How do I use DLLImport with structs as parameters in C#?

Question

All the examples I can find using DLLImport to call C++ code from C# passes ints back and forth. I can get those examples working just fine. The method I need call takes two structs as its import parameters, and I'm not exactly clear how I can make this work.

Here's what I've got to work with:

I own the C++ code, so I can make any changes/additions to it that I need to.

A third party application is going to load my DLL on startup and expects the DLLExport to be defined a certain way, so i can't really change the method signature thats getting exported.

The C# app I'm building is going to be used as a wrapper so i can integrate this C++ piece into some of our other applications, which are all written in C#.

The C++ method signature I need to call looks like this

DllExport int Calculate (const MathInputStuctType *input, 
    MathOutputStructType *output, void **formulaStorage)

And MathInputStructType is defined as the following

typedef struct MathInputStuctTypeS {
    int    _setData;
    double    _data[(int) FieldSize];
    int    _setTdData;
} MathInputStuctType;

Answer 1

From the declaration you posted, your C# code will look something like this:

[DllImport("mydll.dll")]
static extern int (ref MathInputStructType input,
    ref MathOutputStructType output, ref IntPtr formulaStorage);

Depending on the structure of MathInputStructType and MathOutputStructType in C++, you are going to have to attribute those structure declarations as well so that they marshal correctly.

Answer 2

put the struct in a simple header file.

call swig on your header file

swig -c++ -csharp myheader.h

look at the output

Answer 3

For the struct:

struct MathInputStuctType 
{
    int       _setData;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = FieldSize)]
    double[]  _data;
    int       _setTdData;
}

Answer 4

You might want to look at this project on CodePlex, http://www.codeplex.com/clrinterop/Release/ProjectReleases.aspx?ReleaseId=14120. It should help you marshal the structures correctly.

Answer 5

The MSDN topic Passing Structures has a good introduction to passing structures to unmanaged code. You'll also want to look at Marshaling Data with Platform Invoke, and Marshaling Arrays of Types.