NullableComboBox

• Download demo project - 13.6 Kb
• Download source - 4.65 Kb

Introduction

This is a combo box which can bind to business objects and can show null values (empty).

Background

The problem with the combo box is that after you bind data to it, it is not possible to show an empty Text property. It is also not possible to delete an already selected value. This combo box can do both. It is designed that you bind business objects to it, but I think (never tried it) it should also work with .NET DataTables.

Using the code

I call the little trick I do "lazy binding". With this I mean, that I don't bind the data in the first place when you set the datasource. I wait until the user actually tries to drop down the combo box. So you have the effect that the user sees an empty combo box in the beginning. The deleting of already selected items works nearly similar. In that case (if the user press DEL or BACK button), I remove the databinding of the combo box (but still remember the datasource). The code itself is really simple:

The only interesting method is the one that does the data binding:

public void SetDataBinding(IList dataSource, object objectToModify, 
                                      string propertyName, string displayMember)
{
    // init combo box and delete all databinding stuff
    this.DisplayMember = String.Empty;
    this.Items.Clear();
    this.ValueMember = String.Empty;
    this.Text = String.Empty;

    // init private fields
    this.mDataSource = dataSource;
    this.mObjectToModify = objectToModify;
    this.mPropertyName = propertyName;
    this.mProperty = 
      this.mObjectToModify.GetType().GetProperty(this.mPropertyName);
    this.mDisplayMember = displayMember;
    this.mNullableMode = true;
    
    // get selected item
    object selectedItem = 
      this.mProperty.GetValue(this.mObjectToModify, null);

    // if not null, bind to it
    if (selectedItem != null)
    {
        this.DataSource = this.mDataSource;
        this.SelectedItem = selectedItem;
    }
    // do nothing and set datasource to null
    else
        this.DataSource = null;
}

The drop down event:

protected override void OnDropDown(EventArgs e)
{
    // if no datasource is set, set it
    if (this.mNullableMode && this.mDataSource 
           != null && this.DataSource == null)
        this.DataSource = this.mDataSource;

    base.OnDropDown(e);
}

And the key down event:

protected override void OnKeyDown(KeyEventArgs e)
{
    // if DEL or BACK is pressed set property to null and data source to null
    if (this.mNullableMode && (e.KeyCode == 
             Keys.Delete || e.KeyCode == Keys.Back))
    {
        // next line is very important:
        // without you may get an OutOfRangeException
        this.DroppedDown = false;
        this.DataSource = null;
    }

    base.OnKeyDown(e);
}