Friday, 23 March 2007

Easy Drag&Drop from DataGridView control

The DataGridView control does not implement any method for doing Drag&Drop of items or rows out of it, but it can easily be done with some custom code. This first example will hande dragging of entire rows out of the DataGridView:

private void dataGridView1_MouseMove(object sender, MouseEventArgs e)
{
if (e.Button == MouseButtons.Left)
{
// If the datagrid has any selected row, and the mouse moves outside the datagridView,
// start the drag.
if (this.dataGridView1.SelectedRows.Count > 0 &&
!this.dataGridView1.ClientRectangle.Contains(e.X, e.Y))
{
// Proceed with the drag and drop, passing in the list of selected rows. You can
// assign any DragDropEffect you´d like here.
dataGridView1.DoDragDrop(dataGridView1.SelectedRows, DragDropEffects.Move);
}
}
}

This method determines that a dragging operation is being performed if the cursor gets out of the client rectangle of the grid, with the left button pressed (and any row selected).

It handles correctly a multi-select situation but it´s behavior is more "natural" if multiSelect is set to false, because while the cursor is inside the DataGrid, a drag will make the grid to select multiple rows. As soon as the cursor gets out the control, a Drag&Drop operation is started.

This is a verey simple and efficient way of handling this, but there are other possibilities.

Tipically, a windows application decides that a dragging operation is being performed if the mouse coursor travels (with the left button pressed) a certain distance, not necessarily out of the DataGridView like in the previous example. This minimum distance is a system configuration variable that can be found in: SystemInformation.DragSize.

Doing so is a more standard way and it also allows to drag and drop items inside the DataGridView. An easy way to handle this, is to keep track of a Rectangle defined at mouse down, with the dimensions of that DragSize. Some sample code:

private void dataGridView1_MouseDown(object sender, MouseEventArgs e)
{
// Get the index of the item the mouse is below.
DataGridView.HitTestInfo hInfo = this.dataGridView1.HitTest(e.X, e.Y);

// Will just handle if hitted a cell. You can test whatever kind of intersection you like here
if(hInfo.Type == DataGridViewHitTestType.Cell)
{
// The DragSize indicates the size that the mouse must move to consider a drag operation
Size dragSize = SystemInformation.DragSize;

// Create a rectangle using the DragSize, with the mouse position being
// at the center of the rectangle.
mMinDistanceRectangle = new Rectangle(
new Point(e.X - (dragSize.Width / 2), e.Y - (dragSize.Height / 2)), dragSize);
}
else
{
// Reset the rectangle if the mouse is not over an item in the ListBox.
mMinDistanceRectangle = Rectangle.Empty;
}
}

So, in the MouseMove event, you will want to test if the mouse is outside of that rectangle to start a Drag&Drop. Something like this:

if (mMinDistanceRectangle != Rectangle.Empty &&
!mMinDistanceRectangle .Contains(e.X, e.Y))
{
dataGridView1.DoDragDrop(...whatever...);
}

Hope this helps.

Cheers !

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