在excel中编写单元格的函数

我从这里得到一点代码:http://forum.codecall.net/topic/71788-reading-excel-files-in-c/它工作得很好，很容易理解。它只能读取excel单元格，我也想写一些。所以我想在main中这样做:excel_setValue("C10"， "520");

这是我尝试的函数:

    private static Microsoft.Office.Interop.Excel.ApplicationClass appExcel;
    private static Workbook newWorkbook = null;
    private static _Worksheet objsheet = null;

static string excel_setValue(string cellname, string value)  
        {
            if (objsheet.get_Range(cellname).get_Value().ToString() == string.Empty) // Here is error
            {
                Console.WriteLine("watch out u trying to overwrite files");
            }
            else
            {
                objsheet.get_Range(cellname).set_Value(value);
            }         
        }

它告诉我:NullReferenceExpection未处理-对象引用未设置为对象的实例。

另一个错误，当我只把这个:

static void excel_setValue(string cellname, string value)
        {
            //if (objsheet.get_Range(cellname).get_Value().ToString() == string.Empty)
            //{
            //    Console.WriteLine("kijk uit je probeerd cellen te overschrijven");
            //}
            //else
            //{
                objsheet.get_Range(cellname).set_Value(value);
            //}         
        }

错误:COMException was unhandled: Exception from HRESULT: 0x800A03EC

如何调用excel_setValue:

excel_init("C:''");
excel_setValue("B10","520");

excel_init:

static void excel_init(String path)
        {
            appExcel = new Microsoft.Office.Interop.Excel.ApplicationClass();
            if (System.IO.File.Exists(path))
            {
                // then go and load this into excel
                newWorkbook = appExcel.Workbooks.Open(path, true, true);
                objsheet = (_Worksheet)appExcel.ActiveWorkbook.ActiveSheet;
            }
            else
            {
                Console.WriteLine("Unable to open file!");
                System.Runtime.InteropServices.Marshal.ReleaseComObject(appExcel);
                appExcel = null;
            }
        }

objsheet = (_Worksheet)appExcel.ActiveWorkbook.ActiveSheet;

How I call my excel_setValue:
excel_setValue("B10","520");
excel_init("C:''");