實現下面效果  窗體：Form1：

一：窗體佈局：文字框一個或多個，下拉框一個，初始化隱藏，定義全域性變數

string click_value = "";

 private void Form1_Load(object sender, EventArgs e)
{
    this.comboBox1.Visible = false;
}

二：文字框點選事件：

private void txt_one_Click(object sender, EventArgs e)
{
    //清空下拉框資料
    this.comboBox1.Items.Clear();
    this.comboBox1.Text = "==請選擇==";
    //追加賦值
    double[] d = new double[3] { 2, 1.5, 0.5 };
    foreach (double val in d)
    {
       this.comboBox1.Items.Add(val);
    }
     //改變下拉框的位置        
    Point point = this.txt_one.Location;
    this.comboBox1.Visible = true;
    this.comboBox1.BringToFront();
    this.comboBox1.Location = new Point(point.X, point.Y + 30);
    //定義一個全域性變數（因為我這裡要實現多個文字框觸發顯示下拉框）
    click_value = "one";
}

private void txt_two_Click(object sender, EventArgs e)
{
    //清空下拉框資料
    this.comboBox1.Items.Clear();
    this.comboBox1.Text = "==請選擇==";
    //追加賦值
    double[] d = new double[3] { 2, 1.5, 0.5 };
    foreach (double val in d)
    {
       this.comboBox1.Items.Add(val);
    }
     //改變下拉框的位置        
    Point point = this.txt_one.Location;
    this.comboBox1.Visible = true;
    this.comboBox1.BringToFront();
    this.comboBox1.Location = new Point(point.X, point.Y + 30);
    //定義一個全域性變數（因為我這裡要實現多個文字框觸發顯示下拉框）
    click_value = "two";
}

private void txt_three_Click(object sender, EventArgs e)
{
    //清空下拉框資料
    this.comboBox1.Items.Clear();
    this.comboBox1.Text = "==請選擇==";
    //追加賦值
    double[] d = new double[3] { 2, 1.5, 0.5 };
    foreach (double val in d)
    {
       this.comboBox1.Items.Add(val);
    }
     //改變下拉框的位置        
    Point point = this.txt_one.Location;
    this.comboBox1.Visible = true;
    this.comboBox1.BringToFront();
    this.comboBox1.Location = new Point(point.X, point.Y + 30);
    //定義一個全域性變數（因為我這裡要實現多個文字框觸發顯示下拉框）
    click_value = "three";
}
 

三：下拉框選中改變事件

 /// <summary>
/// 值改變
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void comboBox1_SelectedValueChanged(object sender, EventArgs e)
 {
    switch (click_value)
    {
        case "one":
            this.txt_one.Text = this.comboBox1.Text;
            this.comboBox1.Visible = false;
            break;
        case "two":
            this.txt_two.Text = this.comboBox1.Text;
            this.comboBox1.Visible = false;
            break;
        case "three":
             this.txt_three.Text = this.comboBox1.Text;
             this.comboBox1.Visible = false;
             break;
        default:
            break;
      }
  }