Exposing a custom type as a JSON string in an ASP.NET Core API

Sometimes your API needs to expose a non-primitive type that has a “natural” string representation. For instance, a standard representation for a duration is the ISO 8601 format, where “1 month, 2 days, 3 hours and 4 minutes” can be represented as P1M2DT3H4M (note that this isn’t the same as a Timespan, which has no notion of calendar months and years). A duration could be represented in C# as a custom type, like the Duration structure in my Iso8601DurationHelper project.