running on CheckedListBox and check what in the file
AA
BB
CC
DD

private void CheckEveryOther_Click(object sender, System.EventArgs e) {
    // Cycle through every item and check every other.

    // Set flag to true to know when this code is being executed. Used in the ItemCheck
    // event handler.
    insideCheckEveryOther = true;

    for (int i = 0; i < checkedListBox1.Items.Count; i++) {

          // here you need to compare with that string....
                checkedListBox1.SetItemCheckState(i, CheckState.Indeterminate);
            else
                checkedListBox1.SetItemChecked(i, true);
        }
    }

    insideCheckEveryOther = false;
}

string myStr = "CC";
        for (int it = 0; it < checkedListBox1.Items.Count; it++)
        {
            checkedListBox1.SetItemChecked(
               it, myStr.Contains(checkedListBox1.Items[it].ToString()));
        }