I love that this job allows me to learn new stuff every day. In this case, I was building a simple API to use for some upcomingPluralsight courses. I wanted to use Minimal APIs to expose some data for an old dataset fromFiveThirtyEight on Bechdel Tests for Films. While I was adding paging, I got confused.

So, I started with a minimal API for getting all the films like so:

app.MapGet("api/films", async (BechdelDataService ds) =>{  if (ds is null) return Results.BadRequest();  FilmResult data = await ds.LoadAllFilmsAsync();  if (data.Results is null) return Results.NotFound();  return Results.Ok(data);}).WithTags("By Film").Produces(200).ProducesProblem(404);

Pretty simple Minimal API to get some data (my BechdelDataService is just a repository-like class). When I added paging, I wanted to use the typical Web API trick of using optional parameters:

app.MapGet("api/films", async (BechdelDataService ds,   int page = 1,   int pageSize = 50) =>{...}).WithTags("By Film").Produces(200).ProducesProblem(404);

To my surprise this didn’t work. What I expected was that if I used a query string, it would bind to the extra parameters by name:

/api/films?page=1/api/films

In this case, these should be the same. What is going on. At first I dove into seeing why Minimal APIs didn’t support this. I didn’t see much about it, I saw options to read the query strings manually:

app.MapGet("api/films", async (    HttpRequest request,     BechdelDataService ds) =>{  var page = request.Query["page"] ?? 1;  var size = request.Query["pageSize"] ?? 50;...}).WithTags("By Film").Produces(200).ProducesProblem(404);

This works, but I found it clunky. I wanted binding to work. I saw some examples that broke out the lambda to a method:

app.MapGet("api/films", GetAllFilms);async Task<IResult> GetAllFilms(BechdelDataService ds,   int page = 1,   int pageSize = 50){    ...}

This works, so the problem wasn’t with Minimal APIs, the problem is that Lambdas do not support default values (since you’re passing in a lambda, the spec assumes that you’ll supply all values).

So this is a fix? I don’t like the messiness of breaking it out into separate methods. So how do we fix it? Let’s use nullable values:

app.MapGet("api/films", async (BechdelDataService ds,   int? page,   int? pageSize) =>{  if (ds is null) return Results.BadRequest();  int pageNumber = page ?? 1;  int pagerTake = pageSize ?? 50;  FilmResult data = await ds.LoadAllFilmsAsync(pageNumber, pagerTake);  if (data.Results is null) return Results.NotFound();  return Results.Ok(data);}).WithTags("By Film").Produces(200).ProducesProblem(404);

By supplying the values as nullable types (int?), we have to do our own defaulting, but I think it’s acceptable. One note here, is that I’m defining the types to non-nullable types to make my calls to the data service easier, but this isn’t necessary.

So, was I the only one who didn’t know that default values weren’t supported in lambda’s?

This work byShawn Wildermuth is licensed under aCreative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Based on a work atwildermuth.com.

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