Updating from *new* ComboBox text, rather than the old text

Question

I have a combobox I'm selecting a value from. I want to use the newly selected value in the combobox as a comparison to my database, but I've only been able to return the value that matches with the text that was in the combobox before the selection change.

I've tried changing .text to selected item & selected value, neither of which returns a selection at all.

This is called upon the ComboBox selection change:

    private void StationComboBox_1_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        GetIP();
    }

    private void GetIP()
    {
        string connectionString = "SERVER=localhost;DATABASE=db; UID=PC;Password=pw;";

        MySqlConnection connection = new MySqlConnection(connectionString);

        MySqlCommand cmd = new MySqlCommand("SELECT IP_stations from stations WHERE stationNumber_stations='" + stationComboBox_1.Text + "'", connection); 
        connection.Open();

        string IP = (string)cmd.ExecuteScalar();


        DisplayIP.Text = IP;

This is the code from that ComboBox, which gets its values from the same database, but a different table.

<ComboBox 
    Grid.Column="1" 
    x:Name="stationComboBox_1" 
    FontSize="25" 
    Width="60" 
    HorizontalAlignment="Left" 
    DisplayMemberPath="stationNumber_stations" 
    ItemsSource="{Binding}" 
    Text="0" 
    SelectionChanged="StationComboBox_1_SelectionChanged"
    />

DisplayIP Xaml:

<TextBox x:Name="DisplayIP"/>

Setting up the ComboBoxes:

        public void SQLSetup()
        {
            string connectionString = "SERVER=localhost;DATABASE=db; UID=PC;Password=pw;"; 

            MySqlConnection connection = new MySqlConnection(connectionString); 

            MySqlCommand cmd = new MySqlCommand("Select stationNumber_stations from stations", connection); //Command to select all the station numbers
            connection.Open();


            DataTable dt = new DataTable(); //Creates a new data table
            dt.Load(cmd.ExecuteReader()); //Loads the data table with the values returned from the MySQL command
            connection.Close(); //Closes the MySQL connection

            //Sets the values in the station dropdowns to the values from the data table with the station numbers
            stationComboBox_1.DataContext = stationComboBox_2.DataContext = stationComboBox_3.DataContext = stationComboBox_4.DataContext = stationComboBox_5.DataContext =
                stationComboBox_6.DataContext = stationComboBox_7.DataContext = stationComboBox_8.DataContext = dt;
            stationComboBox_1.ItemsSource = stationComboBox_2.ItemsSource = stationComboBox_3.ItemsSource = stationComboBox_4.ItemsSource = stationComboBox_5.ItemsSource =
                stationComboBox_6.ItemsSource = stationComboBox_7.ItemsSource = stationComboBox_8.ItemsSource = dt.DefaultView;


            //sets an int to the value selected in the station dropdown
            string stationSelection_1 = stationComboBox_1.Text;
            string stationSelection_2 = stationComboBox_2.Text;
            string stationSelection_3 = stationComboBox_3.Text;
            string stationSelection_4 = stationComboBox_4.Text;
            string stationSelection_5 = stationComboBox_5.Text;
            string stationSelection_6 = stationComboBox_6.Text;
            string stationSelection_7 = stationComboBox_7.Text;
            string stationSelection_8 = stationComboBox_8.Text;
        }

I want the DisplayIP TextBox to display the IP of the current selection. However, it is displaying the IP of the previous selection.

Right now, on startup, both the ComboBox & TextBox are empty. I select a station in the ComboBox & the TextBox remains empty. When I select a new station in the ComboBox, the TextBox updates to display the IP address of the first selection.

Answer 1

It looks like when you change the selection in the ComboBox, you want the textbox to display the IP address of the selected station. We know that's in the IP_stations column in the stations table.

We already discussed the fact that ItemsSource="{Binding}" is redundant, so I'm omitting that. I'm omitting the selection-changed handler because that job will be done by a binding. There's no need to re-query the database when the selection changes, because we already have all the columns of all the rows in the DataView displayed in the combobox.

You've already learned DisplayMemberPath, and we'll be using his evil twin, SelectedValuePath.

SelectedValuePath="IPStations" tells the combobox that when the selection changes, it should look at the selected item (in this case a DataRowView from the DataView), and try to find a property or column (column, in our case) by that name. If it finds it, the ComboBox will assign the value of that column to its own SelectedValue property. Then we teach the textbox to update itself by binding its Text property to stationComboBox_1.SelectedValue.

To make that work you'll need to select both values from the table (I neglected this the first time around):

//Command to select all the station numbers and IPs
MySqlCommand cmd = 
    new MySqlCommand("Select stationNumber_stations, IP_stations from stations", 
        connection); 

Done.

<ComboBox 
    Grid.Column="1" 
    x:Name="stationComboBox_1" 
    Width="60" 
    HorizontalAlignment="Left" 
    DisplayMemberPath="stationNumber_stations" 
    SelectedValuePath="IP_stations"
    />
<TextBox
    Text="{Binding SelectedValue, ElementName=stationComboBox_1}" 
    />

ComboBox.Text is only used in WPF in cases where the text of the combobox is editable.

Incidentally, here's what you could have done in the selection changed handler:

private void stationComboBox_1_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    var cb = (sender as ComboBox);

    //  If SelectedValuePath is "IP_Stations", this will be the IP address. 
    //  It will be the correct current selected value. 
    var selectedValue = cb.SelectedValue;

    //  Alternatively, we could do it this way:
    var row = cb.SelectedItem as DataRowView;

    if (row != null)
    {
        var selectedIP = row["IP_stations"];
    }
}

The next issue here, at least in my mind, is that you're copying and pasting everything eight times. MVVM could get you out of that mess.