调用SendMessage操作RichEdit时调试中断，回调未触发求助

尝试使用C#通过SendMessage向RichEdit控件发送EM_STREAMOUT消息，搭配SF_BINARY和SF_HTML标志获取HTML内容，但回调函数从未被调用。调用SendMessage时Visual Studio调试中断且未抛出任何异常，请问原因是什么？如何获取调试信息？

相关代码 #

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;
}

1. 委托定义缺失/签名不匹配 #

代码仅声明了EditStreamCallback类型，但未定义符合Win32规范的委托签名，也未指定StdCall调用约定。Win32的EDITSTREAMCALLBACK要求固定调用约定，签名不匹配会导致栈不平衡，触发调试中断，同时RichEdit控件无法正确调用回调函数。

2. RichEdit控件版本不支持HTML导出 #

SF_HTML标志仅在RichEdit 4.1及以上版本（如msftedit.dll中的RICHEDIT50W控件）支持，若操作的是旧版本控件（如riched32.dll），EM_STREAMOUT会直接失败，不会触发回调。

3. 缺少错误检查逻辑 #

代码未检查SendMessage的返回值和EDITSTREAM.dwError字段，无法及时发现调用失败的情况。

1. 修正委托和结构体定义 #

添加符合Win32规范的委托定义，确保调用约定正确：

// 匹配Win32 EDITSTREAMCALLBACK的委托定义
[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate int EditStreamCallback(IntPtr dwCookie, IntPtr pbBuff, int cb, out int pcb);

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

2. 验证RichEdit控件版本 #

确认目标控件是支持HTML的版本：

• 调用GetClassName函数获取控件类名，应为RICHEDIT50W或RICHEDIT20W（需引入user32.dll的GetClassName方法）。
• 若使用自定义控件，确保加载的是msftedit.dll而非旧版本的riched32.dll。

3. 添加错误检查 #

在SendMessage调用后添加结果验证：

var sendResult = SendMessage(handle, WMConstants.EM_STREAMOUT, WMConstants.SF_BINARY | WMConstants.SF_HTML, ref editStream);
if (sendResult == IntPtr.Zero || editStream.dwError != 0)
{
    throw new InvalidOperationException($"EM_STREAMOUT调用失败，错误码: {editStream.dwError}");
}

• 启用本机代码调试：在Visual Studio的工具 -> 选项 -> 调试 -> 常规中勾选启用本机代码调试，可捕获P/Invoke时的本机异常，查看具体错误信息。
• 验证控件句柄有效性：通过GetClassName确认控件类名，确保句柄指向正确的RichEdit控件。
• 检查回调触发情况：在EditStreamProc开头添加断点，若未触发，说明EM_STREAMOUT未执行成功，重点排查控件版本和参数组合是否正确。

内容的提问来源于stack exchange，提问作者Steve Andrews