How to check in listbox how many items are selected?

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C#

Visual-Studio

ListBox

I have a ListBox display all data from the database. Can do a multiple selections as well but only one selected item possible to edit in a same time.

I tried the following code but working only with the else branch but I want put my full code into the if branch.

What I missed? How to revers?

What I have tried:

if (listBox.SelectedItems.Count != 1)
{
Do modificate selected item.
}
else
{
MessageBox.Show("Only one item can be edit!", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}