The Constrained Application Protocol (CoAP) (https://datatracker.ietf.org/doc/draft-ietf-core-coap/) is a RESTful web transfer protocol for resource-constrained networks and nodes. CoAP.NET is an implementation in C# providing CoAP-based services to .NET applications. Reviews and suggestions would be appreciated.

• [Quick Start] (#quick-start)
• [Build] (#build)
• [License] (#license)
• [Acknowledgements] (#acknowledgements)

CoAP sessions are considered as request-response pair.

Access remote CoAP resources by issuing a [Request] (CoAP.NET/Request.cs) and receive its [Response] (CoAP.NET/Request.cs)(s).

  // new a GET request
  Request request = new Request(Method.GET);
  request.URI = new Uri("coap://[::1]/hello-world");
  request.Send();
  
  // wait for one response
  Response response = request.WaitForResponse();

There are 4 types of request: GET, POST, PUT, DELETE, defined as Method.GET, Method.POST, Method.PUT, Method.DELETE.

Responses can be received in two ways. By calling request.WaitForResponse() a response will be received synchronously, which means it will block until timeout or a response is arrived. If more responses are expected, call WaitForResponse() again.

To receive responses asynchronously, register a event handler to the event request.Respond before executing.

Parsing Link Format

Use LinkFormat.Parse(String) to parse a link-format response. The returned enumeration of WebLink contains all resources stated in the given link-format string.

Request request = new Request(Method.GET); request.URI = new Uri("coap://[::1]/.well-known/core"); request.Send(); Response response = request.WaitForResponse(); IEnumerable links = LinkFormat.Parse(response.PayloadString);

See [CoAP Example Client] (CoAP.Client) for more.

### CoAP Server

A new CoAP server can be easily built with help of the class
[**CoapServer**] (CoAP.NET/Server/CoapServer.cs)

```csharp
static void Main(String[] args)
{
  CoapServer server = new CoapServer();
  
  server.Add(new HelloWorldResource("hello"));
  
  server.Start();
  
  Console.ReadKey();
}

See [CoAP Example Server] (CoAP.Server) for more.

CoAP resources are classes that can be accessed by a URI via CoAP. In CoAP.NET, a resource is defined as a subclass of [Resource] (CoAP.NET/Server/Resources/Resource.cs). By overriding methods DoGet, DoPost, DoPut or DoDelete, one resource accepts GET, POST, PUT or DELETE requests.

The following code gives an example of HelloWorldResource, which can be visited by sending a GET request to "/hello-world", and respones a plain string in code "2.05 Content".

  class HelloWorldResource : Resource
  {
      public HelloWorldResource()
          : base("hello-world")
      {
          Attributes.Title = "GET a friendly greeting!";
      }

      protected override void DoGet(CoapExchange exchange)
      {
          exchange.Respond("Hello World from CoAP.NET!");
      }
  }
  
  class Server
  {
      static void Main(String[] args)
      {
          CoapServer server = new CoapServer();
          server.Add(new HelloWorldResource());
          server.Start();
      }
  }

See [CoAP Example Server] (CoAP.Server) for more.

A few compile symbols are introduced to build for different drafts of CoAP:

• COAP03 -- [draft-ietf-core-coap-03] (http://tools.ietf.org/html/draft-ietf-core-coap-03)
• COAP08 -- [draft-ietf-core-coap-08] (http://tools.ietf.org/html/draft-ietf-core-coap-08)
• COAP12 -- [draft-ietf-core-coap-12] (http://tools.ietf.org/html/draft-ietf-core-coap-12)
• COAP13 -- [draft-ietf-core-coap-13] (http://tools.ietf.org/html/draft-ietf-core-coap-13)
• COAP18 -- [draft-ietf-core-coap-18] (http://tools.ietf.org/html/draft-ietf-core-coap-18)
• COAPALL -- all supported drafts above

By default (with no symbol defined), CoAP.NET will be compiled with the latest version of CoAP protocol. To enable drafts, define one or more of those compile symbols.

With drafts enabled, an interface ISpec will be introduced, representing draft specification. Define COAPXX to enable draft XX, or COAPALL to enable all supported drafts. All enabled drafts will be available in class [Spec] (CoAP.NET/Spec.cs):

  public static class Spec
  {
    public static readonly ISpec Draft03;
    public static readonly ISpec Draft08;
    public static readonly ISpec Draft12;
    public static readonly ISpec Draft13;
    public static readonly ISpec Draft18;
  }

With none of the symbols defined, only the latest version of draft will be compiled as the class [Spec] (CoAP.NET/Spec.cs), with static members instead of various drafts:

  public static class Spec
  {
    public static readonly String Name = "draft-ietf-core-coap-18";
    public static readonly Int32 DefaultPort = 5683;
    public static IMessageEncoder NewMessageEncoder();
    public static IMessageDecoder NewMessageDecoder(Byte[] data);
    public static Byte[] Encode(Message msg);
    public static Message Decode(Byte[] bytes);
  }

See [LICENSE] (LICENSE) for more info.

CoAP.NET is based on [Californium] (https://github.com/mkovatsc/Californium), a CoAP framework in Java by Matthias Kovatsch, Dominique Im Obersteg, and Daniel Pauli, ETH Zurich. See http://people.inf.ethz.ch/mkovatsc/californium.php. Thanks to the authors and their great job.