The interfaces package

Install the package

The Meziantou.Framework.StronglyTypedId.Interfaces package contains two interfaces that the generated IDs implement. Add it to your project:

dotnet package add Meziantou.Framework.StronglyTypedId.Interfaces

When the project references it, every ID implements:

The interfaces are in the Meziantou.Framework namespace. You do not change the declaration of the IDs: the generator adds the interfaces when it finds them.

Use an ID without knowing its type

[StronglyTypedId(typeof(int))]
public partial struct UserId
{
}

// The generated type implements IStronglyTypedId and IStronglyTypedId<int>
var userId = UserId.FromInt32(42);

// Access via interface
IStronglyTypedId stronglyTypedId = userId;
Assert.Equal("42", stronglyTypedId.ValueAsString);
Assert.Equal(typeof(int), stronglyTypedId.UnderlyingType);

// Access via generic interface
Assert.Equal(42, GetValue(userId));

static T GetValue<T>(IStronglyTypedId<T> id) => id.Value;

• IStronglyTypedId gives the value as a string and its type, for code that handles every ID the same way: logging, auditing, building cache keys or URLs.
• IStronglyTypedId<T> gives the typed value, for generic code that needs it: IStronglyTypedId<int> matches every ID that wraps an int. T is the underlying type, not the type of the ID.
• UnderlyingType is implemented explicitly: it is only visible through the interface.

Accessing an ID of a struct type through an interface boxes it. In generic code, use a constraint (where TId : IStronglyTypedId<int>) to avoid the allocation.