Question

Following is my code which is binding a List() to a ComboBox control. I tried to add few items to the CombBox control using Insert() method which is not allowed as it has been assigned to a datasource. So, how can I get the datasouce back to a new variable (say, var colours2) from cmbColour.DataSource which is returning an object. Thanx !

var colours= new Dictionary<string, string>
            {
              {"1x","Green"},
              {"2x","Red"},
              {"3y","Blue"},
              {"4y","Black"}
            }.ToList();

cmbColour.ValueMember = "Key";
cmbColour.DisplayMember = "Value";
cmbColour.DataSource = colours;

var colours2 = //how can I get the DataSource back 

Answer 1

The following code will return a new dictionary containing the same data you bound to the combo box.

var list = (List<KeyValuePair<String, String>>)cmbColor.DataSource;

var dictionary = list.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);

The property DataSource will return the same instance you assigned but because it is typed Object you have to cast it back to the actual type before you can access any members.

But why don’t you just keep the original dictionary? And it is definitely supported to modify a list bound to a data source – that is the whole point of data binding.

I guess my answer does not really solve your actual problem, only what you think what your problem is. Maybe you can give some additional information about what you are trying to achieve and I or someone else will be able to help you with your underlying problem.

UPDATE

This should work for your scenario – I stick with the user example.

public class User
{
    public String Id { get; set; }

    public String Name { get; set; }
}

And the code for the form.

public partial class MainForm : Form
{
    private readonly BindingList<User> recentlyAddedUsers = new BindingList<User>();

    private void MainFormLoad(Object sender, EventArgs e)
    {
        this.comboBoxRecentlyAddedUsers.DataSource = this.recentlyAddedUsers;

        this.comboBoxRecentlyAddedUsers.ValueMember = "Id";
        this.comboBoxRecentlyAddedUsers.DisplayMember = "Name";

        var recentlyAddedUsersFromService = this.GetRecentlyAddedUsers();

        foreach (var user in recentlyAddedUsersFromService)
        {
            this.recentlyAddedUsers.Add(user);
        }
    }

    private void ButtonAddNewUserClick(Object sender, EventArgs e)
    {
        var newUser = new User();

        newUser.Id = this.textBoxUserId.Text;
        newUser.Name = this.textBoxUserName.Text;

        this.SaveNewUser(newUser);

        this.recentlyAddedUsers.RemoveAt(0);
        this.recentlyAddedUsers.Insert(newUser);
    }

    private List<User> GetRecentlyAddedUsers()
    {
        // Get a list of recently added users from the database.
    }

    private void SaveNewUser(User user)
    {
        // Save the new user to the database.
    }
}

Note the usage of BindingList<T> – this will notify the combo box about any changes to the list. A simple List<T> would also work but then you have to explicitly tell the combo box to refresh the data binding.