What's new in C# 7.1

C# 7.1 is the first point release to the C# language. It marks an increased release cadence for the language. You can use the new features sooner, ideally when each new feature is ready. C# 7.1 adds the ability to configure the compiler to match a specified version of the language. That enables you to separate the decision to upgrade tools from the decision to upgrade language versions.

C# 7.1 adds the language version selection configuration element, three new language features and new compiler behavior.

The new language features in this release are:

• async Main method
  • The entry point for an application can have the async modifier.
• default literal expressions
  • You can use default literal expressions in default value expressions when the target type can be inferred.
• Inferred tuple element names
  • The names of tuple elements can be inferred from tuple initialization in many cases.

Finally, the compiler has two options /refout and /refonly that control reference assembly generation.

Language version selection

The C# compiler supports C# 7.1 starting with Visual Studio 2017 version 15.3, or the .NET Core SDK 2.0. However, the 7.1 features are turned off by default. To enable the 7.1 features, you need to change the language version setting for your project.

In Visual Studio, right-click on the project node in Solution Explorer and select Properties. Select the Build tab and select the Advanced button. In the dropdown, select C# latest minor version (latest), or the specific version C# 7.1 as shown in the image following. The latest value means you want to use the latest minor version on the current machine. The C# 7.1 means that you want to use C# 7.1, even after newer minor versions are released.

Alternatively, you can edit the "csproj" file and add or modify the following lines:

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

<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
  <LangVersion>latest</LangVersion>
</PropertyGroup>

<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
  <LangVersion>latest</LangVersion>
</PropertyGroup>

Valid settings for the LangVersion element are:

• ISO-1
• ISO-2
• 3
• 4
• 5
• 6
• 7
• 7.1
• default
• latest

The special strings default and latest resolve to the latest major and minor language versions installed on the build machine, respectively.

This setting decouples installing new versions of the SDK and tools in your development environment from choosing to incorporate new language features in a project. You can install the latest SDK and tools on your build machine. Each project can be configured to use a specific version of the language for its build.

Async main

An async main method enables you to use await in your Main method. Previously you would need to write:

static int Main()
{
    return DoAsyncWork().GetAwaiter().GetResult();
}

You can now write:

static async Task<int> Main()
{
    // This could also be replaced with the body
    // DoAsyncWork, including its await expressions:
    return await DoAsyncWork();
}

If your program doesn't return an exit code, you can declare a Main method that returns a Task:

static async Task Main()
{
    await SomeAsyncMethod();
}

You can read more about the details in the async main topic in the programming guide.

Default literal expressions

Default literal expressions are an enhancement to default value expressions. These expressions initialize a variable to the default value. Where you previously would write:

Func<string, bool> whereClause = default(Func<string, bool>);

You can now omit the type on the right-hand side of the initialization:

Func<string, bool> whereClause = default;

You can learn more about this enhancement in the C# Programming Guide topic on default value expressions.

This enhancement also changes some of the parsing rules for the default keyword.

Inferred tuple element names

This feature is a small enhancement to the tuples feature introduced in C# 7.0. Many times when you initialize a tuple, the variables used for the right side of the assignment are the same as the names you'd like for the tuple elements:

int count = 5;
string label = "Colors used in the map";
var pair = (count: count, label: label);

The names of tuple elements can be inferred from the variables used to initialize the tuple in C# 7.1:

int count = 5;
string label = "Colors used in the map";
var pair = (count, label); // element names are "count" and "label"

You can learn more about this feature in the Tuples topic.

Reference assembly generation

There are two new compiler options that generate reference-only assemblies: /refout and /refonly. The linked topics explain these options and reference assemblies in more detail.