Casting Results<T1, ... Tn> to another Results<T1, ... Tn>

[kieronlanning]

I've a helper method for performing list operations. All of the list APIs take a ListRequest record (via AsParameters), or something that inherits from it - all taking a DBContext, Validator, PageNum and PageSize.

This works for 90% of the list endpoints I have:

groups.MapGet("/", ListAsync)
    .WithName("ListGroups")
    .WithDescription("List all groups");
;

static async Task<Results<Ok<PagedResponse<GroupModel>>, NoContent, ValidationProblem>> ListAsync([AsParameters] ListRequest request, CancellationToken cancellationToken)
{
	var result = await Http.ListAsync(
		request,
		() => request.Context.Groups
			.Where(m => m.UserProfileId == request.User.UserId)
			.OrderBy(m => m.DisplayOrder)
			.Skip((request.PageNum - 1) * request.PageSize)
			.Take(request.PageSize)
			.ToArrayAsync(cancellationToken),
		() => request.Context.Groups
			.CountAsync(m => m.UserProfileId == request.User.UserId, cancellationToken),
		entity => entity.ToModel(),
		cancellationToken);

	return result;
}

However... if I need to introduce another IResult type, like NotFound for example there is no casting I can use to make this work:

static async Task<Results<Ok<PagedResponse<TaskModel>>, NotFound, NoContent, ValidationProblem>> ListByGroupAsync([AsParameters] ListByGroupRequest request, CancellationToken cancellationToken)
{
	var expression = PredicateBuilder.New<GroupEntity>(g => g.UserProfileId == request.User.UserId);
	expression = Guid.TryParse(request.Group, out var groupId)
		? (ExpressionStarter<GroupEntity>)expression.And(g => g.GroupId == groupId)
		: (ExpressionStarter<GroupEntity>)expression.And(g => g.NormalizedName == request.Group.ToUpperInvariant());

	var group = await request.Context.Groups.SingleOrDefaultAsync(expression, cancellationToken);
	if (group == null)
                // New Result type here... as defined in the return value.
		return TypedResults.NotFound();

	var result = await Http.ListAsync(
		request,
		() => request.Context.Tasks
			.Include(m => m.Group)
			.Include(m => m.Tags)
			.Include(m => m.Steps)
			.Where(m => m.Group.UserProfileId == request.User.UserId)
			.OrderBy(m => m.Group.DisplayOrder)
			.ThenBy(m => m.DisplayOrder)
			.Skip((request.PageNum - 1) * request.PageSize)
			.Take(request.PageSize)
			.ToArrayAsync(cancellationToken),
		() => request.Context.Tasks
			.CountAsync(m => m.Group.UserProfileId == request.User.UserId, cancellationToken),
		entity => entity.ToModel(),
		cancellationToken);

        // COMPILE ERROR: Unable to cast..
	return result;
}

In this instance, there is a parent Group on the ListRequest derived ListByGroupRequest. And if it's not found then it needs to return NotFound - this situation seems impossible.

I'm a huge fan of the minimal APIs, and the fact I can use it to accurately generation OpenAPI docs is a real win. But I'm struggling to find out how to do some of these basic reusable methods.

For the sake of clarify, the Http.ListAsync and ListRequest method looks like this:

public static async Task<Results<Ok<PagedResponse<TModel>>, NoContent, ValidationProblem>> ListAsync<TEntity, TModel>(
	ListRequest request,
	Func<Task<TEntity[]>> whereResult,
	Func<Task<int>> countResult,
	Func<TEntity, TModel> convert,
	CancellationToken cancellationToken)
	where TEntity : class
	where TModel : class
{
	var validated = await request.Validator.ValidateAsync(request, cancellationToken);
	if (!validated.IsValid)
		return TypedResults.ValidationProblem(validated.ToDictionary());

	var results = await whereResult();
	if (results.Length == 0)
		return TypedResults.NoContent();

	var totalCount = await countResult();
	PagedResponse<TModel> pagedResult = new()
	{
		Items = results.Select(convert).ToList(),
		PageNum = request.PageNum,
		PageSize = request.PageSize,
		TotalCount = totalCount
	};

	return TypedResults.Ok(pagedResult);
}


record ListRequest(
    [FromServices] TaskStoreContext Context,
    [FromServices] CurrentUser User,
    [FromServices] IValidator<ListRequest> Validator,
    int PageNum = HttpGeneral.PageNum_Default,
    int PageSize = HttpGeneral.PageSize_Default
};