System.IO namespace Stream class issues

Project Type: Mobile Apps / Class Library / Xamarin.Forms portable

I'm trying to serialise an object into JSON format by DataContractJsonSerializer:

DataContractJsonSerializer serializer = new DataContractJsonSerializer (type);
Stream s = new MemoryStream ();
serializer.WriteObject (stream, graph);

When I post the data to the server, internal server error returns because content is empty but the same logic works other than Xamarin.Forms projects:

HttpContent httpContent = new StreamContent (s);
HttpResponseMessage httpResponseMessage = await httpClient.PostAsync (uri, httpContent);

Besides I want to close stream s by:

s.Close ();

But XS throws:

Type `System.IO.Stream' does not contain a definition for `Close' and no extension method `Close' of type `System.IO.Stream' could be found. Are you missing an assembly reference? (CS1061)

This is a library project and has:

.NET Portable Subset
From Packages
    System.Net.Http
    System.Net.Http.Extensions
    System.Net.Http.Primitives
    Xamarin.Forms.Code
    Xamarin.Forms.Xaml

as references.

Is it really a missing assembly issue or does System.IO.Stream class have issues?