CODEWITHSUNDEEP
c#pinvoke
Jonathon Reinhart
Jonathon Reinhart

Reputation: 137467

PInvoke struct with array of unspecified length

C definitions

typedef struct {
    const uint8_t* buf;
    int bufLen;
} Info;


int Foo(Info* info);

C# definitions

[StructLayout(LayoutKind.Sequential)]
public struct Info
{
     // [MarshalAs( ??? )]
     public byte[] buf;
     int bufLen
}    

[DllImport(...)]
public static extern int Foo(ref Info info);

I'm having trouble figuring out what to specify for the MarshalAs attribute on byte[] buf in the C# struct definition. The buffer is allocated on the .NET side, and its length is known at call time.

In a simple little test:

var info = new Info {
    buf = new byte[] {0x40, 0x50, 0x60, 0x70},
    bufLen = 4,
};

Foo(ref info);

Everything seems to be working correctly, but in fact the data in my buffer is incorrect. Printing it out from the DLL I see 01 00 80 00 - not sure what that is.

I've tried:

and nothing works.

In general, I really don't know the best way to debug these kinds of problems either.

Upvotes: 5

Views: 2870

Answers (2)

Mike Znaet
Mike Znaet

Reputation: 89

I know it is too late.. but just in case if some one come here for an answer.

I finally came up with next solution for similar exercise.

[StructLayout(LayoutKind.Sequential)]
public struct Info
{
     [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
     public byte[] buf;
     int bufLen
}

From MSDN

Arrays can be marshaled as UnmanagedType.ByValArray, which requires you to set the MarshalAsAttribute.SizeConst field. The size can be set only as a constant.

In my case the buffer size is 8 bytes.

Upvotes: 0

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137467

Per Hans Passant's suggestion, I implemented the following:

[StructLayout(LayoutKind.Sequential)]
public struct Info : IDisposable
{
    private IntPtr buf;
    private int bufLen;

    public Info(byte[] buf) : this() {
        this.buf = Marshal.AllocHGlobal(buf.Length);
        Marshal.Copy(buf, 0, this.buf, buf.Length);
        this.bufLen = buf.Length;
    }

    public void Dispose() {
        if (buf != IntPtr.Zero) {
            Marshal.FreeHGlobal(buf);
            buf= IntPtr.Zero;
        }
    }
}

Upvotes: 4

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