public static void SetWatermark(this TextEdit textEdit, string watermark)
{
  textEdit.Properties.NullValuePromptShowForEmptyValue = true;
  textEdit.Properties.NullValuePrompt = watermark;
}
public static void ClearWatermark(this TextEdit textEdit)
{
  if (textEdit.Properties.NullValuePromptShowForEmptyValue)
 textEdit.Properties.NullValuePrompt = string.Empty;
}

private void Form1_Load(object sender, EventArgs e)
{
  textEdit1.SetWatermark("请输入用户名称....");
  textEdit2.SetWatermark("请输入用户密码....");
}

private void simpleButton1_Click(object sender, EventArgs e)
{
  textEdit1.ClearWatermark();
  textEdit2.ClearWatermark();
}