使用NTWAIN库时C#数据源管理器(DSM)未显示引发myDS空引用错误的求助

Hey there, let's work through this NTWAIN scanning issue you're hitting. The root problem here is that myDS is coming back as null because your code isn't triggering the Data Source Manager (DSM) selection dialog, and session.GetSources() isn't finding any available scanners. Even with the DSM installed, there are a few common fixes we can try:

1. Fix the Data Source Selection Logic #

Your current code tries to grab the first available source directly, but if no sources are detected (or the DSM isn't initialized properly), you end up with a null reference. Instead, use NTWAIN's built-in SelectSource() method—it explicitly triggers the DSM selection dialog, ensuring you get a valid source from the user:

Replace this section:

session.Open();
IEnumerable<DataSource> lesSources = session.GetSources();
myDS = lesSources.FirstOrDefault();
myDS.Open();

With this:

session.Open();
// Trigger DSM selection dialog
myDS = session.SelectSource();

// Handle case where user cancels or no sources exist
if (myDS == null)
{
    session.Close();
    return new List<Image>(); // Or throw a meaningful exception based on your needs
}

myDS.Open();

2. Verify DSM and Scanner Compatibility #

• Check bitness match: NTWAIN is strict about 32/64-bit compatibility. If your application is built as 64-bit, make sure you installed the 64-bit DSM, and vice versa. Mismatched bitness is one of the most common reasons for missing sources.
• Confirm scanner driver support: Ensure your scanner's driver supports NTWAIN (not just WIA). Test with Windows' built-in scanning tool first—if that works but NTWAIN doesn't, the driver might lack NTWAIN support.
• Check device status: In Windows, go to Settings > Devices > Printers & scanners to confirm your scanner is listed and shows as "Ready".

3. Tweak Session Initialization and Enable Parameters #

• Expand DataGroups: When creating your TWIdentity, try including both Control and Image data groups to ensure full DSM communication:

  var appId = TWIdentity.CreateFromAssembly(DataGroups.Control | DataGroups.Image, Assembly.GetExecutingAssembly());
  

• Provide a valid parent window handle: When calling myDS.Enable(), pass your application's window handle instead of IntPtr.Zero. For WinForms, use this.Handle; for WPF, use new WindowInteropHelper(Application.Current.MainWindow).Handle. This ensures the DSM dialog is properly attached to your app, which can resolve visibility issues.

4. Add Debug Logging #

Enable NTWAIN's debug logging to see exactly where the DSM initialization is failing:

// Add this before creating your TwainSession
TwainLogger.LogLevel = LogLevel.Debug;
TwainLogger.LogToConsole = true;

The logs will show details about DSM connection attempts, source detection, and any errors that occur—this is invaluable for narrowing down the issue.

Modified Scan Method (Full Example) #

Here's how your updated Scan method might look with these fixes:

public List<Image> Scan(decimal dpi) {
    ImagesScan.Clear();
    var appId = TWIdentity.CreateFromAssembly(DataGroups.Control | DataGroups.Image, Assembly.GetExecutingAssembly());
    session = new TwainSession(appId);
    session.TransferReady += session_TransferReady;
    session.DataTransferred += session_DataTransferred;
    session.SourceDisabled += session_SourceDisable;
    session.Open();

    // Trigger DSM selection dialog
    myDS = session.SelectSource();
    if (myDS == null)
    {
        session.Close();
        return ImagesScan;
    }

    myDS.Open();
    PixelType typeCouleur = PixelType.Gray;
    if (myDS.Capabilities.ICapPixelType.CanSet && myDS.Capabilities.ICapPixelType.GetValues().Contains(typeCouleur)) {
        myDS.Capabilities.ICapPixelType.SetValue(typeCouleur);
    }
    TWFix32 DPI = (float)dpi;
    if (myDS.Capabilities.ICapXResolution.CanSet && myDS.Capabilities.ICapXResolution.GetValues().Contains(DPI)) {
        myDS.Capabilities.ICapXResolution.SetValue(DPI);
    }
    if (myDS.Capabilities.ICapYResolution.CanSet && myDS.Capabilities.ICapYResolution.GetValues().Contains(DPI)) {
        myDS.Capabilities.ICapYResolution.SetValue(DPI);
    }

    // Replace IntPtr.Zero with your app's actual window handle
    IntPtr parentHandle = IntPtr.Zero; 
    myDS.Enable(SourceEnableMode.ShowUI, false, parentHandle);
    EventWaitHandle session_SourceDisable_Wait = new EventWaitHandle(false, EventResetMode.AutoReset);
    session_SourceDisable_Wait.WaitOne();
    return ImagesScan;
}

Give these steps a try—most likely the bitness mismatch or missing SelectSource() call is the culprit.

内容的提问来源于stack exchange，提问作者Gizmor