Excel open 3 times and wont close until i use the task manag

Question

Hi

 

Im pretty new to the C# language. Right now im writing a Programm to calculate different

cable dimensioning. Till now everything just worked fine, but now i realised that when i use the programm it opens me three times the excel process and will only close if i propperly close the programm. otherwise everytime i hit the calculate button it will add 3 more excel process to the allready 3. I'm using excel for all the constants  i need for my calculation. I just cant find out where the Problem is so i thought you might help me.

I know the style of my programming isnt very good even bad:-) but im no programmer and most of the stuff i have in my programm is from the internet, because im an newby and dont know or understand some of that.

So if you could help me finding the error i would be very thankfull. The comments and other stuff is written in german, because thats my nativ language i hope this wont be a problem. I guess the error is because im trying befor i open the excel file to catch all possible exceptions with if and else.

thank you