System.Runtime.InteropServices.COMException

This type of exception is thrown when an unrecognised HRESULT is returned from a COM method call
You do not have to catch a COMException specifically, it will be caught by the general System.Exception.

The CLR transforms well known HRESULTS to .NET Exceptions

Check the ErrorCode property of the exception to determine the HRESULT returned by the COM object

COM Exception Catch

try 
{ 
   Globals.ThisAddIn.Application.Workbooks.Open(@"C:\temp\nofile.txt"); 
} 
catch (System.Runtime.InteropServices.ComException ex) 
{ 
   System.Windows.Forms.MessageBox.Show(ex.Message); 
} 
catch (System.Runtime.InteropServices.ComException ex) 
{ 
   System.Windows.Forms.MessageBox.Show(ex.Message); 
}