How to change Visual Studio 2017 default language version for all projects?

Question

How to set language version on C#7.0 for all projects?

Answer 1

The meaning of default value comes from the C# compiler itself. So in order to change it's meaning you need to change the compiler.

But in compiler that comes with Visual Studio 2017 default actually means C# 7.0, so you don't need to do anything.

Visual Studio Project System merely passes language version value to MSBuild. And MSBuild passes it further to C# compiler as a /langversion option. /langversion option lets you specify an upper language version that compiler accepts. In another words it allows you to restrict language features usage to a certain version. If you use feature from language version higher than you specified, C# compiler will emit error. That's all. If you specify /langversion as default, C# compiler will accept all valid syntax that last major C# language version include (see /langversion (C# Compiler Options) page on MSDN). Last major C# version that comes with Visual Studio 2017 is 7.0. See Features Added in C# Language Versions page on C# GitHub repository.

If you need to enable features of latest minor versions (7.1, 7.2 etc.) or forbid usage of some new C# features for multiple projects or solutions at once you can use MSBuild 15 Directory.Build.props customization file. Relevant excerpt from Customize your build article on MSDN:

...now you can add a new property to every project in one step by defining it in a single file called Directory.Build.props at the root of your repo. When MSBuild runs, Microsoft.Common.props searches your directory structure for the Directory.Build.props file (and Microsoft.Common.targets looks for Directory.Build.targets). If it finds one, it imports the property. Directory.Build.props is a user-defined file that provides customizations to projects under a directory.

Following example of a Directory.Build.props file instructs C# compiler to accept all valid syntax of a latest minor C# version (C# 7.2 in Visual Studio 2017 version 15.5.3) in all projects given their .csproj file doesn't include <LangVersion> tag which takes precedence:

<Project>
    <PropertyGroup>
        <LangVersion>latest</LangVersion>
    </PropertyGroup>
</Project>

For more information check:

• What does the langversion switch do? article by Eric Lippert
• Project System source code
• Common MSBuild project properties page on MSDN, CscToolPath property in particular