如何在System.Windows.Forms.ToolStripControlHost.SetVisibleCore?中修复NullReferenceException

Question

在设置控件可见性时，我有时会得到这样的结果：

CRASH! Failed to refresh job control:
Exception:NullReferenceException
Message:Object reference not set to an instance of an object.
Source:     System.Windows.Forms
TargetSite: SetVisibleCore
StackTrace:    at System.Windows.Forms.ToolStripControlHost.SetVisibleCore(Boolean visible)
   at System.Windows.Forms.ToolStripItem.set_Visible(Boolean value)
   at Cwc.WaterWorks.Console.frmMain.SetToolStripOptionalButtons()
   at Cwc.WaterWorks.Console.frmMain.UpdateToolStrip(ToolStripButton toolStripButton)
   at Cwc.WaterWorks.Console.frmMain.RefreshJobControl(Boolean fetch)

从以下位置调用：

private void SetToolStripOptionalButtons()
{

    NewJobButtonsVisibility(newJobStripButton.Checked);
    JobListButtonsVisibility(jobListStripButton.Checked);
    jumpToJob.Visible = !newJobStripButton.Checked;

    if (newJobStripButton.Checked)
    {
        JobCreateStatus(false);
            uxLockedToolStrip.Visible = false;
        HideJobControlButtons();
    }
    else if (jobListStripButton.Checked)
    {
        UpdateFilterStatus();
        HideJobControlButtons();
    }
        else if (controlStripButton.Checked)
    {
        ShowJobControlButtons();
        }

        findCustomerStripButton.Enabled = newJobStripButton.Checked && basis2ServiceLayer.Basis2Status.IsConnected==true;
    filterStripDropDown.Enabled = jobListStripButton.Checked;
}

你知道为什么吗？

Answer 1

我可以用一个小的测试表单重现这个崩溃：

public partial class Form1 : Form {
    public Form1() {
        var strip = new ToolStrip();
        test = new ToolStripControlHost(new TextBox());
        strip.Items.Add(test);
        this.Controls.Add(strip);
    }
    protected override void OnMouseClick(MouseEventArgs e) {
        test.Dispose();
        test.Visible = true;
    }
    ToolStripItem test;
}

单击该窗口，然后单击kaboom：

System.NullReferenceException was unhandled
  Message="Object reference not set to an instance of an object."
  Source="System.Windows.Forms"
  StackTrace:
       at System.Windows.Forms.ToolStripControlHost.SetVisibleCore(Boolean visible)
       at WindowsFormsApplication1.Form1.OnMouseClick(MouseEventArgs e) in C:\Users\hpass_000\AppData\Local\Temporary Projects\WindowsFormsApplication1\Form1.cs:line 21
       ...
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at WindowsFormsApplication1.Program.Main() in C:\Users\hpass_000\AppData\Local\Temporary Projects\WindowsFormsApplication1\Program.cs:line 15

因此，您正在尝试在释放Toolstrip项之后对其进行更新。除此之外，我们不清楚你的程序是如何进入这种状态的。线程问题的概率非常高，即使在用户关闭窗口后，线程仍会继续调用。this answer的主题。

你真的应该解决核心问题，但Q&D修复可能会解决你的崩溃是：

private void SetToolStripOptionalButtons()
{
    if (this.Disposed) return;
    // etc...
}

尽管它现在很可能会在其他地方崩溃。