AppActivate, which many of you may recognize from VB activates a window for an application which is already running, bringing the window of that app to the foreground.
In C#, there is no exact equivalent to this function, which leaves two alternatives (see link at the bottom for code examples):
Option 1 (recommended):
Use SetForeGroundWindow, described in my similar post (scroll down a bit for the code examples).
Add a reference to Microsoft.VisualBasic and call Interactive.AppActivate.
Although Option 2 is a little simpler/faster if you are not familiar with or ready yet to delve into Win32 API, it is not a recommended solution as there can be potential side effects of this approach.
For code examples of both options, see my similarly titled post below and scroll through for code references.
Start a Process in the Foreground in C# .Net Without AppActivate https://ronniediaz.com/2011/05/03/start-a-process-in-the-foreground-in-c-net-without-appactivate/