C# 使用Windows窗体组合框的双向绑定在更改时引发NullReferenceException

我有一个Windows窗体应用程序，其中一个窗体有两个

组合框

e，它们都绑定到我通过实体框架映射的对象的同一个

绑定列表

Form\u Load

事件调用以下内容：

private void SetFacilityDropdowns()
{
    dbContext.Facilities.Load();
    var bindingSource = dbContext.Facilities.Local.ToBindingList();
    Dictionary<ComboBox, string> selectedDropDownsAndBoundFields = new Dictionary<ComboBox, string>
        {
            {FacilityId,    DataConstants.Facility.FacilityId},
            {FacilityName,  DataConstants.Facility.FacilityName}
        };
    SetDropdowns(bindingSource, selectedDropDownsAndBoundFields);            
}

一切看起来都很好，

ComboBox

es似乎都包含了我所期望的所有项目。但是，

Facility

表中有一些

Facility

对象，其中

FacilityName

属性为

null

。这在组合框中显示为空字符串，尽管我不知道它是否仅以这种方式显示，或者在绑定时是否转换

通常，当我更改

FacilityName

下拉列表时，

FacilityId

下拉列表将更改为其匹配值。但是，当我将

FacilityName

更改为null/空字符串值之一时，会抛出

NullReferenceException

。它似乎是由Windows窗体UI线程引发的，因此我无法在任何

组合框

“已更改”事件中设置断点，因为在触发任何事件之前会引发异常

知道为什么会这样吗

编辑：

以下是堆栈跟踪：

>   NITS.exe!NITS.Program.CurrentDomain_UIThreadException(object sender, System.Threading.ThreadExceptionEventArgs t) Line 50   C#
    System.Windows.Forms.dll!System.Windows.Forms.Application.ThreadContext.OnThreadException(System.Exception t) + 0x8e bytes  
    System.Windows.Forms.dll!System.Windows.Forms.Control.WndProcException(System.Exception e) + 0x16 bytes 
    System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.OnThreadException(System.Exception e) + 0xa bytes 
    System.Windows.Forms.dll!System.Windows.Forms.NativeWindow.Callback(System.IntPtr hWnd, int msg, System.IntPtr wparam, System.IntPtr lparam) + 0x9b bytes   
    [Native to Managed Transition]  
    [Managed to Native Transition]  
    System.Windows.Forms.dll!System.Windows.Forms.Control.SendMessage(int msg, System.IntPtr wparam, System.IntPtr lparam) + 0x21 bytes 
    System.Windows.Forms.dll!System.Windows.Forms.Control.ReflectMessageInternal(System.IntPtr hWnd, ref System.Windows.Forms.Message m) + 0x3b bytes   
    System.Windows.Forms.dll!System.Windows.Forms.Control.WmCommand(ref System.Windows.Forms.Message m) + 0x1b bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Control.WndProc(ref System.Windows.Forms.Message m) + 0x3b7 bytes 
    System.Windows.Forms.dll!System.Windows.Forms.ScrollableControl.WndProc(ref System.Windows.Forms.Message m) + 0x2a bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Form.WndProc(ref System.Windows.Forms.Message m) + 0x5e bytes 
    System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.OnMessage(ref System.Windows.Forms.Message m) + 0x11 bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.WndProc(ref System.Windows.Forms.Message m) + 0x35 bytes  
    System.Windows.Forms.dll!System.Windows.Forms.NativeWindow.Callback(System.IntPtr hWnd, int msg, System.IntPtr wparam, System.IntPtr lparam) + 0x80 bytes   
    [Native to Managed Transition]  
    [Managed to Native Transition]  
    System.Windows.Forms.dll!System.Windows.Forms.NativeWindow.DefWndProc(ref System.Windows.Forms.Message m) + 0x56 bytes  
    System.Windows.Forms.dll!System.Windows.Forms.Control.DefWndProc(ref System.Windows.Forms.Message m) + 0xa bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Control.WmMouseDown(ref System.Windows.Forms.Message m, System.Windows.Forms.MouseButtons button, int clicks) + 0x3a bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Control.WndProc(ref System.Windows.Forms.Message m) + 0x8da bytes 
    System.Windows.Forms.dll!System.Windows.Forms.ComboBox.WndProc(ref System.Windows.Forms.Message m) + 0x864 bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.OnMessage(ref System.Windows.Forms.Message m) + 0x11 bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Control.ControlNativeWindow.WndProc(ref System.Windows.Forms.Message m) + 0x35 bytes  
    System.Windows.Forms.dll!System.Windows.Forms.NativeWindow.Callback(System.IntPtr hWnd, int msg, System.IntPtr wparam, System.IntPtr lparam) + 0x80 bytes   
    [Native to Managed Transition]  
    [Managed to Native Transition]  
    System.Windows.Forms.dll!System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(System.IntPtr dwComponentID, int reason, int pvLoopData) + 0x24d bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(int reason, System.Windows.Forms.ApplicationContext context) + 0x155 bytes  
    System.Windows.Forms.dll!System.Windows.Forms.Application.ThreadContext.RunMessageLoop(int reason, System.Windows.Forms.ApplicationContext context) + 0x4a bytes    
    System.Windows.Forms.dll!System.Windows.Forms.Application.Run(System.Windows.Forms.Form mainForm) + 0x31 bytes  
    NITS.exe!NITS.Program.Main() Line 37 + 0x1d bytes   C#
    [Native to Managed Transition]  
    [Managed to Native Transition]  
    mscorlib.dll!System.AppDomain.ExecuteAssembly(string assemblyFile, System.Security.Policy.Evidence assemblySecurity, string[] args) + 0x6b bytes    
    Microsoft.VisualStudio.HostingProcess.Utilities.dll!Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() + 0x27 bytes  
    mscorlib.dll!System.Threading.ThreadHelper.ThreadStart_Context(object state) + 0x6f bytes   
    mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) + 0xa7 bytes  
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) + 0x16 bytes  
    mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x41 bytes    
    mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() + 0x44 bytes   
    [Native to Managed Transition]  

引发的异常如下所示：

private static void SetDropdowns<T>(BindingList<T> dataSource, Dictionary<ComboBox, string> dropdownsAndBoundFields)
{
    if (dropdownsAndBoundFields == null)
    {
        return;
    }

    foreach (var dropdown in dropdownsAndBoundFields)
    {
        dropdown.Key.DataSource = dataSource;
        dropdown.Key.DisplayMember = dropdown.Value;
        dropdown.Key.ValueMember = dropdown.Value;
        dropdown.Key.AutoCompleteSource = AutoCompleteSource.ListItems;
        dropdown.Key.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
        dropdown.Key.SelectedIndexChanged += Dropdown_SelectedIndexChanged;
    }
}

System.NullReferenceException Object reference not set to an instance of an object. System.String get_Text()    at System.Windows.Forms.ComboBox.get_Text()
   at System.Windows.Forms.ComboBox.WmReflectCommand(Message& m)
   at System.Windows.Forms.ComboBox.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

---

您可以尝试使用IntelliTrace获取更多信息？

您的主要异常显然是

System.NullReferenceException对象引用未设置为对象的实例。System.Windows.Forms.ComboBox.get_Text（）中的System.String get_Text（）

这让我相信您的combobox试图将

DisplayMember

属性设置为空值。由于您自己说过，您不确定从数据库返回的空列在

Facility

类中是否表示为空字符串或null，因此您可以在getter中明确地转换它们。此外，如果要向FacilityID属性传递一个可为null的值，则需要将其转换为int，以便combobox可以处理它

所以，在你的

设施中

    public class Facility
    {
        private string _facilityName;
        private int _facilityID;

        public string FacilityName
        {
            get
            {
                if (_facilityName == null)
                    return String.Empty;
                else
                    return _facilityName;
            }
            set { _facilityName = value; }
        }

        public int FacilityID
        {
            get { return _facilityID; }
            set { _facilityID = value == null ? default(int) : value};
        }
    }

使用此选项，您将确保您的控件永远不会尝试绑定到空值，并且它将消除异常。
您可以显示stacktrace吗？刚刚添加了调用堆栈和异常消息。然后绑定一个没有空项（即空值）的列表或ObservableCollection到组合框。@RaviPatel然后我失去了将两个组合框链接在一起的能力，因此当我更改一个组合框时，另一个组合框也会相应地更改。@Douglas Barbin，但是您可以将两个组合框绑定到同一个集合。因此，如果集合更改，两个组合框都将相应更改。
Facility
是一个由实体框架自动生成的类。您可以更改此类的行为以确保不返回空值吗？我可以，这实际上是可行的。我将尝试一些方法，然后返回结果。好的，这解决了异常问题，但引入了一个新的异常。当我选择一个空值时，另一个下拉列表中的伴随值变为
System.Data.Entity.DynamicProxies.Facility_F4B14BF33FD7C299E70286AAFB2DE3802F40B0A50A6BF0EFD80BD6C4385597B6
（或结尾的其他长十六进制字符串），而不是正确的
FacilityId
值。我将此标记为答案，因为它删除了异常。