HashTable问题，产生异常是什么原因呢？
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace HashTable
{
/// <summary>
/// Form1 的摘要说明。
/// </summary>
public class Form1 : System.Windows.Forms.Form
{
private Hashtable _hashtable=new Hashtable();
private System.Windows.Forms.GroupBox groupBox1;
private System.Windows.Forms.Label label1;
private System.Windows.Forms.Label label2;
private System.Windows.Forms.Label label3;
private System.Windows.Forms.Label label4;
private System.Windows.Forms.TextBox txtKey;
private System.Windows.Forms.TextBox txtValue;
private System.Windows.Forms.Button btnAdd;
private System.Windows.Forms.ListBox lstAddedKey;
private System.Windows.Forms.TextBox txtSelectedValue;
/// <summary>
/// 必需的设计器变量。
/// </summary>
private System.ComponentModel.Container components = null;

public Form1()
{
//
// Windows 窗体设计器支持所必需的
//
InitializeComponent();

//
// TODO: 在 InitializeComponent 调用后添加任何构造函数代码
//
}

/// <summary>
/// 清理所有正在使用的资源。
/// </summary>
protected override void Dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.Dispose();
}
}
base.Dispose( disposing );
}

#region Windows 窗体设计器生成的代码
/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.groupBox1 = new System.Windows.Forms.GroupBox();
this.btnAdd = new System.Windows.Forms.Button();
this.txtValue = new System.Windows.Forms.TextBox();
this.txtKey = new System.Windows.Forms.TextBox();
this.label2 = new System.Windows.Forms.Label();
this.label1 = new System.Windows.Forms.Label();
this.label3 = new System.Windows.Forms.Label();
this.lstAddedKey = new System.Windows.Forms.ListBox();
this.label4 = new System.Windows.Forms.Label();
this.txtSelectedValue = new System.Windows.Forms.TextBox();
this.groupBox1.SuspendLayout();
this.SuspendLayout();
//
// groupBox1
//
this.groupBox1.Controls.Add(this.btnAdd);
this.groupBox1.Controls.Add(this.txtValue);
this.groupBox1.Controls.Add(this.txtKey);
this.groupBox1.Controls.Add(this.label2);
this.groupBox1.Controls.Add(this.label1);
this.groupBox1.Location = new System.Drawing.Point(8, 8);
this.groupBox1.Name = "groupBox1 ";
this.groupBox1.Size = new System.Drawing.Size(280, 120);
this.groupBox1.TabIndex = 0;
this.groupBox1.TabStop = false;

this.groupBox1.Text = "建立键值 ";
//
// btnAdd
//
this.btnAdd.Location = new System.Drawing.Point(96, 88);
this.btnAdd.Name = "btnAdd ";
this.btnAdd.TabIndex = 4;
this.btnAdd.Text = "添加 ";
this.btnAdd.Click += new System.EventHandler(this.btnAdd_Click);
//
// txtValue
//
this.txtValue.Location = new System.Drawing.Point(96, 56);
this.txtValue.Name = "txtValue ";
this.txtValue.Size = new System.Drawing.Size(176, 21);
this.txtValue.TabIndex = 3;
this.txtValue.Text = " ";
//
// txtKey
//
this.txtKey.Location = new System.Drawing.Point(96, 24);
this.txtKey.Name = "txtKey ";
this.txtKey.Size = new System.Drawing.Size(176, 21);
this.txtKey.TabIndex = 2;
this.txtKey.Text = " ";
//
// label2
//
this.label2.AutoSize = true;
this.label2.Location = new System.Drawing.Point(16, 56);
this.label2.Name = "label2 ";
this.label2.Size = new System.Drawing.Size(29, 17);
this.label2.TabIndex = 1;
this.label2.Text = "值： ";
//
// label1
//
this.label1.AutoSize = true;
this.label1.Location = new System.Drawing.Point(16, 24);
this.label1.Name = "label1 ";
this.label1.Size = new System.Drawing.Size(29, 17);
this.label1.TabIndex = 0;
this.label1.Text = "键： ";
//
// label3
//
this.label3.AutoSize = true;
this.label3.Location = new System.Drawing.Point(8, 136);
this.label3.Name = "label3 ";
this.label3.Size = new System.Drawing.Size(72, 17);
this.label3.TabIndex = 1;
this.label3.Text = "已经添加值: ";
//
// lstAddedKey
//
this.lstAddedKey.ItemHeight = 12;
this.lstAddedKey.Location = new System.Drawing.Point(96, 136);
this.lstAddedKey.Name = "lstAddedKey ";
this.lstAddedKey.Size = new System.Drawing.Size(192, 112);
this.lstAddedKey.TabIndex = 2;
this.lstAddedKey.SelectedValueChanged += new System.EventHandler(this.lstAddedKey_SelectedValueChanged);
this.lstAddedKey.SelectedIndexChanged += new System.EventHandler(this.lstAddedKey_SelectedIndexChanged);
//
// label4
//
this.label4.AutoSize = true;
this.label4.Location = new System.Drawing.Point(16, 264);
this.label4.Name = "label4 ";
this.label4.Size = new System.Drawing.Size(60, 17);
this.label4.TabIndex = 3;
this.label4.Text = "选中的值: ";
//
// txtSelectedValue

//
this.txtSelectedValue.Location = new System.Drawing.Point(96, 256);
this.txtSelectedValue.Name = "txtSelectedValue ";
this.txtSelectedValue.Size = new System.Drawing.Size(192, 21);
this.txtSelectedValue.TabIndex = 4;
this.txtSelectedValue.Text = " ";
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
this.ClientSize = new System.Drawing.Size(292, 293);
this.Controls.Add(this.txtSelectedValue);
this.Controls.Add(this.label4);
this.Controls.Add(this.lstAddedKey);
this.Controls.Add(this.label3);
this.Controls.Add(this.groupBox1);
this.Name = "Form1 ";
this.Text = "HashTableExam ";
this.Load += new System.EventHandler(this.Form1_Load);
this.groupBox1.ResumeLayout(false);
this.ResumeLayout(false);

}
#endregion

/// <summary>
/// 应用程序的主入口点。
/// </summary>
[STAThread]
static void Main()
{
Application.Run(new Form1());
}

private void Form1_Load(object sender, System.EventArgs e)
{

}

private void btnAdd_Click(object sender, System.EventArgs e)
{
try
{
_hashtable.Add(txtKey.Text,txtValue.Text);
}
catch(ArgumentException)
{
MessageBox.Show( "不能添加重负的值 ", "错误 ",System.Windows.Forms.MessageBoxButtons.OK,
System.Windows.Forms.MessageBoxIcon.Information);
//有重复的键值给用户提示，插入动作终止
return;
}
catch(Exception)
{
MessageBox.Show( "产生系统错误 ");//产生未知错误
Application.Exit();//则关闭应用程序
}
lstAddedKey.Items.Add(txtKey.Text);
//txtKey.Text= " ";
//txtValue.Text= " ";
}

private void lstAddedKey_SelectedIndexChanged(object sender, System.EventArgs e)
{

}

private void lstAddedKey_SelectedValueChanged(object sender, System.EventArgs e)
{
string selectedKey=lstAddedKey.SelectedValue.ToString();
string selectedValue=(string)_hashtable[selectedKey];
txtSelectedValue.Text=selectedValue;
}
}
}


[解决办法]
private void lstAddedKey_SelectedValueChanged(object sender, System.EventArgs e)
{
string selectedKey=lstAddedKey.SelectedItem.ToString(); //修改的地方
string selectedValue=(string)_hashtable[selectedKey];
txtSelectedValue.Text=selectedValue;
}
[解决办法]
报什么错误 哪行报错？
[解决办法]
namespace HashTable ----------------------??????改一下看看
{
/// <summary>
/// Form1 的摘要说明。
/// </summary>
public class Form1 : System.Windows.Forms.Form

[解决办法]
同一楼

string selectedKey=lstAddedKey.SelectedValue.ToString();

改为

string selectedKey=lstAddedKey.SelectedItem.ToString();