Optional query parameters in webapi

[pavelm]

I was converting a C# webapi project to F# using the F# ASP.NET templates. Everything is working great except optional query parameters. I keep getting this error

{
    "message": "The request is invalid.",
    "messageDetail": "The parameters dictionary contains an invalid entry for parameter 'start' for method 'System.Threading.Tasks.Task`1[System.Net.Http.HttpResponseMessage] GetVendorFiles(Int32, System.Nullable`1[System.DateTime])' in 'Thor.WebApi.VendorFilesController'. The dictionary contains a value of type 'System.Reflection.Missing', but the parameter requires a value of type 'System.Nullable`1[System.DateTime]'."
}

F# function signature:

        [<HttpGet; Route("")>]
        member x.GetVendorFiles( [<Optional; DefaultParameterValue(100)>] count, [<Optional; DefaultParameterValue(null)>] start : Nullable<DateTime> ) =

C# function signature:

        [HttpGet]
        [Route("")]
        public async Task<HttpResponseMessage> GetVendorFiles(int count = 100,DateTime? start = null)

Does any one know of any work arounds?

[panesofglass]

For what it's worth, I rarely use the parameter bindings. I find request.GetQueryNameValuePairs() a more reliable mechanism, especially when you want optional parameters. F# doesn't seem to map well to the MVC conventions for parameter binding. I'm sure this is not the answer for which you hoped, but I've never been able to get parameter binding to work reliably. You ma want to raise this in https://github.com/Microsoft/visualfsharp/issues.