idc_Enter

private void idc_Enter(object sender, EventArgs e)
{
    InputDigitControl idc = (InputDigitControl)sender;
    string fldID = idc.Name.Substring(3, 1);

    switch (fldID)
    {
        case "C":
            {
                enC = true;
                enK = false;
                enF = false;
            }; break;
        case "K":
            {
                enC = false;
                enK = true;
                enF = false;
            }; break;
        case "F":
            {
                enC = false;
                enK = false;
                enF = true;
            }; break;
    }
}