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Closing a Modeless Dialog

P: n/a
I get a Null Reference Exception if I close a modeless form (that is, a form
displayed using Show()) when a selection is made from a ComboxBox. If the
form is modal (displayed using ShowDialog()) or the selection is made from,
say, a ListBox, no exception is thrown.

I have included a simple example below. The error message refers to Unsafe
Native Methods, but the code is 100% managed. What is going on ?

I am using C#.NET 2003, Standard Edition.

Thanks,
Andrew.

// sample code

using System;
using System.Drawing;
using System.ComponentModel;
using System.Windows.Forms;

namespace Main
{
/// <summary>
/// Summary description for Form1.
/// </summary>
///

public class Form1 : System.Windows.Forms.Form
{
/// <summary>
/// Required designer variable.
/// </summary>

private System.ComponentModel.Container components = null;
private System.Windows.Forms.Button button1;
private A.FormA A;

public Form1()
{
//
// Required for Windows Form Designer support
//
InitializeComponent();

//
// TODO: Add any constructor code after InitializeComponent call
//

}

/// <summary>
/// Clean up any resources being used.
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.button1 = new System.Windows.Forms.Button();
this.SuspendLayout();
//
// button1
//
this.button1.Location = new System.Drawing.Point(56, 16);
this.button1.Name = "button1";
this.button1.TabIndex = 0;
this.button1.Text = "open A";
this.button1.Click += new System.EventHandler(this.button1_Click);
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
this.ClientSize = new System.Drawing.Size(184, 61);
this.Controls.Add(this.button1);
this.Name = "Form1";
this.Text = "Form1";
this.ResumeLayout(false);

}
#endregion

/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void button1_Click(object sender, System.EventArgs e)
{
A = new A.FormA();
A.Show(); // modeless dialog : callback function crashes when selection
made from ComboBox in form A.
//A.ShowDialog(); // modal case works.
}

}

}
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;

namespace A
{
/// <summary>
/// Summary description for FormA.
/// </summary>
public class FormA : System.Windows.Forms.Form
{
private System.Windows.Forms.ComboBox comboBox1;

/// <summary>
/// Required designer variable.
/// </summary>
private System.ComponentModel.Container components = null;

public FormA()
{
//
// Required for Windows Form Designer support
//
InitializeComponent();

//
// TODO: Add any constructor code after InitializeComponent call
//
}

/// <summary>
/// Clean up any resources being used.
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if(components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows Form Designer generated code
/// <summary>
/// Required method for Designer support - do not modify
/// the contents of this method with the code editor.
/// </summary>
private void InitializeComponent()
{
this.comboBox1 = new System.Windows.Forms.ComboBox();
this.SuspendLayout();
//
// comboBox1
//
this.comboBox1.Items.AddRange(new object[] {
"1",
"2"});
this.comboBox1.Location = new System.Drawing.Point(88, 16);
this.comboBox1.Name = "comboBox1";
this.comboBox1.Size = new System.Drawing.Size(80, 21);
this.comboBox1.TabIndex = 0;
this.comboBox1.Text = "1";
this.comboBox1.SelectedIndexChanged += new
System.EventHandler(this.comboBox1_SelectedIndexCh anged);
//
// FormA
//
this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
this.ClientSize = new System.Drawing.Size(232, 61);
this.Controls.Add(this.comboBox1);
this.Name = "FormA";
this.Text = "FormA";
this.ResumeLayout(false);

}
#endregion

private void comboBox1_SelectedIndexChanged(object sender,
System.EventArgs e)
{
this.Close(); // does not work if FormA is a modeless dialog.
//MessageBox.Show(" Closing..."); // has not yet crashed
}

}
}

// *** ERROR MESSAGE ***

See the end of this message for details on invoking
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of
an object.
at System.Windows.Forms.UnsafeNativeMethods.CallWindo wProc(IntPtr wndProc,
IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at System.Windows.Forms.NativeWindow.DefWndProc(Messa ge& m)
at System.Windows.Forms.Control.DefWndProc(Message& m)
at System.Windows.Forms.Control.WmCommand(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ComboBox.WndProc(Message& m)
at System.Windows.Forms.ControlNativeWindow.OnMessage (Message& m)
at System.Windows.Forms.ControlNativeWindow.WndProc(M essage& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg,
IntPtr wparam, IntPtr lparam)
************** Loaded Assemblies **************
mscorlib
Assembly Version: 1.0.5000.0
Win32 Version: 1.1.4322.2032
CodeBase: file:///c:/winnt/microsoft.net/framework/v1.1.4322/mscorlib.dll
----------------------------------------
Dummy2
Assembly Version: 1.0.1794.22479
Win32 Version: 1.0.1794.22479
CodeBase: file:///C:/Source/Dummy2/bin/Release/Dummy2.exe
----------------------------------------
System.Windows.Forms
Assembly Version: 1.0.5000.0
Win32 Version: 1.1.4322.2032
CodeBase:
file:///c:/winnt/assembly/gac/system.windows.forms/1.0.5000.0__b77a5c561934e089/system.windows.forms.dll
----------------------------------------
System
Assembly Version: 1.0.5000.0
Win32 Version: 1.1.4322.2032
CodeBase:
file:///c:/winnt/assembly/gac/system/1.0.5000.0__b77a5c561934e089/system.dll
----------------------------------------
System.Drawing
Assembly Version: 1.0.5000.0
Win32 Version: 1.1.4322.2032
CodeBase:
file:///c:/winnt/assembly/gac/system.drawing/1.0.5000.0__b03f5f7f11d50a3a/system.drawing.dll
----------------------------------------

************** JIT Debugging **************
To enable just in time (JIT) debugging, the config file for this
application or machine (machine.config) must have the
jitDebugging value set in the system.windows.forms section.
The application must also be compiled with debugging
enabled.

For example:

<configuration>
<system.windows.forms jitDebugging="true" />
</configuration>

When JIT debugging is enabled, any unhandled exception
will be sent to the JIT debugger registered on the machine
rather than being handled by this dialog.
Nov 16 '05 #1
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