How can I download an Excel file to the user's download folder?
<p>In a C#, asp.net 4.0 project, I've created an Excel file using Microsoft.Office.Interop.Excel.  The file gets created correctly and is placed in a folder on the server.  However, I've been unable to come up with a way to display the file to the client.  Can someone help me?</p>

<p>Some background: I'm trying to work around the Microsoft change that is blocking Excel files from opening.  Our web application uses a Telerik grid and that grid is exported to an Excel file using the ExportToExcel function.  The file downloads (goes to the user's downloads file), but when the user tries to open it, they get a blank Excel application.  There are work arounds such as uninstalling the patch that created this problem, turning off Excel security options, and clicking unblock in the file's properties; however, our client doesn't want to do any of these.  So I'm rewriting exports for 40+ grids.</p>

<p>I'm got the data from the radGrid to a datatable and written it to Excel using this code which works:</p>

<pre><code>            Microsoft.Office.Interop.Excel.Application m_objExcel = null;
            Microsoft.Office.Interop.Excel.Workbooks m_objBooks = null;
            Microsoft.Office.Interop.Excel._Workbook m_objBook = null;
            Microsoft.Office.Interop.Excel.Sheets m_objSheets = null;
            Microsoft.Office.Interop.Excel._Worksheet m_objSheet = null;
            object m_objOpt = System.Reflection.Missing.Value;

            m_objExcel = new Microsoft.Office.Interop.Excel.Application();
            m_objBooks = (Microsoft.Office.Interop.Excel.Workbooks)m_objExcel.Workbooks;
            m_objBook = (Microsoft.Office.Interop.Excel._Workbook)(m_objBooks.Add(m_objOpt));

            m_objSheets = (Microsoft.Office.Interop.Excel.Sheets)m_objBook.Worksheets;
            m_objSheet = (Microsoft.Office.Interop.Excel._Worksheet)(m_objSheets.get_Item(1));

            int colcount = 1;
            foreach (DataColumn col in dt.Columns)
            {
                m_objSheet.Cells[1, colcount] = col.ColumnName;
                colcount++;
            }

            int rowcount = 2;
            foreach (DataRow row in dt.Rows)
            {
                for (int i = 1; i &lt; dt.Columns.Count; i++)
                {
                    m_objSheet.Cells[rowcount, i] = row[i - 1].ToString();
                }
                rowcount++;
            }

            string currentDateTime = DateTime.Now.ToString("yyyyMMddHHmmss");

            m_objBook.SaveAs("C:\\Temp\\MD" + currentDateTime + ".xlsx");
            m_objBook.Close();

            m_objExcel.DisplayAlerts = false;
            m_objExcel.Quit();
</code></pre>

<p>My problem comes when I try to get the user to download the file.  I tried the code below, but I get an error saying "The process cannot access the file 'C:\Temp\MD20160802161458.xlsx' because it is being used by another process."  Can anyone explain how to either unblock the file after it's created by Excel or show me another way to download the file to the user?</p>

<pre><code> string fileName = "C:\\Temp\\MD" + currentDateTime + ".xlsx", myStringWebResource = null;
 WebClient myWebClient = new WebClient();
 myStringWebResource = fileName;
 myWebClient.DownloadFile(myStringWebResource, fileName);
</code></pre>

<p>I also tried the code below to open the Excel file. The file is created on the server, but it never opens.  When I try to open the file on the server, Excel crashes.  I suspect this is because I have Excel 2013 on my development machine and Excel 2007 is on the server.  This raises another issue because I can't guarantee what version of Excel would be on eventual production server.  Any suggestions would be appreciated.</p>

<pre><code>            var excelApp = new Microsoft.Office.Interop.Excel.Application();
            excelApp.Visible = true;
            excelApp.Workbooks.Open("C:\\Temp\\MD" + currentDateTime + ".xlsx");
            m_objExcel.Quit();
</code></pre>