I am attempting to follow the guidance in this blog post, which talks about using SendMessage to RichEdit with EM_STREAMOUT, and SF_BINARY and SF_HTML flags, to get HTML content from a RichEditControl. I am working in C#.

RichEdit HTML Support

When I call SendMessage, debugging stops in Visual Studio and no exception is thrown. Any idea why? How can I get debugging information?

private static MemoryStream? memoryStream;

public static class WMConstants
{
    public const uint WM_USER = 0x0400;
    public const uint EM_STREAMOUT = WM_USER + 74;

    public const uint SF_HTML = 0x00100000;
    public const uint SF_BINARY = 0x0008;
    public const uint SF_CLIPBOARD = 0x80000000;
}

[DllImport("user32.dll", CharSet = CharSet.Auto)]
[SuppressMessage("Interoperability", "SYSLIB1054:Use 'LibraryImportAttribute' instead of 'DllImportAttribute' to generate P/Invoke marshalling code at compile time", Justification = "<Pending>")]
private static extern IntPtr SendMessage(IntPtr hwnd, uint msg, uint wParam, ref EDITSTREAM lParam);

internal static string ReadHtml(IntPtr handle)
{
    memoryStream = new MemoryStream();
    var result = string.Empty;

    var editStream = new EDITSTREAM
    {
        dwCookie = IntPtr.Zero,
        pfnCallback = EditStreamProc,
    };

    try
    {
        _ = SendMessage(
            hwnd: handle, 
            msg: WMConstants.EM_STREAMOUT,
            wParam: WMConstants.SF_BINARY | WMConstants.SF_HTML, // | WMConstants.SF_CLIPBOARD,
            lParam: ref editStream);

        memoryStream.Seek(0, SeekOrigin.Begin);
        using (var reader = new StreamReader(memoryStream))
        {
            result = reader.ReadToEnd();
        }
    }
    catch (Exception ex)
    {
        // Log the exception if needed
        throw new InvalidOperationException("Failed to read HTML from the RichTextBox.", ex);
    }
    finally
    {
        memoryStream?.Dispose();
        memoryStream = null;
    }

    return result;
}

public static int EditStreamProc(IntPtr dwCookie, IntPtr pbBuff, int cb, out int pcb)
{
    pcb = cb;
    var buffer = new byte[cb];
    Marshal.Copy(pbBuff, buffer, 0, cb);
    memoryStream!.Write(buffer, 0, cb);
    return 0;
}

[StructLayout(LayoutKind.Sequential)]
private struct EDITSTREAM
{
    public IntPtr dwCookie;
    public uint dwError;
    public EditStreamCallback pfnCallback;
}