Check if list is Populated

Question

Im writing a program that allows people to input a postcode and get different medical services displayed with how far away they are from the inputted postcode.

The postcodes are meant to be stored in a list however I do not know how to check if the list gets populated.

could anyone give me any ideas.

This is the list

 private List<string> GetPostcodes(string table)
    {
        connect = new MySqlConnection(connectionString);
        connect.Open();
        string selectString = "select postcode from " + table;

        MySqlCommand cmd = new MySqlCommand(selectString,connect);
        reader = cmd.ExecuteReader();
        while (reader.Read())
        {
            postcodes.Add(reader.GetString("postcode"));
        }
        connect.Close();

        return postcodes;
    }

And this is a tester button however it doesnt work

 private void Button_Click1(object sender, RoutedEventArgs e)
    {
        var test1 = GetPostcodes("gpSurgery").ToString();
        MessageBox.Show(test1);

    }

@gunr2171 gives error message saying could not convert int to string — Fawcett Fawcett, Dec 11 '19 at 14:46
@FawcettFawcett What do you mean nothing happened? Did the code get hit? Did you run it with the debugger attached? Was an exception thrown? You need to be very specific when describing the behavior of your code. Remember we are not at your computer to see what's happening, so it's up to you to describe what's going on. — mason, Dec 11 '19 at 14:46
Also see https://stackoverflow.com/questions/18867180/check-if-list-is-empty-in-c-sharp — gunr2171, Dec 11 '19 at 14:46
@FawcettFawcett: That's because you are calling `ToString()` on the return value. See my answer for a working example. — mm8, Dec 11 '19 at 14:48

score 2 · Accepted Answer · answered Dec 11 '19 at 14:47

The GetPostcodes method should create a new List<string> on each invocation instead of reusing a global variable:

private List<string> GetPostcodes(string table)
{
    List<string> postcodes = new List<string>();
    using (var connect = new MySqlConnection(connectionString))
    {
        connect.Open();
        string selectString = "select postcode from " + table;
        using (MySqlCommand cmd = new MySqlCommand(selectString, connect))
        {
            using (var reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    postcodes.Add(reader.GetString("postcode"));
                }
            }
        }
    }
    return postcodes;
}

In the "test", you could simply check the value of the Count property of the list if you want to determine the number of items in it:

private void Button_Click1(object sender, RoutedEventArgs e)
{
    var test1 = GetPostcodes("gpSurgery");
    MessageBox.Show(test1.Count);

}

score 1 · Answer 2 · answered Dec 11 '19 at 14:46

What you need to do is check the List being returned by the GetpostCodes. Dont convert that to String because that function doesnt work with Lists. You have to correctly convert the list to its corresponding string.

GetPostcodes returns a list. Use that list to check its Count.

 private void Button_Click1(object sender, RoutedEventArgs e)
    {
        var test1 = GetPostcodes("gpSurgery");
        if (test1.Count > 0) // <---- Check the Count here
        {
            MessageBox.Show(string.Join(",", test1)); // <-- Display the list
        }
        else
        {
            MessageBox.Show("Nothing found"); // <-- Shows nothing found.
        }
    }

Check if list is Populated

2 Answers2