C# 11 Preview Features: Multiline Interpolated Strings, List Patterns, and Parameter Null Checking

The .NET chief project manager Catherine posted a blog outlining several preview features of C# 11 that can be tried in Visual Studio 17.1 and .NET SDK 6.0.200. The article extracts three notable additions.

C# 11 Preview: Multiline Interpolated String Expressions

Interpolated strings, introduced in C# 6.0, allow embedding expressions inside a string using "$"" (non‑verbatim) or "$@"" (verbatim). Previously, non‑verbatim interpolated strings could not contain line breaks, requiring escape sequences like \r or \n. In C# 11, line breaks are permitted inside the expression part of an interpolated string, removing this limitation. The following code, which previously caused a compilation error, now compiles:

var v = $"Count is\t: { this.Is.A.Really()
                            .That.I.Should(
                                be + able)[
                                    to.Wrap()] }.";

Reference: GitHub Issue #4935.

C# 11 Preview: List Patterns

List patterns enable matching arrays or lists against a sequence of patterns, e.g., array is [1, 2, 3] matches an integer array of length 3 with those exact elements. Slice patterns ( ..) allow discarding or capturing zero or more elements. Examples:

int[] arr1 = { 1, 2, 10 };
int[] arr2 = { 1, 2, 5, 10 };
int[] arr3 = { 1, 2, 5, 6, 7, 8, 9, 10 };

A switch expression using list patterns:

public static int CheckSwitch(int[] values) => values switch
{
    [1, 2, .., 10] => 1,
    [1, 2] => 2,
    [1, _] => 3,
    [1, ..] => 4,
    [..] => 50,
};

Calling the switch with different arrays yields the corresponding results (1, 2, 3, 4, or 50). Slice patterns can also be captured:

public static string CaptureSlice(int[] values) => values switch
{
    [1, .. var middle, _] => $"Middle {String.Join(", ", middle)}",
    [.. var all] => $"All {String.Join(", ", all)}",
};

List patterns apply to any type with a Length or Count property and an indexer taking int or System.Index. Slice patterns require a type supporting Range or an indexer with two int parameters. Support for IEnumerable is under consideration.

C# 11 Preview: New Parameter Null‑Checking

C# 11 adds a concise syntax to validate that a method parameter is not null and automatically throw an ArgumentNullException. By appending !! to a parameter name, the compiler injects the null‑check before the method body (or before base/this constructor calls).

public static void M(string s!!)
{
    // Body of the method
}

The equivalent pre‑C# 11 code required an explicit if (s is null) throw new ArgumentNullException(nameof(s)); block.

How to Try the Preview Features

Create a C# project and set LangVersion to preview in the .csproj file:

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <OutputType>Exe</OutputType>
        <TargetFramework>net6.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
        <LangVersion>preview</LangVersion>
    </PropertyGroup>
</Project>

For more details, refer to the original blog or the discussions in the CSharpLang GitHub repository.