The name `String' does not exist in the current context

The error resulted from the following c# code:

void Update () {
		print( String.Format("mouse position: %f %f %f", 

I thought String class would always be available since it is included in the System namespace? I am learning C# right now and I know Java and Javascript already.

The System namespace isn’t imported automatically; use System.String.Format. You can import System, but it can create conflicts (such as System.Random vs. UnityEngine.Random), so unless you’re using a lot of stuff in System, it’s often better just to specify System manually.

(By the way, “%f” is not anything that works in .NET; you might want to look up .NET string formatting examples.)

Try using string.Format if that’s not working.