C# import C Dll. Array pointer in struct. How to?

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?

Hans Passant · Accepted Answer · 2010-12-19 19:22:51Z

3

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.

answered Dec 19, 2010 at 19:22

Hans Passant

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4 Comments

MaoWoo Over a year ago

If I allocate memory for the Data member only in C# and pass the struct to C to modify Data's value. And call Marshal.FreeHGlobal in C# to free the memory after it's done. Will that be OK?

Hans Passant Over a year ago

Yes, as long as you modify Data's content, not value. You however won't know how much memory to allocate until you know the size of the bitmap. Short from using the SAFEARRAY api, you could fix the problem in C by calling GlobalAlloc so you can use Marshal.FreeHGlobal in C#.

MaoWoo Over a year ago

Fortunately both C and C# parts are implemented by myself so I can gain more control over it. Is there any better way to do this?

Hans Passant Over a year ago

The approaches I gave in the previous comment will work. You could also add a ReleaseBitmap export. Other than that, lose the struct and replace with 3 functions.

Samuel · Accepted Answer · 2010-12-19 19:19:02Z

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;
}

answered Dec 19, 2010 at 19:19

Samuel

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