Make a parameter in minimal api as an optional in .NET Core - api

Question

I have this API as you can see:
app.MapGet("/api/fund/{fundCode}", ([FromServices] IMediator mediator,string fundCode)
=> mediator.Send(new GetFundsQuery(fundCode)));
I want to set fundcode as an optional parameter to my API, so I changed it to
app.MapGet("/api/fund/{fundCode?}", ([FromServices] IMediator mediator,string fundCode)
=> mediator.Send(new GetFundsQuery(fundCode)));
But it didn't work, and when I call this address
https://localhost:7147/api/fund
I get an http 404 error. Why?

Answer 1

When I used code below, I'll get "null" as a response when I call localhost:port/hello. But when I use string id as the parameter, I got 400 bad request...
app.MapGet("/hello/{id?}", (string? id) =>
{
if (id == null)
{
return "null";
}
else {
return id;
}
});
I also tried to use code below and when I call localhost:port/hello, I get "empty" as the response.
app.MapGet("/hello/{id?}", (string? id) =>
{
if (id == null)
{
return "null";
}
else {
return id;
}
});
app.MapGet("/hello", () =>
{
return "empty";
});

Answer 2

You can try the query parameter. For example:
app.MapGet("/api/fund", GetFundsAsync).Produces<IList<Fund>>();
private async Task<IList> GetFundsAsync(IMediator mediator,string fundCode = null)
{
return await mediator.Send(new GetFundsQuery(fundCode));
}
This way https://localhost:7147/api/fund will return results
And https://localhost:7147/api/fund?fundCode=ABC will work too