Excel crashes

A simple code to export data to excel
<pre>

public partial class Form1 : Form
{

Microsoft.Office.Interop.Excel.Application _objAppln;
Workbook _objWorkBook;
Workbooks _objWorkBooks;
Worksheet _objWorkSheet;


public Form1()
{
InitializeComponent();
InitializeExcelObjectModel();
}

~Form1()
{
DisposeExcelObjects();
}

void SaveExcel()
{

_objAppln.DisplayAlerts = false;//Since, we are using SaveFileDialog's overwrite prompt(control is on view).

_objWorkBook.SaveAs("C:\\tmp.xls",
XlFileFormat.xlWorkbookNormal, Type.Missing, Type.Missing, false, Type.Missing, XlSaveAsAccessMode.xlNoChange,
Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
_objWorkBook.Close(true, "C:\\tmp.xls", false);
_objAppln.DisplayAlerts = true;//restore back for other display alerts
}

private void InitializeExcelObjectModel()
{

_objAppln = new Microsoft.Office.Interop.Excel.Application(); // To initialize excel file
//_objAppln.Visible = true;
if (_objAppln != null)
{
_objWorkBooks = _objAppln.Workbooks;
_objWorkBook = _objWorkBooks.Add(Type.Missing); // To add workbook with sheets in excel file
_objWorkSheet = (Worksheet)_objAppln.ActiveSheet; // To get the current active sheet in excel file

}
}

public void DisposeExcelObjects()
{

System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkSheet);
System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkBook);
System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkBooks);
System.Runtime.InteropServices.Marshal.ReleaseComObject(_objAppln);
_objWorkSheet = null; _objWorkBooks = null; _objWorkBooks = null; _objAppln = null;
}

private void button1_Click(object sender, EventArgs e)
{

Range objRange=null;
string cell1 = string.Empty, cell2 = string.Empty;
string[] chars = { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T" };


for (int row = 1; row &lt; 1000; row++)
{
for (int column = 0; column &lt; 20; column++)
{
cell1 = chars[column] + row.ToString();
objRange = _objWorkSheet.get_Range(cell1, cell1);
objRange.Value2 = cell1;


}
}
System.Runtime.InteropServices.Marshal.ReleaseComObject(objRange);
objRange = null;
SaveExcel();
}
}
</pre>

Step 1: Add the above code to a solution and add reference for Microsoft.Office.Interop.Excel.dll
Step 2: Run the application and Execute the code in button1_Click.
Step 3: While the processing is on, open any other excel file and just click on cells here and there.
COM exception occurs. Anything I am doing wrong above?

How to resolve?

Try using the following code, it looks like the interop/automation server is busy when you're trying to hit it because the dll's it needs are already in use.

I made a wrapper function that basically takes any other function (in this case, your unsafe COM calls) as a delegate, and returns an object that you can then cast back to the type you need.

The wrapper function checks to see if it gets a COM exception, and if it does, waits a slice, and tries again; you'll probably want to limit that to certain errors, or a certain number of retries, and have it throw the real error after 50 tries or something like that.

Code:

---

using System;
using System.Windows.Forms;
using Microsoft.Office.Interop.Excel;
using System.Runtime.InteropServices;

namespace ExcelExport
{
    public partial class Form1 : Form
    {
        Microsoft.Office.Interop.Excel.Application _objAppln;
        Workbook _objWorkBook;
        Workbooks _objWorkBooks;
        Worksheet _objWorkSheet;
        public Form1()
        {
            InitializeComponent();
            InitializeExcelObjectModel();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Range objRange = null;
            string cell1 = string.Empty, cell2 = string.Empty;
            string[] chars = { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T" };

            for (int row = 1; row < 1000; row++)
            {
                for (int column = 0; column < 20; column++)
                {
                    //simple method call
                    cell1 = chars[column] + row.ToString();
                    objRange = (Range) COMExecute(() => _objWorkSheet.get_Range(cell1, cell1));
                    objRange.Value2 = cell1;
                }
            }
            System.Runtime.InteropServices.Marshal.ReleaseComObject(objRange);
            objRange = null;
            SaveExcel();

        }
        ~Form1()
        {
            DisposeExcelObjects();
        }

        void SaveExcel()
        {
            _objAppln.DisplayAlerts = false;//Since, we are using SaveFileDialog's overwrite prompt(control is on view).
            _objWorkBook.SaveAs("C:\\tmp.xls",
            XlFileFormat.xlWorkbookNormal, Type.Missing, Type.Missing, false, Type.Missing, XlSaveAsAccessMode.xlNoChange,
            Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            _objWorkBook.Close(true, "C:\\tmp.xls", false);
            _objAppln.DisplayAlerts = true;//restore back for other display alerts
        }

        private void InitializeExcelObjectModel()
        {
            _objAppln = new Microsoft.Office.Interop.Excel.Application(); // To initialize excel file
            //_objAppln.Visible = true;
            if (_objAppln == null) return;
            _objWorkBooks = _objWorkBooks = _objAppln.Workbooks;
            _objWorkBook = _objWorkBook = _objWorkBooks.Add(Type.Missing); // To add workbook with sheets in excel file
            _objWorkSheet = (Worksheet) _objAppln.ActiveSheet; // To get the current active sheet in excel file
        }

        public void DisposeExcelObjects()
        {
            System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkSheet);
            System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkBook);
            System.Runtime.InteropServices.Marshal.ReleaseComObject(_objWorkBooks);
            System.Runtime.InteropServices.Marshal.ReleaseComObject(_objAppln);
            _objWorkSheet = null; _objWorkBooks = null; _objWorkBooks = null; _objAppln = null;
        }

        delegate object COMWrapper();
        private object COMExecute(COMWrapper executeCommand)
        {
            bool executeFailed = true;
            while (executeFailed)
            {
                try
                {
                    return executeCommand();
                }
                catch (COMException ex)
                {
                    System.Threading.Thread.Sleep(50);
                    if (executeFailed) continue;
                }
            }
            return false;
        }
    }
}

---

Hope that helps, it fixed the errors on my box at least.