Communicating With the Server


The simplest way to implement client-server communication with WebSharper is by remote method calls. Add the [Remote] attibute on a static method and you will be able to call it from JavaScript-translated code, all serialization will be handled automatically by the WebSharper runtime. Customization of the remoting mechanism are possible for both the server and client side.

Remote calls can fall into three categories, depending on their return type.

Send message

Remote methods returning void are sending data to the server without blocking on the client. No callback is registered so there is no way to confirm success or error state.

Synchronous request

Synchronous remote calls are possible, but not recommended, as they are blocking JavaScript execution on the client. They give a deprecated warning with most current JavaScript engines.

Asynchronous request

Remote methods with return types Task or Task<T> (or FSharpAsync) allow waiting on the result asynchronously.

Here is an example of an RPC call which runs a database query asynchronously on the server:

public static class Server
    public static async Task<string[]> GetTitlesByAuthor(string author)
            using (var ctx = new MyDataContext())
                return await (
                    from a in ctx.Articles
                    where a.Author.Name = author
                    select a.Title).ToArrayAsync();

public class SomeClientSideClass

	public async Task DisplayTitles(string author)
		var titles = await Server.GetTitlesByAuthor(author);
		// ...

Here await on the client-side code means that execution of the rest of the async method will be resumed by a callback after the server has responded. Task based remoting also propagates errors produced by the underlying XMLHttpRequest as exceptions, such as the server responding with an error code.


Supported types

Remote method arguments are serialized from/to JSON. All types occurring in method parameters and return must be one of the following:

  • System namespace numeric types, string, bool, DateTime, TimeSpan, Guid, Nullable<T>, Tuple<...>. Using decimal on the client side needs the WebSharper.MathJS package referenced.
  • Enums.
  • Collection types: Array<T> (one-dimensional arrays), List<T>, Queue<T>, Stack<T>, LinkedList<T>. Other collection types have to be converted to a supported one.
  • Classes with a default constructor for which the values of all fields are themselves serializable or marked with the NonSerialized attribute. The Serializable attribute is not required and does not affect behavior.
  • F# union and record types (including those defined in FSharp.Core), FSharpSet<>, FSharpMap<_>.


Remote methods are exposed as http endpoints, so any security measures have to be integrated into the method body itself. WebSharper.Web.Remoting.GetContext().UserSession exposes some utilities for tracking users. This uses System.Web.Security.FormsAuthentication on the server and cookies in the browser. See here for more information.

//using WebSharper.Web;
public static async Task<bool> Login(string user, string password)
    var ctx = WebSharper.Web.Remoting.GetContext();
    if (await VerifyLogin(user, password)) {
        await ctx.UserSession.LoginUserAsync(user);
        return true;
    return false;

Server-side customization

You can also use instance methods for client-server communication. The syntax for this is:

//using static WebSharper.JavaScript.Pervasives;

The method invoked should be annotated with the Remote attribute and follows the same convention as static methods:

public class MyType
    public a this.MyMethod(...) = //...

In this case, you must provide a single instance of each type you use, preferably when the web application starts (such as in Global.asax).

WebSharper.Core.Remoting.AddHandler(typeof(MyType), new MyType());

Note: on ASP.Net Core, instead of calling WebSharper.Core.Remoting.AddHandler, you can add the handler to the dependency injection graph using services.AddWebSharperRemoting<MyType>(). See here for more details.

Remote annotated methods can be abstract or virtual. The instance that you provide by AddHandler can be of a subclass of the type in the first argument. This way you can have parts of server-side RPC functionality undefined in a library, and have them implemented in your web project for better separation of concerns.

// Common server and client code:

public abstract class MyType
    public abstract ... MyMethod(...);

// Client consumer:


// Server implementation:

public class MyTypeImpl
    public ... MyMethod(...) { ... }

// App startup:

WebSharper.Core.Remoting.AddHandler(typeof(MyType), new MyTypeImpl());

Client-side customization

If you want to change the URL that remoting calls target by default, set the value at the WebSharper.Remoting.EndPoint property at the startup of your client-side code.

You can You can further customize how RPC methods are called from the client-side code by writing a class implementing the customize how RPC methods are called from the client-side code by writing a class implementing the WebSharper.Remoting.IRemotingProvider interface, then adding a RemotingProvider attribute on the remote method with the custom remoting provider type as argument.

The IRemotingProvider provider has 4 methods for implementing synchronous, asynchronous (Task and F# Async), and send calls: Sync, Task, Async, Send respectively. You have to implement only the methods that your RPC methods will be using depending on their signatures.

The RemotingProvider attribute can be also used on classes and assemblies, overriding client-side remote handling for all RPC methods found in their scope.

The default implementation used is WebSharper.Remoting.AjaxRemotingProvider.

Communication Protocol

The communication protocol used by WebSharper is a custom protocol built on top of HTTP and JSON. The client sends HTTP POST requests marked with a special HTTP header x-websharper-rpc to the current URL of the page (?), with the bodies of the requests containing the JSON-serialized method arguments. The server responds with a JSON reply.

The URL to which the requests are sent can be customized modifying the value of WebSharper.Remoting.EndPoint static property on client-side, the default implementation of IRemotingProvider uses this string value.