, i.e. I can’t do this:

int retVal = DoSomething(IntPtr.Zero)

What are my options here?

Answer 1

If you’re trying to pass a value, then out is not the right keyword; change it to ref. You’ll still need to explicitly pass a variable, but it can be a null reference.

For example…

[DllImport("lulz.so")]
public static extern int DoSomething(ref IntPtr SomeParam);

You can then call it like this:

IntPtr retVal = IntPtr.Zero;

DoSomething(ref retVal);

However

What is telling you that it needs to be either out or ref? Passing an IntPtr as out or ref is really akin to passing a double pointer. It would actually seem more appropriate to pass the parameter as an IntPtr.

The typical procedure is either to allocate the necessary memory in managed code and pass an IntPtr representing that allocated memory, or IntPtr.Zero to represent a null pointer. You do not need to pass the IntPtr as out or ref in order to send data back to .NET; you only need to do that if the function you’re calling would actually change the pointer’s address.