EF Core 3.1 - Database scalar function - string.Join - sql-server

Question

Since string.Join(...) is not translatable in the latest EF Core and a lot of our queries do use that method, I'm currently trying to implement it with user-defined functions. I can see that the function actually works in the ssms but I can't get it to work in c#.
The method needs to accept an array of strings as an input so I created a user-defined table type:
IF TYPE_ID('[system].[stringList]') IS NULL
BEGIN
CREATE TYPE system.stringList AS TABLE (val nvarchar(max))
END
GO
Then I created the function itself:
create function toCommaSeparatedString(#strings [system].[stringList] readonly)
returns nvarchar(max)
as
begin
DECLARE #values nvarchar(max)
SELECT #values = COALESCE(#values + ', ', '') + [val]
FROM #strings
return #values
end
I can verify that it works by simply executing the following sql in the ssms:
declare #input [system].[stringList]
insert into #input values ('1'), ('2'), ('3'), ('4')
select dbo.toCommaSeparatedString(#input)
In c# I declared the function on my DbContext (I tried both string[] and IEnumerable<string>):
[DbFunction("toCommaSeparatedString", "dbo")]
public static string ToCommaSeparatedString(string[] strings) => throw new NotImplementedException();
And used it:
...
var output = MyDbContext.ToCommaSeparatedString(new [] { "1", "2" });
...
but I'm getting an exception:
"The parameter 'strings' for the DbFunction 'BusinessUnitDbContext.ToCommaSeparatedString' has an invalid type 'string[]'. Ensure the parameter type can be mapped by the current provider.",
Is it possible to achieve what I'm trying to achieve here? Should I also configure that function on the DbContext?
EDIT:
Here is my custom projection that i want to use to create the viewmodel:
private static IQueryable<IView> ProjectToJobOverView(this IQueryable<Job> entities)
{
return entities.Select(j => new JobOverView
{
Id = j.EntityId,
Deleted = j.Deleted.HasValue,
Name = j.Name,
Status = j.Status.ToString(),
NumberOfInstructionSets = j.Tasks.Count(t => t.InstructionSet != null),
NumberOfCompletedInstructionSets = j.Tasks.Count(t => t.InstructionSet.IsCompleted),
NumberOfOrderLines = j.Tasks
.SelectMany(t => t.TaskOrderLines).Select(x => x.OrderLineId)
.Distinct()
.Count(),
Details = j.Tasks
.OfType<StockMovementTask>()
.Where(t => t.InstructionSet != null)
.Select(t => t.InstructionSet)
.Select(s => new JobOverviewDetail
{
Id = s.EntityId,
Deleted = s.Deleted.HasValue,
State = s.Tasks.First().OperationState.ToString(),
DeliveryMethod = BusinessUnitDbContext.ToCommaSeparatedString(
s.Tasks
.First()
.TaskOrderLines.Where(x => x.OrderLine.Order is OutgoingOrder && (x.OrderLine.Order as OutgoingOrder).DeliveryType != null)
.Select(x => (x.OrderLine.Order as OutgoingOrder).DeliveryType.Name).ToArray()),
}),
Categories = j.Tasks.Select(t => t.Process.ProcessDefinition.ProcessDefinitionGroup.ProcessDefinitionCategory.ToString()).Distinct().ToArray(),
OrderNumbers = j.Tasks.OfType<StockMovementTask>().SelectMany(t => t.TaskOrderLines).Select(j => j.OrderLine.Order.Number).Distinct().ToArray(),
});
}