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StronglyTypedId makes creating strongly-typed IDs as easy as adding an attribute! No more accidentally passing arguments in the wrong order to methods - StronglyTypedId uses .NET 7+'s compile-time incremental source generators to generate the boilerplate required to use strongly-typed IDs.

Simply, install the required package add the [StronglyTypedId] attribute to a struct (in the StronglyTypedIds namespace):

using StronglyTypedIds;
 
[StronglyTypedId] // <- Add this attribute to auto-generate the rest of the type
public partial struct FooId { }

and the source generator magically generates the backing code when you save the file! Use Go to Definition to see the generated code:

<img src="https://raw.githubusercontent.com/andrewlock/StronglyTypedId/master/docs/strongly_typed_id.gif" alt="Generating a strongly-typed ID using the StronglyTypedId packages"/>

StronglyTypedId requires the .NET Core SDK v7.0.100 or greater.

Why do I need this library?

I have written a blog-post series on strongly-typed IDs that explains the issues and rational behind this library. For a detailed view, I suggest starting there, but I provide a brief introduction here.

This library is designed to tackle a specific instance of primitive obsession, whereby we use primitive objects (Guid/string/int/long etc) to represent the IDs of domain objects. The problem is that these IDs are all interchangeable - an order ID can be assigned to a product ID, despite the fact that is likely nonsensical from the domain point of view. See here for a more concrete example.

By using strongly-typed IDs, we give each ID its own Type which wraps the underlying primitive value. This ensures you can only use the ID where it makes sense: ProductIds can only be assigned to products, or you can only search for products using a ProductId, not an OrderId.

Unfortunately, taking this approach requires a lot of boilerplate and ceremony to make working with the IDs manageable. This library abstracts all that away from you, by generating the boilerplate at build-time by using a Roslyn-powered code generator.

Requirements

The StronglyTypedId NuGet package is a .NET Standard 2.0 package.

You must be using the .NET 7+ SDK (though you can compile for other target frameworks like .NET Core 2.1 and .NET Framework 4.8)

Installing

To use StronglyTypedIds, install the StronglyTypedId NuGet package into your csproj file, for example by running

dotnet add package StronglyTypedId --version 1.0.0-beta08

This adds a <PackageReference> to your project. You can additionally mark the package as PrivateAsets="all" and ExcludeAssets="runtime".

Setting PrivateAssets="all" means any projects referencing this one will not also get a reference to the StronglyTypedId package. Setting ExcludeAssets="runtime" ensures the StronglyTypedId.Attributes.dll file is not copied to your build output (it is not required at runtime).

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
  </PropertyGroup>
  
  <ItemGroup>
    <!-- Add the package -->
    <PackageReference Include="StronglyTypedId" Version="1.0.0-beta08" PrivateAssets="all" ExcludeAssets="runtime" />
    <!-- -->
  </ItemGroup>

</Project>

Usage

To create a strongly-typed ID, create a partial struct with the desired name, and decorate it with the [StronglyTypedId] attribute, in the StronglyTypedIds namespace:

using StronglyTypedIds;

[StronglyTypedId] // Add this attribute to auto-generate the rest of the type
public partial struct FooId { }

This generates the "default" strongly-typed ID using a Guid backing field. You can use your IDE's Go to Definition functionality on your ID to see the_exact code generated by the source generator. The ID implements the following interfaces automatically:

And it additionally includes two converters/serializers:

This provides basic integration for many use cases, but you may want to customize the IDs further, as you'll see shortly.

Using different types as a backing fields

The default strongly-typed ID uses a Guid backing field:

using StronglyTypedIds;

[StronglyTypedId]
public partial struct FooId { }

var id = new FooId(Guid.NewGuid());

You can choose a different type backing field, by passing a value of the Template enum in the constructor.

using StronglyTypedIds;

[StronglyTypedId(Template.Int)]
public partial struct FooId { }

var id = new FooId(123);

Currently supported built-in backing types are:

Changing the defaults globally

If you wish to change the template used by default for all the [StronglyTypedId]-decorated IDs in your project, you can use the assembly attribute [StronglyTypedIdDefaults] to set all of these. For example, the following changes the default backing-type for all IDs to int

// Set the defaults for the project
[assembly:StronglyTypedIdDefaults(Template.Int)]

[StronglyTypedId] // Uses the default 'int' template
public partial struct OrderId { }

[StronglyTypedId] // Uses the default 'int' template
public partial struct UserId { } 

[StronglyTypedId(Template.Guid)] // Overrides the default to use 'Guid' template
public partial struct HostId { }

Using custom templates

In addition to the built-in templates, you can provide your own templates for use with strongly typed IDs. To do this, do the following:

For example, you could create a template that provides an EF Core ValueConverter implementation called guid-efcore.typedid like this:

partial struct PLACEHOLDERID
{
    public class EfCoreValueConverter : global::Microsoft.EntityFrameworkCore.Storage.ValueConversion.ValueConverter<PLACEHOLDERID, global::System.Guid>
    {
        public EfCoreValueConverter() : this(null) { }
        public EfCoreValueConverter(global::Microsoft.EntityFrameworkCore.Storage.ValueConversion.ConverterMappingHints? mappingHints = null)
            : base(
                id => id.Value,
                value => new PLACEHOLDERID(value),
                mappingHints
            ) { }
    }
}

Note that the content of the guid-efcore.typedid file is valid C#. One easy way to author these templates is to create a .cs file containing the code you want for your ID, then rename your ID to PLACEHOLDERID, change the file extension from .cs to _.typedid, and then set the build action.

After creating a template in your project you can apply it to your IDs like this:

// Use the built-in Guid template and also the custom template
[StronglyTypedId(Template.Guid, "guid-efcore")] 
public partial struct GuidId {}

This shows another important feature: you can specify multiple templates to use when generating the ID.

Using multiple templates

When specifying the templates for an ID, you can specify

For example:

[StronglyTypedId] // Use the default templates
public partial struct MyDefaultId {}

[StronglyTypedId(Template.Guid)] // Use a built-in template only
public partial struct MyId1 {}

[StronglyTypedId("my-guid")] // Use a custom template only
public partial struct MyId2 {}

[StronglyTypedId("my-guid", "guid-efcore")] // Use multiple custom templates
public partial struct MyId2 {}

[StronglyTypedId(Template.Guid, "guid-efcore")] // Use a built-in template _and_ a custom template
public partial struct MyId3 {}

// Use a built-in template _and_ multiple custom template
[StronglyTypedId(Template.Guid, "guid-efcore", "guid-dapper")]
public partial struct MyId4 {}

Similarly, for the optional [StronglyTypedIdDefaults] assembly attribute, which defines the default templates to use when you use the raw [StronglyTypedId] attribute, you use a combination of built-in and/or custom templates:

//⚠ You can only use _one_ of these in your project, they're all shown here for comparison

[assembly:StronglyTypedIdDefaults(Template.Guid)] // Use a built-in template only

[assembly:StronglyTypedIdDefaults("my-guid")] // Use a custom template only

[assembly:StronglyTypedIdDefaults("my-guid", "guid-efcore")] // Use multiple custom templates

[assembly:StronglyTypedIdDefaults(Template.Guid, "guid-efcore")] // Use a built-in template _and_ a custom template

// Use a built-in template _and_ multiple custom template
[assembly:StronglyTypedIdDefaults(Template.Guid, "guid-efcore", "guid-dapper")]

[StronglyTypedId] // Uses whatever templates were specified!
public partial struct MyDefaultId {}

To simplify the creation of templates, the StronglyTypedId package includes a code-fix provider to generate a template.

Creating a custom template with the Roslyn CodeFix provider

As well as the source generator, the StronglyTypedId NuGet package includes a CodeFix provider that looks for cases where you have specified a custom template that the source generator cannot find. For example, in the following code,the "some-int" template does not yet exist:

[StronglyTypedId("some-int")] // does not exist
public partial struct MyStruct { }

In the IDE, you can see the generator has marked this as an error:

An error is shown when the template does not exist

The image above also shows that there's a CodeFix action available. Clicking the action reveals the possible fix: Add some-int.typedid template to the project, and shows a preview of the file that will be added:

Showing the CodeFix in action, suggesting you can add a project

Choosing this option will add the template to your project.

Unfortunately, due to limitations with the Roslyn APIs, it's not possible to add the new template with the required AdditionalFiles/C# analyzer additional file build action already set. Until you change the build-action, the error will remain on your [StronglyTypedId] attribute.

Right-click the newly-added template, choose Properties, and change the Build Action to either C# analyzer additional file (Visual Studio 2022) or AdditionalFiles (JetBrains Rider). The source generator will then detect your template and the error will disappear.

The CodeFix provider does a basic check against the name of the template you're trying to create. If it includes int, long, or string, the template it creates will be based on one of those backing types. Otherwise, the template is based on a Guid backing type.

Once the template is created, you're free to edit it as required.

"Community" templates package StronglyTypedId.Templates

The "template-based" design of StronglyTypedId is intended to make it easy to get started, while also giving you the flexibility to customise your IDs to your needs.

To make it easier to share templates with multiple people, and optional StronglyTypedId.Templates NuGet package is available that includes various converters and other backing types. To use these templates, add the StronglyTypedId.Templates package to your project:

dotnet add package StronglyTypedId.Templates --version 1.0.0-beta08

You will then be able to reference any of the templates it includes. This includes "complete" implementations, including multiple converters, for various backing types:

It also includes "standalone" EF Core, Dapper, and Newtonsoft JSON converter templates to enhance the Guid/int/long/string built-in templates. For example

For the full list of available templates, see GitHub.

You can also create your own templates package and distribute it on NuGet.

Embedding the attributes in your project

By default, the [StronglyTypedId] attributes referenced in your application are contained in an external dll. It is also possible to embed the attributes directly in your project, so they appear in the dll when your project is built. If you wish to do this, you must do two things:

  1. Define the MSBuild constant STRONGLY_TYPED_ID_EMBED_ATTRIBUTES. This ensures the attributes are embedded in your project
  2. Add compile to the list of excluded assets in your <PackageReference> element. This ensures the attributes in your project are referenced, instead of the StronglyTypedId.Attributes.dll library.

Your project file should look something like this:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <!--  Define the MSBuild constant    -->
    <DefineConstants>STRONGLY_TYPED_ID_EMBED_ATTRIBUTES</DefineConstants>
  </PropertyGroup>

  <!-- Add the package -->
  <PackageReference Include="StronglyTypedId" Version="1.0.0-beta08" 
                    PrivateAssets="all"
                    ExcludeAssets="compile;runtime" />
<!--                               ☝ Add compile to the list of excluded assets. -->

</Project>

Preserving usages of the [StronglyTypedId] attribute

The [StronglyTypedId] and [StronglyTypedIdDefaults] attributes are decorated with the [Conditional] attribute, so their usage will not appear in the build output of your project. If you use reflection at runtime on one of your IDs, you will not find [StronglyTypedId] in the list of custom attributes.

If you wish to preserve these attributes in the build output, you can define the STRONGLY_TYPED_ID_USAGES MSBuild variable. Note that this means your project will have a runtime-dependency on StronglyTypedId.Attributes.dll so you need to ensure this is included in your build output.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <!--  Define the MSBuild constant to preserve usages   -->
    <DefineConstants>STRONGLY_TYPED_ID_USAGES</DefineConstants>
  </PropertyGroup>

  <!-- Add the package -->
  <PackageReference Include="StronglyTypedId" Version="1.0.0-beta08" PrivateAssets="all" />
  <!--              ☝ You must not exclude the runtime assets in this case -->

</Project>

Error CS0436 and [InternalsVisibleTo]

In the latest version of StronglyTypedId, you should not experience error CS0436 by default.

In previous versions of the StronglyTypedId generator, the [StronglyTypedId] attributes were added to your compilation as internal attributes by default. If you added the source generator package to multiple projects, and used the [InternalsVisibleTo] attribute, you could experience errors when you build:

warning CS0436: The type 'StronglyTypedIdImplementations' in 'StronglyTypedIds\StronglyTypedIds.StronglyTypedIdGenerator\StronglyTypedIdImplementations.cs' conflicts with the imported type 'StronglyTypedIdImplementations' in 'MyProject, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

In the latest version of StronglyTypedId, the attributes are not embedded by default, so you should not experience this problem. If you see this error, compare your installation to the examples in the installation guide.