ComboBoxes and Non-Modal Forms

[Hector]

I have a ComboBox set up in a non-modal form. When a selection is made from the ComboBox, the handler code closes the form, but then the system crashes because of an unhandled NullReferenceException. There is a reference to Unsafe Native Methods and Callback functions, but Interop Services are not used. What is going on ? The problem does not occur if the form is made modal. Any thoughts would be appreciated. The code appears below for a simple example of this problem.

I am using C#.NET 2003.

System.NullReferenceException: Object reference not set to an instance of an object.
at System.Windows.Forms.UnsafeNativeMethods.CallWindo wProc(…)
…
at System.Windows.Forms.ComboBox.WndProc(Message& m)
…
at System.Windows.Forms.NativeWindow.Callback(..)�

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;

public 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 A.
//A.ShowDialog(); // modal // no bug in modal case.
}

}

}

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[] {
"A",
"B"});
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 = "A";
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, 69);
this.Controls.Add(this.comboBox1);
this.Name = "FormA";
this.Text = "FormA";
this.ResumeLayout(false);

}
#endregion

private void comboBox1_SelectedIndexChanged(object sender, System.EventArgs e)
{
Close();
}

}
}