dotNET (OK, C#) gets union types

# .NET (OK, C#) finally gets union types🎉: Exploring the .NET 11 preview - Part 2 Source: [https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/) May 19, 2026~10 min read [Exploring the \.NET 11 preview \- Part 2](https://andrewlock.net/series/exploring-the-dotnet-11-preview/) This is the second post in the series:[Exploring the \.NET 11 preview](https://andrewlock.net/series/exploring-the-dotnet-11-preview/)\. 1. [Part 1 \- Running background tasks in Blazor with Web Workers](https://andrewlock.net/exploring-the-dotnet-11-preview-1-running-background-tasks-in-blazor-with-web-workers/) 2. Part 2 \- \.NET \(OK, C\#\) finally gets union types🎉 \(this post\) Unions are one of those features that have been requested for years, and in \.NET 11 \(or rather, C\# 15\) they're*finally*here\. In this post I describe what that support looks like, how you can use them, how they're implemented, and how you can implement your own custom types\. > This post was written using the features available in \.NET 11 preview 4\. Many things may change between now and the final release of \.NET 11\. ## [What are union types?](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#what-are-union-types-) Unions are one of those basic data structures which are used all the time in the functional programming world; they're available in F\#, TypeScript, Rust…pretty much any functional\-first language\. There are many different*types*of union, but at their core they allow having a type that can represent two different things\. Some of the simplest union types are the`Option<T\>`and`Result<TSuccess, TError\>`types\. There's no "standard" version of these, but it's*super*common to see custom implementations\.`Result<\>`is one of the easiest to explain as it can be in one of two states: - Success—in this case the`Result<\>`object contains a`TSuccess`value representing the "success" result for an operation that succeeded\. - Error—in this case the`Result<\>`object contains a`TError`value representing the "error" for an operation that failed\. You return a`Result<\>`object from your method, and then the caller has to*explicitly*handle both cases instead of assuming success\. > This pattern is often called the result pattern and it has both pros and cons in C\#\. I wrote a series about using this pattern,[as well as considering whether it's worth it here](https://andrewlock.net/series/working-with-the-result-pattern/)\. Union types don't have to be the super generic form like this though\. They can be used to represent any arbitrary combined set of types\. ## [Union types in C\# 15 with the`union`keyword](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#union-types-in-c-15-with-the-union-keyword) In the previous section I used the classic`Result<\>`type as an example of a union, but unions are far more versatile than that\. They're ideal whenever you want to deal with data that could be one of several potentially unrelated types\. For example, imagine we have three different`record`types, containing different properties, representing Operating Systems: ``` public record Windows(string Version); public record Linux(string Distro, string Version); public record MacOS(string Name, int Version); ``` Note that these types*don't*have any values in common\. Prior to C\# 15, the main options for handling something which could be a`Windows`*or*`Linux`*or*`MaxOS`object would be: - Try to create a base class from which all the types derive\. That*might*work, but what if you don't control these types because they come from a library? - Store the type in an`object`instance\. This works, but you lose all the safety of working with types in this case\. - Use some "tag" value for keeping track of which type your object contains, e\.g\. using an`enum`to track this\. In C\# 15, we get direct support for this scenario with the`union`keyword, as shown below: ``` // 👇 Use `union` as the type public union SupportedOS(Windows, Linux, MacOS); // 👆 List the types that are part of the union ``` You can create an instance of the`SupportedOS`type in a couple of ways: ``` // You can call new and pass in an instance SupportedOS os = new SupportedOS(new MacOS("Tahoe", 25)); // Or you can use implict conversion (which calls new() behind the scenes) SupportedOS os = new MacOS("Tahoe", 25); ``` The generated`union`type implements the`IUnion`interface: ``` public interface IUnion { object? Value { get; } } ``` so you can always get the "inner" case value back out as an`object?`if you need to: ``` // You can access the stored "inner" object using `.Value` Console.WriteLine(os.Value); // MacOS { Name = Tahoe, Version = 25 } ``` However, the canonical way to work with unions is to use a`switch`expression: ``` string GetDescription(SupportedOS os) => os switch { Windows windows => $"Windows {windows.Version}", Linux linux => $"{linux.Distro} {linux.Version}", MacOS macOS => $"MacOS {macOS.Name} ({macOS.Version})", }; // note: no discard _ required ``` The`switch`expression automatically extracts the inner case type, and a very neat thing is that you*don't*need to include the`\_ =\>`"discard" case either: the compiler enforces that you check for each of the allowed values, but you*only*need to check these values\. And if you forget one, you'll get a warning: ``` warning CS8509: The switch expression does not handle all possible values of its input type (it is not exhaustive). For example, the pattern 'MacOS' is not covered. ``` > Note that if one of your case types is nullable, e\.g\.`MacOS?`then you'll need to handle`null`in your`switch`expressions too\. To come full circle, we could perhaps implement the`Result<\>`type as the following \(just an example, there's lots of different implementations we could choose\!\) ``` public union Result<T>(T, Exception); ``` or to show another classic, the`Option<T\>`type: ``` public record class None; public union Option<T>(None, T); ``` That's the basics of the`union`types in C\# 15, so next we'll look at how you can use them today, before we look behind the scenes at how they're implemented\. ## [Using`union`types in \.NET 11](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#using-union-types-in-net-11) To use`union`types you need to do two things: - Install \.NET 11 preview 2\+ SDK\. The initial`union`support was added in preview 2, but you'll have a smoother experience if you install preview 4\+\. - Enable preview language support in your \.csproj files, by adding`<LangVersion\>preview</LangVersion\>` ``` <Project Sdk="Microsoft.NET.Sdk"> <PropertyGroup> <OutputType>Exe</OutputType> <!-- 👇 Add this --> <LangVersion>preview</LangVersion> <TargetFrameworks>net11.0;net8.0;net48</TargetFrameworks> <ImplicitUsings>enable</ImplicitUsings> <Nullable>enable</Nullable> </PropertyGroup> </Project> ``` Note that although you need to use the \.NET 11 SDK, you*can*target earlier versions of the runtime, such as I'm doing in the above*\.csproj*file\. The`union`support is implemented as a compiler feature, so it's available on earlier runtimes \([even if it's not*technically*supported on them](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-versioning)\)\. However, if you're targeting earlier runtimes \(or you're using \.NET 11 preview 2 or preview 3\), then you'll*also*need to add some helper types to your project: ``` #if !NET11_0_OR_GREATER namespace System.Runtime.CompilerServices; [AttributeUsage(Class | Struct, AllowMultiple = false, Inherited = false)] public sealed class UnionAttribute : Attribute; public interface IUnion { object? Value { get; } } ``` [These were added](https://github.com/dotnet/runtime/pull/127001)to \.NET 11 in preview 4, so they'll be available automatically if you're using a newer SDK, but you'll need to include them if you're targeting earlier runtimes, regardless\. As you might have guessed, when the compiler creates the`union`types, it uses this attribute and implements this interface\. In the next section we'll take a look at what the generated code looks like, to understand how the`union`types are implemented\. In terms of IDE support, if you're using either Visual Studio Preview, or VS Code's C\# DevKit Insiders, then you should have initial support\.[Support for JetBrains Rider is still pending](https://youtrack.jetbrains.com/projects/RIDER/issues/RIDER-135866/ETA-for-net11-preview-1-support)\. ## [How are`union`types implemented](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#how-are-union-types-implemented) You can see the full spec for`union`types[here](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unions), but the standard generated code is really pretty simple: ``` using System.Runtime.CompilerServices; [Union] public struct SupportedOS : IUnion { public object? Value { get; } // Constructors for each case type public SupportedOS(Windows value) => this.Value = (object) value; public SupportedOS(Linux value) => this.Value = (object) value; public SupportedOS(MacOS value) => this.Value = (object) value; } ``` As you can see, the generated`SupportedOS`type: - Is a`struct`, decorated with the`\[Union\]`attribute\. - Has a single, readonly,`object? Value`property, implementing the`IUnion`interface\. - Has a constructor for each of the case types it supports\. I was somewhat surprised to find there was no implicit conversion from the case types to the`SupportedOS`type, given that we can write code like this: ``` SupportedOS os = new MacOS("Tahoe", 25); ``` However it looks like the compiler simply rewrites this to use the`\[Union\]`constructor: ``` // SupportedOS os = new MacOS("Tahoe", 25); // The compiler emits code that looks like this: SupportedOS os = new SupportedOS(new MacOS("Tahoe", 25)); ``` This implicit conversion is all driven by the`\[Union\]`attribute\. You can see this in action if we rewrite our example to*not*use the`union`keyword, and instead use the implementation code shown previously but we "forget" to include the`\[Union\]`attribute: ``` using System.Runtime.CompilerServices; SupportedOS os = new MacOS("Tahoe", 25); // Cannot implicitly convert type 'MacOS' to 'SupportedOS' var description = os switch { Windows windows => $"Windows {windows.Version}", // An expression of type 'SupportedOS' cannot be handled by a pattern of type 'Windows' Linux linux => $"{linux.Distro} {linux.Version}", // An expression of type 'SupportedOS' cannot be handled by a pattern of type 'Linux' MacOS macOS => $"MacOS {macOS.Name} ({macOS.Version})", // An expression of type 'SupportedOS' cannot be handled by a pattern of type 'MacOS' }; public record Windows(string Version); public record Linux(string Distro, string Version); public record MacOS(string Name, int Version); // 👇 This attribute is required to be a valid Union type, // just removed here for demo purposes // [Union] public struct SupportedOS : IUnion { public object? Value { get; } public SupportedOS(Windows value) => this.Value = (object) value; public SupportedOS(Linux value) => this.Value = (object) value; public SupportedOS(MacOS value) => this.Value = (object) value; } ``` The code above fails to compile with the following, demonstrating how the`\[Union\]`attribute drives the implicit conversions and`switch`expressions: ``` error CS0029: Cannot implicitly convert type 'MacOS' to 'SupportedOS' error CS8121: An expression of type 'SupportedOS' cannot be handled by a pattern of type 'Windows'. error CS8121: An expression of type 'SupportedOS' cannot be handled by a pattern of type 'Linux'. error CS8121: An expression of type 'SupportedOS' cannot be handled by a pattern of type 'MacOS'. ``` If you re\-instate the`\[Union\]`attribute, everything compiles and runs just fine, which shows how you can create your own*custom*union types\. ## [Avoiding boxing with custom Union implementations](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#avoiding-boxing-with-custom-union-implementations) Given we're*just*getting support for`union`types, why might you want to create*custom*`Union`types? One reason is that you might*already*be using custom union types, such as provided by[OneOf](https://www.nuget.org/packages/OneOf), or[Sasa](https://www.nuget.org/packages/Sasa)\(two packages I've used in the past\)\. In these cases, the libraries could benefit from built\-in language support \(e\.g\.`switch`expression support\) by simply implementing the`IUnion`interface and adding the`\[Union\]`attribute\. Another case is when the "store the case type in an`object`instance" just isn't good enough for you\. The generated union type is*always*a`struct`with a single`object`field\. That means that if you're creating a`union`of multiple`struct`types, those types are going to be boxed onto the heap\. For example, imagine you need this`union`, which can represent either an`int`or a`bool`: ``` public union IntOrBool(int, bool); ``` The problem is that the`int`or`bool`passed into the constructor of`IntOrBool`is immediately boxed to an`object`and stored in the`Value`property: ``` [Union] public struct IntOrBool : IUnion { public object? Value { get; } // The struct arguments are always boxed, allocating on the heap public IntOrBool(int value) => this.Value = (object) value; public IntOrBool(bool value) => this.Value = (object) value; } ``` This allocates on the heap, which is generally undesirable, as`union`types are intended to be largely transparent performance\-wise\. Any`switch`expressions using this implementation will similarly use the`Value`property\. For example, with the basic built\-in`union`implementation, the following expression: ``` IntOrBool intOrBool; var description = intOrBool switch { int i => "integer", bool b => "bool", }; ``` would lower to code similar to this: ``` IntOrBool unmatchedValue = new IntOrBool(23); object obj = unmatchedValue.Value; // 👈 Access the boxed value string str; if (obj is int _) { str = "integer"; } else if (obj is bool _) { str = "bool"; } else { ThrowSwitchExpressionException((object) unmatchedValue); // can't happen, but handled anyway } ``` In many cases, the boxing allocation won't really matter, but in other places, such as in hot paths, the boxing is undesirable\. To account for this, the`union`feature allows for a "non\-boxing" implementation, using a`TryGetValue`pattern\. This requires that you implement: - `bool HasValue \{ get; \}`which returns`true`if the stored value is non\-`null` - `bool TryGetValue\(out T value\)`for each case type,`T` For example, the following is a[potential implementation](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unions#examples-of-union-types)of the`IntOrBool`type above that avoids boxing ``` [Union] public struct IntOrBool : IUnion { private readonly bool _isBool; private readonly int _value; public IntOrBool(int value) { _isBool = false; _value = value; } public IntOrBool(bool value) { _isBool = true; _value = value ? 1 : 0; } public bool HasValue => true; // the values are never null public bool TryGetValue(out int value) // get the int value without boxing { value = _value; return !_isBool; } public bool TryGetValue(out bool value) // get the bool value without boxing { value = _isBool && _value is 1; return _isBool; } // 👇 Have to implement this to satisfy IUnion, // and it still boxes, but it won't be used by default. public object Value => _isBool ? _value is 1 : _value; } ``` When you implement the`TryGetValue\(\)`methods, the compiler automatically uses them in`switch`expressions instead of the`Value`property, so the switch expression above becomes the following: ``` IntOrBool unmatchedValue = new IntOrBool(23); string str; // 👇 Calls TryGetValue instead of using the boxing Value property if (unmatchedValue.TryGetValue(out int _)) { str = "integer"; } else if (unmatchedValue.TryGetValue(out bool _)) { str = "bool"; } else { ThrowSwitchExpressionException((object) unmatchedValue); // can't happen, but handled anyway } ``` Depending on your code paths and use\-cases, it may or may not be worth creating custom non\-boxing implementations like this, it depends on what you're using the`union`types for in your code base\. ## [What other features are yet to come?](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#what-other-features-are-yet-to-come-) The`union`implementation is usable as currently shipped, but there's even more to[the language proposal](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unions)than I've covered\. Here are some of the related features that are yet to come: - [Union member providers](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/unions#union-member-providers)\. These provide a way to define the members that are part of the union type on a*different*type to the union itself\. - [Closed enums](https://github.com/dotnet/csharplang/blob/main/proposals/closed-enums.md)\. These are`enum`s in which you*don't*need to include a "catch\-all" expression \(`\_ =\>`\) in the`switch`expression for the`enum`\. - [Closed hierarchies](https://github.com/dotnet/csharplang/blob/main/proposals/closed-hierarchies.md)\. This allows adding the`closed`modifier on a`class`to prevent derived classes from being declared*outside*the defining assembly, which then similarly allows exhaustive`switch`expressions without a catch\-all expression\. These features may or may not make it into \.NET 11, but I'll be sure to cover them if they do\! ## [Summary](https://andrewlock.net/exploring-the-dotnet-11-preview-2-dotnet-gets-union-types/#summary) In this post I described the support for union types introduced in \.NET 11 preview 2\. I described the steps you need to implement them, as well as how to deconstruct union types using`switch`expressions\. I showed the`union`declaration syntax, how they're implemented behind the scenes, as well as how to implement a non\-boxing version of a union type\. Finally I discussed some of the plans and roadmap for union types and for exhaustiveness improvements in C\# that are yet to be released\. [PreviousRunning background tasks in Blazor with Web Workers: Exploring the \.NET 11 preview \- Part 1](https://andrewlock.net/exploring-the-dotnet-11-preview-1-running-background-tasks-in-blazor-with-web-workers/) Andrew Lock \| \.Net Escapades Want an email when there's new posts?