Using a Web Service

Step 1: Using the System.Web.Services.dll assembly

To consume web services, you need to add to your project the System.Web.Services.dll assembly.  You need to reference this assembly in your project;   If using MonoDevelop, right-click on "Reference" in your solution and pick "Edit References", then select ".NET Assembly" and pick the System.Web.Services.dll assembly from the /Developer/MonoTouch/usr/lib/mono/2.1 directory and click the add button.

At this point, you will have the assembly that can invoke web services included in your project.   

Step 2: Consuming a Web Service from a WSDL file.

Web Services publish description files in the form of a WSDL document (this is an XML document that describes the callable API).   To consume one of these, right click on your project and select "Add Web Reference".  

The dialog that pops up will allow you to enter the URL to a WSDL description file.   Enter the url here, once the dialog fetches the WSDL it will preview the published API.  It should look like this:

If you are not using MonoDevelop, but instead just want to use the command line tools, you would use the "wsdl" command line tool that is part of Mono, and this will generate the web service file for you.

If you want to learn more about how to consume a WSDL file, read Microsoft's Creating an XML Web Service Proxy document.

Step 3: Invoking the Web Service

At this point, you can just create an instance of the generate class that was generated by MonoDevelop (or the wsdl tool if you used the command line).   Intellisense will be active, so you will be able to invoke methods directly.

Using the WebService Async Methods

Mono generates two class of methods in addition to the synchronous method. For example, in addition to Add, the following methods are generated: AddAsync and BeginAdd.

Using the async pattern means that you invoke the BeginFoo method and pass it a callback that will be invoked when the invocation has completed.   Your callback then invokes the "EndFoo" method to fetch the result or any errors.  A full explanation is available on Microsoft's Communicating with Web Services Asynhronously page.   Here is how you would use it:

void Run ()
{
    var t = new TestService ();
		
    // Pass "t" as the state to our callback.
    t.BeginAdd (10, 20, callback, t);
}

void callback (IAsyncResult iar)
{
    TestService ts = (TestService) iar.AsyncState;
    int result = ts.EndAdd (iar);

    // Since we are running on a separate thread, we can not access
    // UIKit objects from here, so we need to invoke those on the
    // main thread:
    InvokeOnMainThread (delegate {
        result.Text = "Result: " + res;
    });
}

The above pattern works great for large methods, but for quick hacks, you can use the C# 3.0 syntax:

void Run ()
void Run ()
{
    var t = new TestService ();
    
    t.BeginAdd (10, 20, delegate (IAsyncResult iar) {
        int result = t.EndAdd (iar);

        InvokeOnMainThread (delegate {
            result.Text = "Result: " + res;
        });        
    }, null);
}

The second asynchronous pattern, uses the FooAsync methods that are generated in the stub. These hide the details about IAsyncResult and casting and instead use strong typed event arguments for the results:

void Run ()
    var t = new TestService ();

    // Connect the event
    t.AddCompleted += AddCompleted;
    // Start the request, will run on a separate thread
    t.AddAsync (10, 20);
}

void AddCompleted (object sender, AddCompletedEventArgs args) 
{
    // The result is in args.Result:

    InvokeOnMainThread (delegate {
        result.Text = "Result: " + args.Result;
    });
}

Using threads.

If you want to manage the threads yourself or perhaps if you want to batch multiple operations in one, you would use something like this:

void Run ()
{
    var t = new Thread (doWork);
    t.Start (54);
}

void DoWork (object arg)
{
    // Get the parameter (54 in this sample)
    int value = (int) arg;

    var res = TestService.Add (value, 10);

    // Since we are running on a separate thread, we can not access
    // UIKit objects from here, so we need to invoke those on the
    // main thread:
    InvokeOnMainThread (delegate {
        result.Text = "Result: " + res;
    });
}