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.

	[StronglyTypedId(generateJsonConverter:false)]
	public partial struct ProductID { };
	public class Product {
	public ProductID ProductID { get; set; }
	}

view raw ProductID.cs hosted with ❤ by GitHub
• Build your project. Let’s have a look at what is generated:

	[System.ComponentModel.TypeConverter(typeof(ProductIDTypeConverter))]
	readonly partial struct ProductID : System.IComparable<ProductID>, System.IEquatable<ProductID>
	{
	public System.Guid Value
	{
	get;
	}
	public ProductID(System.Guid value)
	{
	Value = value;
	}
	public static ProductID New() => new ProductID(System.Guid.NewGuid());
	public static readonly ProductID Empty = new ProductID(System.Guid.Empty);
	public bool Equals(ProductID other) => this.Value.Equals(other.Value);
	public int CompareTo(ProductID other) => Value.CompareTo(other.Value);
	public override bool Equals(object obj)
	{
	if (ReferenceEquals(null, obj))
	return false;
	return obj is ProductID other && Equals(other);
	}
	public override int GetHashCode() => Value.GetHashCode();
	public override string ToString() => Value.ToString();
	public static bool operator ==(ProductID a, ProductID b) => a.CompareTo(b) == 0;
	public static bool operator !=(ProductID a, ProductID b) => !(a == b);
	class ProductIDTypeConverter : System.ComponentModel.TypeConverter
	{
	public override bool CanConvertFrom(System.ComponentModel.ITypeDescriptorContext context, System.Type sourceType)
	{
	return sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
	}
	public override object ConvertFrom(System.ComponentModel.ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
	{
	var stringValue = value as string;
	if (!string.IsNullOrEmpty(stringValue) && System.Guid.TryParse(stringValue, out var guid))
	{
	return new ProductID(guid);
	}
	return base.ConvertFrom(context, culture, value);
	}
	}
	}

view raw ProductID.generated.cs hosted with ❤ by GitHub
• By default a Guid is used as the backing field. If you want to use a different type, you can specify this:

	[StronglyTypedId(generateJsonConverter:false, backingType:StronglyTypedIdBackingType.Int)]
	public partial struct ProductID { };

view raw BackingType.cs hosted with ❤ by GitHub