Interop question about passing a struct by ref, null pointer and IntPtr

Question

Colleagues,

Preamble. My question is more about best practices. I know one workaround. This is the first time I have to deal with interop in C#, at the same time I’ve written a fair amount of code in C and C++.

I need to invoke 2 times a function exposed by an unmanaged DLL. The function takes a pointer to a struct as an argument. 1st time I need to pass a null pointer. 2nd time I need to pass a pointer to an instance of the struct.

The function has 6 parameters, so below are simplified declarations that work for the purposes of this question. (I can post the specifics if anyone is interested.) Here’s the 1st variant of the declaration:

[DllImport("setupapi.dll", CharSet = CharSet.Auto, SetLastError = true)]
   static extern bool foo(ref NEEDED_STRUCT_TYPE sDataStruct);

I like it, because it's strongly typed for the NEEDED_STRUCT_TYPE. But to be able to pass a null pointer, I had to change the declaration to

[DllImport("setupapi.dll", CharSet = CharSet.Auto, SetLastError = true)]
   static extern bool foo(IntPtr sDataStruct);

Now I can pass IntPtr.Zero for null pointer, but the parameter is no longer strongly typed. In the given situation, is there a way to have both: strongly typed parameter and the ability to pass null pointer?

Any suggestion, insight or reference is really appreciated!

Cheers,
- Nick

score 3 · Accepted Answer · answered Dec 27 '10 at 06:13

It's not uncommon to use a wrapper method with the exact API you want to expose (strongly typed, etc.) and then to have that wrapper method call the DllImport method, which is private. In other words, there's no reason the raw method needs to force the managed API.

Interop question about passing a struct by ref, null pointer and IntPtr

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