Pinvoking a native function with array arguments

Question

I am completely confused with how to go about calling functions in native dll with array arguments.

Example:

The function is defined in the C# project as:

[DllImport("Project2.dll", SetLastError = true, CallingConvention = CallingConvention.Cdecl)]
static extern void modifyArray([MarshalAs(UnmanagedType.LPArray)] int[] x, int len);

And the function call is:

modifyArray(arr, 3)

where arr = {4,5,6}

The native C++ function definition is as follows:

extern "C" _declspec(dllexport) void modifyArray(int* x,int len)
{   
        int arr[] = {1,2,3};
        x = arr;
}

Why in the C# project, the array is not pointing to the new array after the function call? It still remains {4,5,6}.

I tried this to but failed

[DllImport("Project2.dll", SetLastError = true, CallingConvention = CallingConvention.Cdecl)]
static extern void modifyArray([In,Out] int[] x, int len);

This pinvoke fails whenever I try modifying arguments passed to these functions using pointers. Otherwise, I have had success passing ref array arguments for native dll sort functions where there is no pointer changes to newly created types.

Your C++ code is invalid, don't try to pinvoke code until you've got it working in C++ first. — Hans Passant
– Hans Passant, Commented Jan 3, 2014 at 18:06

David Heffernan · Accepted Answer · 2014-01-03 18:13:25Z

3

Your C++ code is broken. The caller allocates the array, and the callee populates it. Like this:

extern "C" _declspec(dllexport) void modifyArray(int* x, int len)
{   
    for (int i=0; i<len; i++)
        x[i] = i;
}

As far as your p/invoke call goes, SetLastError should not be true. The function is not calling SetLastError. It should be:

[DllImport("Project2.dll", CallingConvention = CallingConvention.Cdecl)]
static extern void modifyArray(int[] x, int len);

answered Jan 3, 2014 at 18:13

David Heffernan

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Comments

JaredPar · Accepted Answer · 2014-01-03 20:28:18Z

1

This has nothing to do with PInvoke, this is just a plain old C issue. You would have the exact same problem if you called modifyArray from C

int* pArray = NULL;
modifyArray(pArray, len);
pArray == NULL;  // true!

In modifyArray you are trying to change where x points to. This change won't be visible to the calling function because the pointer is passed by value. In order to change where it points to you need to pass a double pointer

void modifyArray(int** x, int len) { 
  *x = ...;
}

Note that you are currently trying to return stack allocated memory instead of heap allocated memory. That is incorrect and will lead to problems down the line

edited Jan 3, 2014 at 20:28

answered Jan 3, 2014 at 18:07

JaredPar

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7 Comments

Hans Passant Over a year ago

This gets him into a lot more trouble, best to recommend him to copy the elements instead.

Tyler Durden Over a year ago

I wrote this as a sample code, but the c++ code is much more complex and I can't modify it. So, I think what JaredPar answered will be best in my case. But, when I tried this, with changing the argument type to ref int[], it failed

Tyler Durden Over a year ago

That is, how should modifyArray be defined in the C# file

Tyler Durden Over a year ago

I did modify the C++ code, but only the first element of the array gets modified, I think to the address of the array in the C++ function.

mihi Over a year ago

In addition, even when using a "double pointer", never pass a pointer to something allocated on the stack out of the function - it will be undefined where it points to later...

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