ASP WebAPI generic List optional parameter

Question

I'm really struggling with this one. I need a generic list parameter for my Get method, but it needs to be optional. I just did this:

public dynamic Get(List <long> ManufacturerIDs = null)

Unfortunately on runtime i get the error:

Optional parameter 'ManufacturerIDs' is not supported by 'FormatterParameterBinding'.

How to get a generic list as an optional parameter here?

Answer 1

What's the point of using an optional parameter? List<T> is a reference type and if the client doesn't supply a value it will simply be null:

public HttpResponseMessage Get(List<long> manufacturerIDs)
{
 ...
}

• Augusto Barreto: In my experience, there are many cases where you have to explicitly set optional parameters to null (stackoverflow.com/a/22397723/1454888). This is counter-intuitive for me, but it works. Thanks.
• Nebula: I am using swashbuckle for swagger documentation generation. In that context it does matter how the optional parameters are defined, observe: [FromUri] object[] foo = null --> "Provide multiple values in new lines." [FromUri] object[] foo --> "Provide multiple values in new lines (at least one required)." object[] foo --> "(required)" So in that context it really depends on what you expect from your end user.
• pawellipowczan: I had to insert [FromUri] attribute before the List<string> optional parameter. There is a difference when you use = null as @Nebula noticed. When you don't add = null then you have to specify at least one element.