此C ++代码的C＃等价物是什么（带有InPtr）

Question

I'm implementing a Custom Credential Provider in C# . I'm using a C++ project as example. This piece of C++ code provides an image to Windows. The way I see it phbmp is a pointer to the image-bitmap. The code either updates the pointer so it points to a new bitmap (read from Resource) or it loads the bitmap to the address pointed by phbmp . I'm not sure if the pointer itself is changed or not .

// Get the image to show in the user tile
HRESULT CSampleCredential::GetBitmapValue(DWORD dwFieldID, _Outptr_result_nullonfailure_ HBITMAP *phbmp)
{
    HRESULT hr;
    *phbmp = nullptr;

    if ((SFI_TILEIMAGE == dwFieldID))
    {
        HBITMAP hbmp = LoadBitmap(HINST_THISDLL, MAKEINTRESOURCE(IDB_TILE_IMAGE));
        if (hbmp != nullptr)
        {
            hr = S_OK;
            *phbmp = hbmp;
        }
        else
        {
            hr = HRESULT_FROM_WIN32(GetLastError());
        }
    }
    else
    {
        hr = E_INVALIDARG;
    }

    return hr;
}

Below is the C# equivalent I'm implementing:

public int GetBitmapValue(uint dwFieldID, IntPtr phbmp)
{
    if (dwFieldID == 2)
    {
        Bitmap image = Resource1.TileImage;

        ImageConverter imageConverter = new ImageConverter();
        byte[] bytes = (byte[])imageConverter.ConvertTo(image, typeof(byte[]));

        Marshal.Copy(bytes, 0, phbmp, bytes.Length);

        return HResultValues.S_OK;
    }

    return HResultValues.E_INVALIDARG;
}

What I'm trying to do:

1. Load the image from resource (this works, it has the correct length)
2. Convert the Bitmap to an array of bytes
3. Copy these bytes to the address pointed by phbmp

This crashes, I assume because of memory-allocation.

The parameters in this method are defined by an interface (in CredentialProvider.Interop.dll , which is provided by Microsoft - I think). So I'm pretty sure it's correct and phbmp is not an out -parameter.

Because it is not an out-parameter I can not change phbmp to let it point to my bitmap, right? I have assigned phbmp to Bitmap.GetHbitmap() and that doesn't crash but it isn't working either. I assume that the change to phbmp is only local in this method.

I can understand that it is not possible to alloc memory to a predefined address. It's the other way around: you alloc memory and get an pointer to it. But then this change is local again. How does this work?

Answer 1

Although some people agreed that IntPtr should be an out-parameter (see comments in https://syfuhs.net/2017/10/15/creating-custom-windows-credential-providers-in-net/ ) the answer was actually:

var bmp = new Bitmap(imageStream);  
Marshal.WriteIntPtr(phbmp, bmp.GetHbitmap());