在SQL上找不到值时如何显示消息框

Question

Following is a code to get values from table in sql and set it to relevant fields. However i want to know how to write if condition in below block code so if the datatable contains the USERID it will performs function below but if it didn't find the USERID it should popup a error message saying no User Found.

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);

}

Answer 1

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

Edit based on comment from @dmitry-bychenko

Answer 2

You need to check if(myReader.HasRows)

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