C# 将datagridview数据导出到excel[excel文件应为只读]

我想用下面的代码将数据从datagrid导出到excel文件我正在将数据导出到excel，但我想要只读的excel文件，我用下面的代码无法获取该文件：

Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
Microsoft.Office.Interop.Excel._Worksheet worksheet = null;

app.Visible = true;

worksheet = workbook.ActiveSheet;
for (int i = 1; i < dg1.Columns.Count + 1; i++)
{
    worksheet.Cells[1, i] = dg1.Columns[i - 1].HeaderText;
}
for (int i = 0; i < dg1.Rows.Count - 1; i++)
{
    for (int j = 0; j < dg1.Columns.Count; j++)
    {
        if (dg1.Rows[i].Cells[j].Value != null)
        {
            worksheet.Cells[i + 2, j + 1] = dg1.Rows[i].Cells[j].Value.ToString();
        }
        else
        {
            worksheet.Cells[i + 2, j + 1] = "";
        }

    }
    worksheet.Columns.AutoFit(); 

Microsoft.Office.Interop.Excel.\u Application app=new Microsoft.Office.Interop.Excel.Application（）；
Microsoft.Office.Interop.Excel.\u工作簿=app.Workbooks.Add（Type.Missing）；
Microsoft.Office.Interop.Excel.\u工作表=null；
app.Visible=true；
工作表=工作簿.ActiveSheet；
对于（int i=1；i

Microsoft.Office.Interop.Excel.\u Application app=new Microsoft.Office.Interop.Excel.Application（）；
Microsoft.Office.Interop.Excel.\u工作簿=app.Workbooks.Add（缺少.Value）；
Microsoft.Office.Interop.Excel.\u工作表工作表=工作簿.ActiveSheet；
工作表=工作簿.ActiveSheet；
//app.Visible=true；
//从datagrid获取所有列
对于（int i=1；i

Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Missing.Value);
Microsoft.Office.Interop.Excel._Worksheet worksheet = workbook.ActiveSheet;
worksheet = workbook.ActiveSheet;

//app.Visible = true;

//getting all the columns from datagrid
for (int i = 1; i < dg1.Columns.Count + 1; i++)
{
    worksheet.Cells[1, i] = dg1.Columns[i - 1].HeaderText;
}
//getting row collection
for (int i = 0; i < dg1.Rows.Count - 1; i++)
{
    for (int j = 0; j < dg1.Columns.Count; j++)
        {
            if (dg1.Rows[i].Cells[j].Value != null)
            {
                worksheet.Cells[i + 2, j + 1] = dg1.Rows[i].Cells[j].Value.ToString();
            }
            else
            {
                worksheet.Cells[i + 2, j + 1] = "";
            }
            worksheet.Columns.AutoFit();                                        
        }
}

//to make the excel sheet readonly
((Excel.Worksheet)app.ActiveWorkbook.Sheets[1]).EnableSelection = Excel.XlEnableSelection.xlUnlockedCells;
((Excel.Worksheet)app.ActiveWorkbook.Sheets[1]).Protect(Password: protectionpasswprd, AllowFormattingCells: false);
app.ActiveWorkbook.SaveCopyAs("D:\\a.xls");
app.ActiveWorkbook.Saved = true;
app.Quit();