How do I disable C# 6 Support in Visual Studio 2015?

You can set the language feature for each project separately by going to Properties => Build tab => Advanced button => Language Version and set your preferred version.

You should realize that it will still use the new "C# 6.0" .Net Compiler Platform (codenamed Roslyn). However, that compiler will imitate the behavior of older compilers and will limit you to features only available on that specific language version.

I don't think that there's a solution-wide setting available.

add below in .sln.DotSettings should disable it on solution level

<s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">CSharp50</s:String>

Or if you don't have a .sln.DotSettings file:

1. If your solution file is called Apple.sln, create a file beside it called Apple.sln.DotSettings.

2. Give it the following contents:

   <wpf:ResourceDictionary xml:space="preserve" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib" xmlns:ss="urn:shemas-jetbrains-com:settings-storage-xaml" xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
       <s:String x:Key="/Default/CodeInspection/CSharpLanguageProject/LanguageLevel/@EntryValue">CSharp50</s:String>
   </wpf:ResourceDictionary>
   

3. Close and reopen the solution, Resharper should only warn you about C#5 things.

4. Don't forget to remove this when you eventually start using C#6 features! :)

This tool I wrote might help you if you have many projects that you need to set LangVersion for.