Still more effective solution can be done.
First you may need to iterate through all the processes running in the memory to check whether at all if excel is running.
If it is not running then u may create the new instance of the excel application and open your file as usual.
if it is already running, then get the application handle to the excel application already open and check whether ur file to be opened is already opened or not. if yes then display "File Already Open" message, else open the file.
go for some googling at google.com for help regarding iterating through the running processes, getting the window handles of the applications and attaching them to objects, etc.
good luck.
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