C＃导入C Dll。 struct中的数组指针。 如何？

Question

I need your help.

I am trying to import a C Dll into a C# project. While doing that, I need pass a struct between the Dll and C# project in both directions.

Here is C definition:

struct mwBITMAP 
{
 int bmWidth;
 int bmHeight;
 BYTE* bmData;
}; 

Here is C# definition:

   [StructLayout(LayoutKind.Sequential)] 
    public struct MwRemoteBmp
    {
        public int Width;
        public int Height;
        public byte[] Data;
    }

I tried to pass the a struct (the Data is well initialized) from C# to a dll's test function by reference. The width and height are both right. But the Data is all wrong.

Where did I make mistakes?

Answer 1

Yes, the array gets marshaled as a SAFEARRAY. Not crashing the pinvoke marshaller is pretty unusual. Declare the Data member as IntPtr, then use Marshal.Copy() to copy the data.

Beware that this would be hard to use in C as well. There's a memory management problem, it isn't clear who owns the array. Most typically, the C function would use malloc() to allocate the array. That's a big problem, you cannot release that array in C#, no way to call free(). You'll have an unpluggable memory leak. If you can't rewrite the C code then you'll need to write a wrapper in the C++/CLI language so that you can call free(). Even that is tricky if the C dll doesn't use the same CRT as the C++/CLI code. You have to compile the C code with the /MD option.

Answer 2

Use the IntPtr type instead of byte[] type. In your example:

[StructLayout(LayoutKind.Sequential)] 
public struct MwRemoteBmp
{
    public int Width;
    public int Height;
    public IntPtr Data;
}