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DDD–Strongly typed Ids

One of the core principles of DDD is the usage of value objects to avoid “primitive obsession”. "Primitives" refer to the built-in types in C#,  int, string,guid etc. "Primitive obsession" refers to over-using these types to represent domain concepts that aren't a perfect fit. Some examples are a HouseNumber that is represented by an int or an EmailAddress that is represented by a string.

This concept not only makes sense for your value objects but is also valuable for your Entity and AggregateRoot id’s. A ProductId should not be interchangeable with an OrderId.

Creating a valueobject for every Id type is not that hard but remains cumbersome. Let’s introduce StronglyTypedId as a solution.

From the website:

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 Roslyn-powered build-time code generation to generate the boilerplate required to use strongly-typed IDs.

Getting started

  • To get started, first add the StronglyTypedId nuget package to your project: https://www.nuget.org/packages/StronglyTypedId/
  • Now create a struct type and add the StronglyTypedId attribute on it.
    • Notice that we specify to not generate a JsonConverter. If you want to serialize the type you need to add an extra reference to Newtonsoft.Json or System.Text.Json.
  • Build your project. Let’s have a look at what is generated:
  • By default a Guid is used as the backing field. If you want to use a different type, you can specify this:
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