VS CodeのUnityのVSCode.cs

https://msdn.microsoft.com/ja-jp/library/hh264221.aspx

#if !UNITY_EDITOR_WIN
            // Make sure our reference framework is 4.0, still the base for Unity
            if (content.IndexOf("<TargetFrameworkVersion>v3.5</TargetFrameworkVersion>") != -1)
            {
                content = Regex.Replace(content, "<TargetFrameworkVersion>v3.5</TargetFrameworkVersion>", "<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>");
            }
#endif

Visual Studio Code で Unity コードを扱う環境を作る - しっぽを追いかけて