Определение функции схемы

вы можете попробовать это.

try
        {
            checkedListBox1.SelectedIndexChanged -= checkedListBox1_SelectedIndexChanged;
            if (checkedListBox1.Items.Count != 0)
            {
                if (checkedListBox1.SelectedItem.ToString() == "Select all")
                {
                    for (int i = 0; i < checkedListBox1.Items.Count; i++)
                    {
                        checkedListBox1.SetItemChecked(i, true);
                    }
                    string changed = "Deselect all";
                    checkedListBox1.Items[checkedListBox1.SelectedIndex] = changed;
                }
                else if (checkedListBox1.SelectedItem.ToString() == "Deselect all")
                {
                    for (int i = 0; i < checkedListBox1.Items.Count; i++)
                    {
                        checkedListBox1.SetItemChecked(i, false);
                    }
                    string changed = "Select all";
                    checkedListBox1.Items[checkedListBox1.SelectedIndex] = changed;
                }
            }
        }
        catch (Exception ex)
        {
        }
        finally
        {
            checkedListBox1.SelectedIndexChanged += checkedListBox1_SelectedIndexChanged;
        }