Question

I’m trying to build a quick administrative interface using the built in Windows.Forms PropertyGrid . I managed to decorate my data classes with the appropriate attributes (ExpandableObjectConverter etc.) and all seems to work fine.

There is a use case I’m not figuring out: When i have values set on complex properties the expand button appears and i can edit the content but when i have a null value there seems to be no way to create a new instance of the desired type.
So a solution to this would be of great help. Added bonus if someone knows of a way to present a drop-down to the user of what types it can create from a list of possible derived values.

Answer 1

This is not so complicated, here is a sample code that does this kind of thing:

public class MyEditor : UITypeEditor
{
    private IWindowsFormsEditorService _editorService;

    public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context)
    {
        return UITypeEditorEditStyle.DropDown;
    }

    public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
    {
        if (value != null) // already initialized
            return base.EditValue(context, provider, value);

        _editorService = (IWindowsFormsEditorService)provider.GetService(typeof(IWindowsFormsEditorService));
        ListBox lb = new ListBox();
        lb.SelectionMode = SelectionMode.One;
        lb.SelectedValueChanged += OnListBoxSelectedValueChanged;

        // TODO: add your items/logic here
        lb.Items.Add(typeof(TYPE1));
        lb.Items.Add(typeof(TYPE2));
        ....
        lb.Items.Add(typeof(TYPEX));

        _editorService.DropDownControl(lb);
        if (lb.SelectedItem == null)
            return base.EditValue(context, provider, value); // no selection, no change

        // instantiate an object (add constructor logic if neede)
        return Activator.CreateInstance((Type)lb.SelectedItem);
    }

    private void OnListBoxSelectedValueChanged(object sender, EventArgs e)
    {
        _editorService.CloseDropDown();
    }
}