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.label2.Location = new System.Drawing.Point(48, 56);
this.label2.Name = "label2 ";

this.label2.Size = new System.Drawing.Size(72, 24);
this.label2.TabIndex = 2;
this.label2.Text = "x0= ";
//
// textBox2
//
this.textBox2.Location = new System.Drawing.Point(128, 56);
this.textBox2.Name = "textBox2 ";
this.textBox2.Size = new System.Drawing.Size(200, 21);
this.textBox2.TabIndex = 1;
this.textBox2.Text = " ";
this.textBox2.TextChanged += new System.EventHandler(this.textBox2_TextChanged);
//
// label3
//
this.label3.Location = new System.Drawing.Point(32, 200);
this.label3.Name = "label3 ";
this.label3.Size = new System.Drawing.Size(56, 24);
this.label3.TabIndex = 3;
this.label3.Text = "f(x0)= ";
//
// textBox3
//
this.textBox3.Location = new System.Drawing.Point(112, 200);
this.textBox3.Name = "textBox3 ";
this.textBox3.Size = new System.Drawing.Size(176, 21);
this.textBox3.TabIndex = 4;
this.textBox3.Text = " ";
//
// button1
//
this.button1.Location = new System.Drawing.Point(176, 128);
this.button1.Name = "button1 ";
this.button1.Size = new System.Drawing.Size(120, 48);
this.button1.TabIndex = 5;
this.button1.Text = "计算 ";
this.button1.Click += new System.EventHandler(this.button1_Click);
//
// Form1
//
this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
this.ClientSize = new System.Drawing.Size(368, 368);
this.Controls.Add(this.button1);
this.Controls.Add(this.textBox3);
this.Controls.Add(this.label3);
this.Controls.Add(this.label2);
this.Controls.Add(this.textBox1);
this.Controls.Add(this.label1);
this.Controls.Add(this.textBox2);
this.Name = "Form1 ";
this.Text = "Form1 ";
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 textBox2_TextChanged(object sender, System.EventArgs e)
{

}

private void button1_Click(object sender, System.EventArgs e)
{
if((textBox1.Text== " ")||(textBox2.Text== " "))
MessageBox.Show( "请输入数据！ ");
else
{

Double x=0;
x=Convert.ToDouble(textBox2.Text);
String fx=textBox1.Text;
textBox3.Text=f(x).ToString();

}
}
}
}


[解决办法]
textBox5.Text 在静态方法中是不能用的

------解决方案--------------------

写成
static Double f(String Text)
{
Double fx=Convert.ToDouble(Text);
return fx;
}

或
static Double f(TextBox Text)
{
Double fx=Convert.ToDouble(Text.text);
return fx;
}
[解决办法]
这个要用到编译原理的知识。简单的话可以使用递归下降法。