如何将 COLORREF 中的所有数据返回给 C# 中的调用者

Question

I'm trying to write a .dll and then call this .dll in C#. I followed this article:

https://blog.quickbird.uk/calling-c-from-net-core-759563bab75d

Now, I have the problem of how to return all of the data in a COLORREF to the caller in C#, and then to be able to distinguish the values.

This is the source from the .dll :

#pragma once
#include <windows.h>

extern "C"
{
    __declspec(dllexport) COLORREF __stdcall getColor(int X, int Y) {
        HDC _hdc = GetDC(NULL);
        COLORREF _result = 0;
        if (_hdc)
        {
            _result = GetPixel(_hdc, X, Y);
        /*  int _red = GetRValue(_result);
            int _green = GetGValue(_result);
            int _blue = GetBValue(_result);   */
            ReleaseDC(NULL, _hdc);
            return _result;
        }
        return _result;
    }
}

And here are some parts from the C# source:

public partial class Main_Window : Form
{
    [DllImport(@"test.dll", EntryPoint = "getColor", CallingConvention = CallingConvention.StdCall)]
    private static extern UInt32 getColor(int X, int Y);

    private void ColB_Click(object sender, EventArgs e)
    {
        UInt32 result = getColor(500, 500);
        MessageBox.Show(Convert.ToString(result)); //<--- just to show what pops up.
        //ColB.BackColor = result; //<--- that didnt work.
    }
}

Answer 1

COLORREF is just an alias for DWORD , which is indeed UInt32 in C#. So, your code looks fine, as far as calling your getColor() function in C#. Though, I would suggest making the function return CLR_INVALID ( 0xFFFFFFFF ) on failure rather than returning 0 (Black), especially since GetPixel() returns CLR_INVALID on failure, eg:

COLORREF __stdcall getColor(int X, int Y) {
    COLORREF _result = CLR_INVALID;
    HDC _hdc = GetDC(NULL);
    if (_hdc)
    {
        _result = GetPixel(_hdc, X, Y);
        ReleaseDC(NULL, _hdc);
    }
    return _result;
}

That being said, the BackColor property of a control expects a System.Drawing.Color , not a UInt32 . You can get a Color from a COLORREF by using one of the Color.FromArgb() methods, eg:

UInt32 result = getColor(500, 500);
if (result != 0xFFFFFFFF) {
    int red = (int) (byte) (result & 0xFF);
    int green = (int) (byte) ((result >> 8) & 0xFF);
    int blue = (int) (byte) ((result >> 16) & 0xFF);
    ColB.BackColor = Color.FromArgb(red, green, blue);
}

Or, the System.Drawing.ColorTranslator.FromWin32() method:

UInt32 result = getColor(500, 500);
if (result != 0xFFFFFFFF) {
    ColB.BackColor = ColorTranslator.FromWin32( (int) result );
}