The name 'File' does not exist in the current context

For anyone using Visual Studio 2015 this error will still occur even if System.IO is referenced. The problem is that by default a Visual Studio 2015 project will target both the dnx451 and dnxcore50 frameworks, and the System.IO assembly doesn't seem to be available for dnxcore50.

If you look in the project.json file you'll see a "frameworks" section. The quick fix is to comment out the "dnxcore50" entry so that you're only targeting dnx451:

"dnxcore50": {
    "dependencies": {
        "System.Console": "4.0.0-beta-22816",
        "System.Collections": "4.0.10-beta-22816",
        "System.Linq": "4.0.0-beta-22816",
        "System.Threading": "4.0.10-beta-22816",
        "Microsoft.CSharp": "4.0.0-beta-22816"
    }
}

Just add this using statement to the top of you file

using System.IO;

The compiler only recognises classes from namespaces that you have in the current context. You add namespace to the context using using statements. You can also use a fully qualified type name such as System.IO.File to refer to the class.

If you are using Visual Studio you can also place the cursor on the problematic symbol (File) and press Shift + Alt + F10