Make ComboBox accept only specific type

Question

I currently have a ComboBox in my Windows Forms Application. In order to specify which values the ComboBox will contain, I set DataSource property of the ComboBox to some array so that ComboBox contains values from that array. I could also use Items.Add() to add new values to ComboBox. However, I want to make sure that ComboBox can be populated with objects of some specific type. So, if I have a class called X, then I want to make it so that only an array of type X can be used as a data source for the ComboBox. Right now, ComboBox accepts objects of type System.Object. How can I achieve it? Is there a property of ComboBox that I need to set to be equal to my data type's name? Or is there an event that will check whether an object added to my ComboBox is of the needed type and will throw an exception if not?

I was thinking of creating a new class as a subtype of ComboBox, and overriding the Add method of Items property so that Add checks whether its argument is of the needed type (not sure if and how I can do it). Even if I do that, there are still other ways to add new values into ComboBox (AddRange, CopyTo, etc.), so I think there should be a more elegant solution to this problem.

Answer 1

If you want to control the type of item that the ComboBox can contain, you could try creating a new class derived form ComboBox, but you'd run into the problem that it still has the ComboBox.ObjectCollection Items property which would still accept any type! And (unfortunately for your idea of overriding) the Add method isn't virtual.

The only practical solution that I could think of would be to abstract the ComboBox somehow. If this isn't shared code, I would recommend just creating a method that you would use to add items to the ComboBox. Something like:

// NOTE: All items that are added to comboBox1 need to be of type `SomeType`.
private void AddItemToComboBox(SomeType item)
{
    comboBox1.Items.Add(item);
}

Any attempt to add a non-SomeType object to the ComboBox would be met with a compiler error. Unfortunately, there's no easy way to prevent someone from still adding a non-SomeType item to ComboBox.Items directly.

Again, if this isn't shared code, it shouldn't really be an issue.

Answer 2

You can hide Items property by your
own Items property of custom type which taking as parameter original ItemsCollection

Example class for testing

public class Order
{
    public Int32 ID { get; set; }
    public string Reference { get; set; }

    public Order() { }
    public Order(Int32 inID, string inReference)
    {
        this.ID = inID;
        this.Reference = (inReference == null) ? string.Empty : inReference;
    }

    //Very important 
    //Because ComboBox using .ToString method for showing Items in the list
    public override string ToString()
    {
        return this.Reference;
    }

}

With next class I tried wrap ComboBox's items collection in own type. Where adding items must be concrete type Here you can add other methods/properties you need (Remove)

public class ComboBoxList<TCustomType>
{
    private System.Windows.Forms.ComboBox.ObjectCollection _baseList;

    public ComboBoxList(System.Windows.Forms.ComboBox.ObjectCollection baseItems)
    {
        _baseList = baseItems;
    }

    public TCustomType this[Int32 index]
    {
        get { return (TCustomType)_baseList[index]; }
        set { _baseList[index] = value; }
    }

    public void Add(TCustomType item)
    {
        _baseList.Add(item);
    }

    public Int32 Count { get { return _baseList.Count; } }

}

Here custom combobox class derived from ComboBox Added: generic type

public class ComboBoxCustomType<TCustomType> : System.Windows.Forms.ComboBox
{
    //Hide base.Items property by our wrapping class
    public new ComboBoxList<TCustomType> Items; 

    public ComboBoxCustomType() : base()
    {
        this.Items = new ComboBoxList<TCustomType>(base.Items);
    }

    public new TCustomType SelectedItem 
    { 
        get { return (TCustomType)base.SelectedItem; } 
    }
}

Next code used in the Form

private ComboBoxCustomType<Order> _cmbCustom;

//this method used in constructor of the Form
private void ComboBoxCustomType_Initialize()
{
    _cmbCustom = new ComboBoxCustomType<Order>();
    _cmbCustom.Location = new Point(100, 20);
    _cmbCustom.Visible = true;
    _cmbCustom.DropDownStyle = ComboBoxStyle.DropDownList;
    _cmbCustom.Items.Add(new Order(0, " - nothing - "));
    _cmbCustom.Items.Add(new Order(1, "One"));
    _cmbCustom.Items.Add(new Order(2, "Three"));
    _cmbCustom.Items.Add(new Order(3, "Four"));
    _cmbCustom.SelectedIndex = 0;
    this.Controls.Add(_cmbCustom);
}