Drag and Drop is not working in C#

Question

I figured out how to drag rows between datagridviews from how to drag gridview row from one grid to another, but now I have a problem. I can drag the row from gridPODetails to DataGridView1. I can drag the row back to gridPODetails from DataGridView1. But after that I get nothing. I would expect to be able to drag back and forth indefinately, but I can only go there and back. What could be causing this and how to correct?

 private void gridPODetails_MouseDown(object sender, MouseEventArgs e)
        {
            DataGridView.HitTestInfo info = gridPODetails.HitTest(e.X, e.Y);

            if (info.RowIndex >= 0)
            {
                DataRow view = ((DataTable)(gridPODetails.DataSource)).Rows[info.RowIndex];
                if (view != null)
                {
                    gridPODetails.DoDragDrop(view, DragDropEffects.Copy);
                }
            }
        }

        private void gridPODetails_DragEnter(object sender, DragEventArgs e)
        {
            e.Effect = DragDropEffects.Copy;
        }

        private void gridPODetails_DragDrop(object sender, DragEventArgs 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();
            }
        }

        private void dataGridView1_MouseDown(object sender, MouseEventArgs e)
        {
            DataGridView.HitTestInfo info = dataGridView1.HitTest(e.X, e.Y);

            if (info.RowIndex >= 0)
            {
                DataRow view = ((DataTable)(dataGridView1.DataSource)).Rows[info.RowIndex];
                if (view != null)
                {
                    dataGridView1.DoDragDrop(view, DragDropEffects.Copy);
                }
            }
        }

        private void dataGridView1_DragEnter(object sender, DragEventArgs e)
        {
            e.Effect = DragDropEffects.Copy;
        }

        private void dataGridView1_DragDrop(object sender, DragEventArgs 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();
            }
        }

Just a guess, but perhaps table.AcceptChanges might help things — Conrad Frix
– Conrad Frix, Commented Aug 30, 2011 at 18:00
@Conrad Frix -Make that an answer, that did the trick! Now I just have to figure out how I can reuse that code (I'm going to end up with 5-8 gridviews I want to be able to drag between). — MAW74656
– MAW74656, Commented Aug 30, 2011 at 18:09
ok I put in as an answer and included my thinking behind it. If I'm wrong enough someone will correct me, or provide a more complete answer — Conrad Frix
– Conrad Frix, Commented Aug 30, 2011 at 21:05
@Shyamsundarshah Please stop adding the visual studio tag to these questions. It is not valid. — Andrew Barber
– Andrew Barber, Commented Jun 30, 2013 at 11:46

Conrad Frix · Accepted Answer · 2011-08-30 21:04:32Z

2

Adding table.AcceptChanges() after row.Delete() should allow you to move the row back forth between tables.

The reason for this may be because importing a row that previously was deleted can cause issues.

answered Aug 30, 2011 at 21:04

Conrad Frix

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Comments

Enigmativity · Accepted Answer · 2011-08-31 05:01:06Z

In response to MAW74656's comment under the question I've put together the method I would use if I were going to wire-up multiple grids to do the drag-and-drop.

In essence I try to create a lambda to group all of this functionality inside a single method - it could be done as it's own method if need be though to allow multiple callers.

Here it is:

Func<DataGridView, IEnumerable<Action>> configureDragDrop = grid =>
{
    var dataTable = grid.DataSource as DataTable;

    /* Event handler definitions here - see below */

    grid.MouseDown += mds;
    grid.DragEnter += des;
    grid.DragDrop += dds;

    return new Action[]
    {
        () => grid.MouseDown -= mds,
        () => grid.DragEnter -= des,
        () => grid.DragDrop -= dds,
    };
};

This code allows me to write this:

        // form-level field
        private List<Action> removeHandlers = new List<Action>();

        // in the method where `configureDragDrop` is defined
        removeHandlers.AddRange(configureDragDrop(gridPODetails));
        removeHandlers.AddRange(configureDragDrop(dataGridView1));
        removeHandlers.AddRange(configureDragDrop(dataGridView2));
        removeHandlers.AddRange(configureDragDrop(dataGridView3));
        removeHandlers.AddRange(configureDragDrop(dataGridView4));
        removeHandlers.AddRange(configureDragDrop(dataGridView5));
        removeHandlers.AddRange(configureDragDrop(dataGridView6));
        removeHandlers.AddRange(configureDragDrop(dataGridView7));
        // etc

When I'm closing down my form I can then remove all the handlers in one line:

        Array.ForEach(removeHandlers.ToArray(), rh => rh.Invoke());

The event handlers look very similar to the original code - just now in lambda form.

MouseDown:

            MouseEventHandler mds = (smd, emd) =>
            {
                var info = grid.HitTest(emd.X, emd.Y);
                if (info.RowIndex >= 0)
                {
                    var dr = dataTable.Rows[info.RowIndex];
                    if (dr != null)
                    {
                        grid.DoDragDrop(dr, DragDropEffects.Copy);
                    }
                }
            };

DragEnter:

            DragEventHandler des = (sde, ede) =>
            {
                ede.Effect = DragDropEffects.Copy;
            };

DragDrop:

            DragEventHandler dds = (sdd, edd) =>
            {
                var dr = edd.Data.GetData(typeof(DataRow)) as DataRow;
                if (dr != null)
                {
                    var dst = dataTable;
                    var src = dr.Table;
                    if (dst != src)
                    {
                        dst.ImportRow(dr);
                        dr.Delete();
                        src.AcceptChanges();
                        dst.AcceptChanges();
                    }
                }
            };

I hope this helps.

@Enigmativity -I'm eager to reuse code here, but I"m new to lambda expressions, any good intro article?
@MAW74656 - To start learning lambdas just think of them as "inline methods". Writing Func<string, int> GetLength = t => t.Length; is the same as private int GetLength(string t) { return t.Length; } when calling the code - int len = GetLength("Hello");. Start from there. Also, any delegate (Func, Action, EventHandler<>, etc) can use the lambda syntax. Otherwise, search StackOverflow as there are tonnes of examples here.

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