Automating Excel and Add-ins Don’t load

There is a common problem I see when automating Excel from an external .NET application:

You will launch Excel, open a workbook, perform work, and then try to close Excel and find that it remains in memory. You might even have code in your solution to try to re-use existing instance of Excel and find that it uses one of these “zombie” instances that do not have any of the add-ins loaded… and maybe an add-in your code relies on.

The issue is that Excel is not able to close because the primary thread of your application is holding on to a COM reference. Even adding Marshal.ComReleaseObject() and GC.Collect() like this probably will not improve the situation either:

	Marshal.ReleaseComObject(excelWorkbook);
	Marshal.ReleaseComObject(excelApp);
	excelApp = null;
	excelWorkbook = null;
	GC.Collect();
	GC.Collect();
	GC.WaitForPendingFinalizers();
	GC.WaitForFullGCComplete();

view raw

COMRelease.cs

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From my experience, and from what I have gathered is that until the thread is stopped, the COM reference will be locked. Not sure where the issue lies (EXCEL, COM, or .NET), but I have found a workaround to this. What you have to do is terminate your thread. But because you do not want to terminate your application, you need to create a NEW thread. So the following pattern is what I have found works:


	OpenFileDialog openFileDialog1 = new OpenFileDialog();
	openFileDialog1.Filter = "Excel Files\|.xls";
	DialogResult result = openFileDialog1.ShowDialog();
	if (result == DialogResult.OK)
	{
	new Thread(() =>
	{
	Excel.Application excelApp = null;
	Excel.Workbook excelWorkbook = null;

	this.Invoke(new Action(() =>
	{
	try
	{
	excelApp = (Microsoft.Office.Interop.Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
	}
	catch
	{
	excelApp = new Excel.Application();
	}

	string workbookPath = openFileDialog1.FileName;
	excelWorkbook = excelApp.Workbooks.Open(workbookPath);

	excelApp.Visible = true;

	}));

	Marshal.ReleaseComObject(excelWorkbook);
	Marshal.ReleaseComObject(excelApp);
	excelApp = null;
	excelWorkbook = null;
	openFileDialog1 = null;
	GC.Collect();
	GC.Collect();
	GC.WaitForPendingFinalizers();
	GC.WaitForFullGCComplete();
	}).Start();
	}