C# Drag and Drop From listBox

问题

I'm trying to build a simple interface that allows users to drop files into a listBox to add them to a process, and to drag them out to remove them. Everything is working fine, but I'd like to add one feature to make it just a tad more sophisticated.

Right now, I have the removal of the item tied to the DragLeave event, which means that as soon as the mouse leaves the box, the item is removed. But I'd like for users to be able to change their minds. In other words, if they realize they're dragging the wrong file out, I'd like them to be able to move the mouse back into the listBox and release the mouse to cancel the action. I'm thinking that means I need to be able to capture the MouseUp event instead of the DragLeave event. But that hasn't been successful so far.

Below is the code I'm currently using for removing files dragged out. How can I modify to keep the files from being removed form the list until the user lets the mouse button go?

private void listBox1_MouseDown(object sender, MouseEventArgs e)
{
    if (listBox1.Items.Count == 0)
    { 
        return; 
    }

    int index = listBox1.IndexFromPoint(e.X, e.Y);
    string s = listBox1.Items[index].ToString();
    DragDropEffects dde1 = DoDragDrop(s, DragDropEffects.All);
}

private void listBox1_DragLeave(object sender, EventArgs e)
{
    ListBox lb = sender as ListBox;
    lb.Items.Remove(lb.SelectedItem);
}

Edit 2013/05/16

The comments and answers so far have been useful, but I realize my question isn't clear enough. In this case, I'm displaying a dialog separate from the parent form that is basically as big as the listBox. When someone drags a file out of the list, they're dragging it off the form completely. Have I backed myself into a corner by doing this? I recognize I'm making it harder than it has to be, but I'd still like to see how it would work if it's possible.

回答1:

Here's a fairly quick hack approach to gaining the functionality you want:

public object lb_item = null;



private void listBox1_DragLeave(object sender, EventArgs e)
{
    ListBox lb = sender as ListBox;

    lb_item = lb.SelectedItem;
    lb.Items.Remove(lb.SelectedItem);
}

private void listBox1_DragEnter(object sender, DragEventArgs e)
{       
    if (lb_item != null)
    {
        listBox1.Items.Add(lb_item);
        lb_item = null;
    }
}


private void listBox1_MouseDown(object sender, MouseEventArgs e)
{
    lb_item = null;

    if (listBox1.Items.Count == 0)
    {
        return;
    }                

    int index = listBox1.IndexFromPoint(e.X, e.Y);
    string s = listBox1.Items[index].ToString();
    DragDropEffects dde1 = DoDragDrop(s, DragDropEffects.All);      
}

private void Form1_DragDrop(object sender, DragEventArgs e)
{            
    lb_item = null;
}

Every time the user drags an item out of the box, it's saved temporarily until the user drops it somewhere else or mouses down on a new item in the list. Note the important part of this is detecting when and where the user let's go of that mouse, which is the rationale behind handling the DragDrop event of Form1, the parent of listBox1.

Depending on the sophistication and density of the rest of your layout, where you handle DragDrop could be much different for you. This is why it's kind of "hacky", but it's also quite simple. It shouldn't matter, though, where or how many times you null lb_item since it pertains only to that specific ListBox.

I suppose another way to do it would be to track the user's mouse states and act accordingly, which may be more appropriate for you if it's inconceivable to handle a lot of DragDrop stuff.

EDIT: If you wanted to be REAL thorough, you could enumerate through every control of the base form using foreach and programmatically append a handler for the DragDrop event to that control, then remove it when done... but that may be getting a little nutty. I'm sure someone has a better approach.

来源：https://stackoverflow.com/questions/16591975/c-sharp-drag-and-drop-from-listbox