从列表中删除项目后如何防止SelectedIndexChanged事件在ListBox中触发

Question

I am working on selecting an item which will cause deletion of selected item. The problem is that when I delete an item I am selecting additional item which cause another deletion...

How do I unselest/deselect after deleting selected item?

This causes my problem:

    void lbMessage_SelectedIndexChanged(object sender, EventArgs e)
    {
        DialogResult result = new DialogResult();

        result = MessageBox.Show("Are you sure you want to remove this item?", "Removal Confirmation", MessageBoxButtons.YesNo, MessageBoxIcon.Information);

        if (result == DialogResult.Yes)
        {
            lbMessage.Items.Remove(lbMessage.SelectedItem); 
            lbMessage.SelectedIndex = -1;
        }
        else
        {
        }
    }

    private void btnAddMessage_Click(object sender, EventArgs e)
    {
        lbMessage.Items.Add(txtMessage.Text);
        txtMessage.Text = string.Empty;
    }

Answer 1

Try removing the SelectedIndexChanged event before removing the item, then add it back in:

private void lbMessage_SelectedIndexChanged(object sender, EventArgs e) {
  if (MessageBox.Show("Are you sure you want to remove this item?", 
                      "Remove Confirmation", 
                      MessageBoxButtons.YesNo,
                      MessageBoxIcon.Question) == DialogResult.Yes) {
    lbMessage.SelectedIndexChanged -= lbMessage_SelectedIndexChanged;
    lbMessage.Items.Remove(lbMessage.SelectedItem);
    lbMessage.SelectedIndexChanged += lbMessage_SelectedIndexChanged;
  }
}

Answer 2

Setting SelectedIndex = -1 is raising the SelectedIndexChanged event. Check if there is nothing selected at the start of the event.

void lbMessage_SelectedIndexChanged(object sender, EventArgs e) 
{
    if (lbMessage.SelectedIndex == -1) return;
    ...
}

Answer 3

If you like it that way without a delete button then just do this:

bool isAfterDelete = false;
void lbMessage_SelectedIndexChanged(object sender, EventArgs e)
{
    if (isAfterDelete)
    {
        isAfterDelete = false;
        return;
    }
    DialogResult result = new DialogResult();

    result = MessageBox.Show("Are you sure you want to remove this item?", 
                             "Removal Confirmation", 
                             MessageBoxButtons.YesNo, 
                             MessageBoxIcon.Information);

    if (result == DialogResult.Yes)
    {
        lbMessage.Items.Remove(lbMessage.SelectedItem); 
        isAfterDelete = true;
        lbMessage.SelectedIndex = -1;
    }
    else
    {
    }
}

Answer 4

What if you just either wrap your code an in if() that checks for lbMessage.SelectedIndex == -1, or write a check for it at the beginning and return if it's true:

void lbMessage_SelectedIndexChanged(object sender, EventArgs e)
{
  if(lbMessage.SelectedIndex == -1)
    return;

    DialogResult result = new DialogResult();

    result = MessageBox.Show("Are you sure you want to remove this item?", "Removal Confirmation", MessageBoxButtons.YesNo, MessageBoxIcon.Information);

    if (result == DialogResult.Yes)
    {
        lbMessage.Items.Remove(lbMessage.SelectedItem); 
        lbMessage.SelectedIndex = -1;
    }
}

Answer 5

The shear act of removing the item from the Items collection causes the items to change their index. This thus calls the "SelectedIndexChanged" event again on the list box. To prevent this, you would need to set a flag before the remove function to indicate that the SelectedIndexChange is being called after the remove and prevent the event from occuring. You can do that by the following.

    private bool afterRemove = false;
    void lbMessage_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (afterRemove)
        {
            afterRemove = false;
            return;
        }

        DialogResult result = new DialogResult();

        result = MessageBox.Show("Are you sure you want to remove this item?", "Removal Confirmation", MessageBoxButtons.YesNo, MessageBoxIcon.Information);

        if (result == DialogResult.Yes)
        {
            afterRemove = true;
            lbMessage.Items.Remove(lbMessage.SelectedItem); 
            // Another call to "lbMessage_SelectedIndexChanged" is made right here.
       }
    }

    private void btnAddMessage_Click(object sender, EventArgs e)
    {
        lbMessage.Items.Add(txtMessage.Text);
        txtMessage.Text = string.Empty;
    }

An alternative that might be easier to understand could be the following.

    private bool afterRemove = false;
    void lbMessage_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (afterRemove)
            return;

        DialogResult result = new DialogResult();

        result = MessageBox.Show("Are you sure you want to remove this item?", "Removal Confirmation", MessageBoxButtons.YesNo, MessageBoxIcon.Information);

        if (result == DialogResult.Yes)
        {
            afterRemove = true;
            lbMessage.Items.Remove(lbMessage.SelectedItem); 
            // Another call to "lbMessage_SelectedIndexChanged" is made right here.
            afterRemove = false;
       }
    }