如何通过textBox输入函数形式，然后求函数值
using   System;
using   System.Drawing;
using   System.Collections;
using   System.ComponentModel;
using   System.Windows.Forms;
using   System.Data;

namespace   TEST
{
///   <summary>
///   Form1   的摘要说明。
///   </summary>
public   class   Form1   :   System.Windows.Forms.Form
{
private   System.Windows.Forms.Label   label1;
private   System.Windows.Forms.TextBox   textBox1;
private   System.Windows.Forms.Label   label2;
private   System.Windows.Forms.TextBox   textBox2;
private   System.Windows.Forms.Label   label3;
private   System.Windows.Forms.TextBox   textBox3;
private   System.Windows.Forms.Button   button1;
///   <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   );
}

static   Double   f(Double   x)
{
Double   fx=Convert.ToDouble(textBox5.Text);//这里出错了，如何改正？
return   fx;
}



#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.label1   =   new   System.Windows.Forms.Label();
this.textBox1   =   new   System.Windows.Forms.TextBox();
this.label2   =   new   System.Windows.Forms.Label();
this.textBox2   =   new   System.Windows.Forms.TextBox();
this.label3   =   new   System.Windows.Forms.Label();
this.textBox3   =   new   System.Windows.Forms.TextBox();
this.button1   =   new   System.Windows.Forms.Button();
this.SuspendLayout();
//  
//   label1
//  
this.label1.Location   =   new   System.Drawing.Point(48,   24);
this.label1.Name   =   "label1 ";
this.label1.Size   =   new   System.Drawing.Size(72,   23);
this.label1.TabIndex   =   0;
this.label1.Text   =   "f(x)= ";
//  
//   textBox1
//  
this.textBox1.Location   =   new   System.Drawing.Point(128,   24);
this.textBox1.Name   =   "textBox1 ";
this.textBox1.Size   =   new   System.Drawing.Size(200,   21);
this.textBox1.TabIndex   =   1;
this.textBox1.Text   =   " ";
//  
//   label2
//  
this.l