c# 엑셀 병합 범위 측정

           Excel.Application  application = new Excel.Application();
            Excel.Workbook readd_wb = application.Workbooks.Open(Filename: @"C:\Users\L\Desktop\test.xlsx");
            Excel.Worksheet ws = readd_wb.Worksheets.Item["Sheet1"];
            Excel.Range rg1 = ws.Range["B5"];
            string c = rg1.MergeArea.get_Address(Excel.XlReferenceStyle.xlA1);

string c = "$B$5:$C$7"