如何在SQL上找不到值时显示消息框

下面是在sql中从表中获取值并将其设置为相关字段的代码。然而，我想知道如何在下面的块代码中编写如果条件，所以如果数据表包含USERID，它将执行下面的功能，但如果它没有找到USERID，它应该弹出一个错误消息说没有用户发现。

SqlCommand myCommand = new SqlCommand
("SELECT * From USER_TABLE WHERE USERID =" + userIdTextBox.Text, con1);
myReader = myCommand.ExecuteReader();
while (myReader.Read())
{
    nameTextBox.Text = (myReader["FIRST_NAME"].ToString());
    lnameTextBox.Text = (myReader["LAST_NAME"].ToString());
    posTextBox.Text = (myReader["POSITION"].ToString());
    emailTextBox.Text = (myReader["E_MAIL"].ToString());
    phoneTextBox.Text = (myReader["PHONE"].ToString());
    usernameTextBox.Text = (myReader["USERNAME"].ToString());
    userLevelTextBox.Text = (myReader["USER_LEVEL"].ToString());
    string filename = (myReader["PROFILE_PICTURE"].ToString());
    profilePicBox.Load(filename);
}

  if(MyReader.HasRows)
    {
         while (myReader.Read())
                {
                   //your code here.
                }
    }
    else
    {
      // your alert.
    }

        if (myReader.Read()) //assuming you only ever have a single result...
        {
            //set form fields.
        }
        else
        {
            //message box 
        }