You need System.Diagnostics.Process.Start()
.
The simplest example:
Process.Start("notepad.exe", fileName);
More Generic Approach:
Process.Start(fileName);
The second approach is probably a better practice as this will cause the windows Shell to open up your file with it's associated editor. Additionally, if the file specified does not have an association, it'll use the Open With...
dialog from windows.
Note to those in the comments, thankyou for your input. My quick n' dirty answer was slightly off, i've updated the answer to reflect the correct way.
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