在C#中怎样把Textbox中文本的文字写入一个.txt文件中?

导入System.IO

按钮单击事件：
private void button1_Click(object sender, EventArgs e)
{
FileStream fs = new FileStream(@"C:/Documents and Settings/user/桌面/t.txt", FileMode.Create);
StreamWriter sw = new StreamWriter(fs);
sw.Write(textBox1.Text);
sw.Close();
fs.Close();

}

using System.IO;

textBox1.Text = "111";
StreamWriter sw = new StreamWriter("1.txt",true);
sw.Write(textBox1.Text);
sw.WriteLine(textBox1.Text);
sw.Flush();

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.IO;
using System.Windows.Forms;

namespace save
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

public void save()
{
string filePath = @"D:\test.txt";
string txt = TextBox.Text;

File.WriteAllText(filePath, txt, Encoding.Default);
}

private void Form1_Load(object sender, EventArgs e)
{

}

private void button1_Click(object sender, EventArgs e)
{
save();
}
}
}