Question

I’ve been able to make 2 C# winform datagridviews drag and drop rows between them, but it uses a lot of code. Now I’m looking at creating a form that will contain 10 such grids, and I’m trying to reuse the code. I thought I could simply make a sub-class of DataGridView and add the event handlers, but I’m having trouble.

Specifically, how can I override the onDragDrop event, when the only event intellisense sees does not have the correct signature.

I see a signature like: protected override void OnDragDrop(DragEventArgs e), but I’m expecting a signature like protected override void OnDragDrop(object sender, DragEventArgs e), because I’m using sender in my code. What am I missing? How should I properly override this event? My non-working code is below:

public class DragGrid : DataGridView
{
    private DragGrid()
    {

    }

    private DragGrid(DataTable table)
    {
        this.DataSource = table;
    }

    protected override void OnDragEnter(DragEventArgs e)
    {
        base.OnDragEnter(e);
        e.Effect = DragDropEffects.Copy;
    }

    protected override void OnDragDrop(DragEventArgs e)
    {
        base.OnDragDrop(e);

        DataGridView grid = sender as DataGridView;
        DataTable table = grid.DataSource as DataTable;
        DataRow row = e.Data.GetData(typeof(DataRow)) as DataRow;

        if (row != null && table != null && row.Table != table)
        {
            table.ImportRow(row);
            row.Delete();
            table.AcceptChanges();
        }
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        base.OnMouseDown(e);
        DragGrid.HitTestInfo info = ((DragGrid)this).HitTest(e.X, e.Y);

        if (info.RowIndex >= 0)
        {
            DataRow view = ((DataTable)(this.DataSource)).Rows[info.RowIndex];

            if (view != null)
            {
                this.DoDragDrop(view, DragDropEffects.Copy);
            }
        }
    }
}

Answer 1

You are making mistake in definition between events and delegates:
protected override void OnDragEnter(DragEventArgs e) { } is a method which invokes the event DragEnter of your datagrid.
And when this event accurs, all the delegates with signature protected override void OnDragEnter(object sender, DragEventArgs e) { } are invoked.

So you can’t override the event – this is just a field. You can override method which invokes it, but I think this is not that you are seeking for. I suggest you to add a handler for the DragEnter event for your script during your constructor:

private DragGrid(DataTable table)
{
    this.DataSource = table;
    this.DragEnter += new DragEventHandler(_onDragEnter);
}
private void _onDragEnter(object sender, DragEventArgs e)
{
    // your code here
}