Tukukkk

I have a ListBox which has an ObservableCollection<string> as its DataSource. Now I want to be able to move every selected item up or down. So if the list looks like this (selected items are prefixed with *):

  Item 1

* Item 2

  Item 3

* Item 4

* Item 5

* Item 6

  Item 7

I want it to look like this after moving down once:

  Item 1

  Item 3

* Item 2

  Item 7

* Item 4

* Item 5

* Item 6

or this after moving up once:

* Item 2

  Item 1

* Item 4

* Item 5

* Item 6

  Item 3

  Item 7

I already stumbled across ObservableCollection<T>.Move(int oldIndex, int newIndex) but I only can get it to work with moving a single item.

What would be a good algorithm?

It would be easier (and in some cases more intuitive to the user) if all selected lines drop with no gaps between them, but it should be possible to implement either way. You can absolutely use the Move() method of ObservableCollection, to move multiple you will have to gather multiple startindexes (for each selected item) into a list and then iterate through that list using the Move() method as well as making some calculations based on what index position the cursor is on and the sorted order of your selected items list to determine what the 'newindex' is for each item.

Edit: also keep in mind the domino effect of changing indexes that you will have to accomodate for in your calculations.

As already mention in my comment. You could use a forward and backward loop to accomplish that. Here is a sample program that does the job. I made to buttons to distinguish the UP and DOWN movement:

ObservableCollection<string> source = new ObservableCollection<string>();

private void Form1_Load(object sender, EventArgs e)

{

    for (int i = 1; i < 10; i++)

    {

        source.Add("Item " + i);

    }



    listBox1.DataSource = source;

}



private void buttonMoveUp_Click(object sender, EventArgs e)

{

    foreach (int index in listBox1.SelectedIndices)

    {

        if (index > 0) // don't move the first element upwards

        {

            source.Move(index, index - 1);

        }

    }



    listBox1.DataSource = null;

    listBox1.DataSource = source;

}



private void buttonMoveDown_Click(object sender, EventArgs e)

{

    for (int i = listBox1.SelectedIndices.Count - 1; i >= 0; i--)

    {

        int index = listBox1.SelectedIndices[i];

        if (index < source.Count-1) // don't move the last element downwards

        {

            source.Move(index, index + 1);

        }

    }            



    listBox1.DataSource = null;

    listBox1.DataSource = source;

}