use c++ unsigned int *array in c#

Question

I have a c++ .dll exporting a function with the following signature:

extern int __stdcall foobar(long ptr, unsigned int *array, unsigned int arraySize);

From c++ Code, I can use it like so:

std::vector<unsigned int> FooBar(6);
int res = foobar(ptr, &FooBar[0], (unsigned int)FooBar.size());

I'd like to use the dll from C# and I've tried this:

[DllImport("mydll.dll")]
public static extern int foobar(long ptr, uint[] arr, uint arrSize);

Call:

uint[] arr = new uint[6];
int count = Obj.foobar(ptr, arr, (uint)arr.GetLength(0)*32)

This throws an "PInvokeStackImbalance" error. What would the correct PInvoke signature look like?

Have you tried public static extern unsafe Int32 foobar(void* ptr, UInt32* array, UInt32 arraySize) and performing the marshalling yourself? — Dai
– Dai, Commented Jan 3, 2014 at 23:40
That ptr is not long, it is IntPtr. size is wrong too, it is * 4. What it actually means is hard to guess at, raw pointers are usually trouble. — Hans Passant
– Hans Passant, Commented Jan 3, 2014 at 23:40
@Dai : Do you have an example for that? I would prefer going without unsafe code if that's possible... — agrafix
– agrafix, Commented Jan 3, 2014 at 23:46
@HansPassant ptr comes from a different function returning long: extern long __stdcall createPtr(int val); — agrafix
– agrafix, Commented Jan 3, 2014 at 23:47
The translation of C++ long to .NET is System.Int32. Using C# long, which is System.Int64, will not work. Also std::vector's size() member returns the number of elements, not the number of bytes, so multiplying GetLength(0) * 32 is wrong. Not sure where 32 came from to begin with. Just (uint)arr.Length will be correct there. — Ben Voigt
– Ben Voigt, Commented Jan 3, 2014 at 23:55

Breealzibub · Accepted Answer · 2014-01-03 23:49:14Z

You're going to need to make use of the MarshalAs attribute, for your array parameter. This essentially describes to the marshalling system how to convert between the C# managed type and your raw C++ function parameter.

[DllImport("mydll.dll")]
public static extern int foobar(
    IntPtr ptr, //Need more details on this parameter
    [MarshalAs(UnmanagedType.LPArray, SizeParamIndex=2)] uint[] arr,
    uint arrSize);

Your "ptr" parameter is a bit confusing, since it's not mentioned in your question, but it's most likely just something you can treat as a regular old C++ pointer, in which case IntPtr is the appropriate type.

In the MarshalAs attribute, we are telling the marshalling system to convert your "uint array" into a raw, long-pointer-compatible C++ array. The SizeParamIndex tells it to use the "arrSize" parameter, when allocating the array's size during conversion.

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