Consuming an ASP.NET Web Service (ASMX)

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ASMX provides the ability to build web services that send messages using the Simple Object Access Protocol (SOAP). SOAP is a platform-independent and language-independent protocol for building and accessing web services. Consumers of an ASMX service do not need to know anything about the platform, object model, or programming language used to implement the service. They only need to understand how to send and receive SOAP messages. This article demonstrates how to consume an ASMX SOAP service from a Xamarin.Forms application.

A SOAP message is an XML document containing the following elements:

  • A root element named Envelope that identifies the XML document as a SOAP message.
  • An optional Header element that contains application-specific information such as authentication data. If the Header element is present it must be the first child element of the Envelope element.
  • A required Body element that contains the SOAP message intended for the recipient.
  • An optional Fault element that's used to indicate error messages. If the Fault element is present, it must be a child element of the Body element.

SOAP can operate over many transport protocols, including HTTP, SMTP, TCP, and UDP. However, an ASMX service can only operate over HTTP. The Xamarin platform supports standard SOAP 1.1 implementations over HTTP, and this includes support for many of the standard ASMX service configurations.

This sample includes the mobile applications that run on physical or emulated devices, and an ASMX service that provides methods to get, add, edit, and delete data. When the mobile applications are run, they connect to the locally-hosted ASMX service as shown in the following screenshot:

Note

In iOS 9 and greater, App Transport Security (ATS) enforces secure connections between internet resources (such as the app's back-end server) and the app, thereby preventing accidental disclosure of sensitive information. Since ATS is enabled by default in apps built for iOS 9, all connections will be subject to ATS security requirements. If connections do not meet these requirements, they will fail with an exception. ATS can be opted out of if it is not possible to use the HTTPS protocol and secure communication for internet resources. This can be achieved by updating the app's Info.plist file. For more information see App Transport Security.

Consume the web service

The ASMX service provides the following operations:

Operation Description Parameters
GetTodoItems Get a list of to-do items
CreateTodoItem Create a new to-do item An XML serialized TodoItem
EditTodoItem Update a to-do item An XML serialized TodoItem
DeleteTodoItem Delete a to-do item An XML serialized TodoItem

For more information about the data model used in the application, see Modeling the data.

Create the TodoService proxy

A proxy class, called TodoService, extends SoapHttpClientProtocol and provides methods for communicating with the ASMX service over HTTP. The proxy is generated by adding a web reference to each platform-specific project in Visual Studio 2019 or Visual Studio 2017. The web reference generates methods and events for each action defined in the service's Web Services Description Language (WSDL) document.

For example, the GetTodoItems service action results in a GetTodoItemsAsync method and a GetTodoItemsCompleted event in the proxy. The generated method has a void return type and invokes the GetTodoItems action on the parent SoapHttpClientProtocol class. When the invoked method receives a response from the service, it fires the GetTodoItemsCompleted event and provides the response data within the event's Result property.

Create the ISoapService implementation

To enable the shared, cross-platform project to work with the service, the sample defines the ISoapService interface, which follows the Task asynchronous programming model in C#. Each platform implements the ISoapService to expose the platform-specific proxy. The sample uses TaskCompletionSource objects to expose the proxy as a task asynchronous interface. Details on using TaskCompletionSource are found in the implementations of each action type in the sections below.

The sample SoapService:

  1. Instantiates the TodoService as a class-level instance
  2. Creates a collection called Items to store TodoItem objects
  3. Specifies a custom endpoint for the optional Url property on the TodoService
public class SoapService : ISoapService
{
    ASMXService.TodoService todoService;
    public List<TodoItem> Items { get; private set; } = new List<TodoItem>();

    public SoapService ()
    {
        todoService = new ASMXService.TodoService ();
        todoService.Url = Constants.SoapUrl;
        ...
    }
}

Create data transfer objects

The sample application uses the TodoItem class to model data. To store a TodoItem item in the web service it must first be converted to the proxy generated TodoItem type. This is accomplished by the ToASMXServiceTodoItem method, as shown in the following code example:

ASMXService.TodoItem ToASMXServiceTodoItem (TodoItem item)
{
    return new ASMXService.TodoItem {
        ID = item.ID,
        Name = item.Name,
        Notes = item.Notes,
        Done = item.Done
    };
}

This method creates a new ASMService.TodoItem instance, and sets each property to the identical property from the TodoItem instance.

Similarly, when data is retrieved from the web service, it must be converted from the proxy generated TodoItem type to a TodoItem instance. This is accomplished with the FromASMXServiceTodoItem method, as shown in the following code example:

static TodoItem FromASMXServiceTodoItem (ASMXService.TodoItem item)
{
    return new TodoItem {
        ID = item.ID,
        Name = item.Name,
        Notes = item.Notes,
        Done = item.Done
    };
}

This method retrieves the data from the proxy generated TodoItem type and sets it in the newly created TodoItem instance.

Retrieve data

The ISoapService interface expects the RefreshDataAsync method to return a Task with the item collection. However, the TodoService.GetTodoItemsAsync method returns void. To satisfy the interface pattern, you must call GetTodoItemsAsync, wait for the GetTodoItemsCompleted event to fire, and populate the collection. This allows you to return a valid collection to the UI.

The example below creates a new TaskCompletionSource, begins the async call in the RefreshDataAsync method, and awaits the Task provided by the TaskCompletionSource. When the TodoService_GetTodoItemsCompleted event handler is invoked it populates the Items collection and updates the TaskCompletionSource:

public class SoapService : ISoapService
{
    TaskCompletionSource<bool> getRequestComplete = null;
    ...

    public SoapService()
    {
        ...
        todoService.GetTodoItemsCompleted += TodoService_GetTodoItemsCompleted;
    }

    public async Task<List<TodoItem>> RefreshDataAsync()
    {
        getRequestComplete = new TaskCompletionSource<bool>();
        todoService.GetTodoItemsAsync();
        await getRequestComplete.Task;
        return Items;
    }

    private void TodoService_GetTodoItemsCompleted(object sender, ASMXService.GetTodoItemsCompletedEventArgs e)
    {
        try
        {
            getRequestComplete = getRequestComplete ?? new TaskCompletionSource<bool>();

            Items = new List<TodoItem>();
            foreach (var item in e.Result)
            {
                Items.Add(FromASMXServiceTodoItem(item));
            }
            getRequestComplete?.TrySetResult(true);
        }
        catch (Exception ex)
        {
            Debug.WriteLine(@"\t\tERROR {0}", ex.Message);
        }
    }

    ...
}

For more information, see Asynchronous Programming Model and TPL and Traditional .NET Framework Asynchronous Programming.

Create or edit data

When you create or edit data, you must implement the ISoapService.SaveTodoItemAsync method. This method detects whether the TodoItem is a new or updated item and calls the appropriate method on the todoService object. The CreateTodoItemCompleted and EditTodoItemCompleted event handlers should also be implemented so you know when the todoService has received a response from the ASMX service (these can be combined into a single handler because they perform the same operation). The following example demonstrates the interface and event handler implementations, as well as the TaskCompletionSource object used to operate asynchronously:

public class SoapService : ISoapService
{
    TaskCompletionSource<bool> saveRequestComplete = null;
    ...

    public SoapService()
    {
        ...
        todoService.CreateTodoItemCompleted += TodoService_SaveTodoItemCompleted;
        todoService.EditTodoItemCompleted += TodoService_SaveTodoItemCompleted;
    }

    public async Task SaveTodoItemAsync (TodoItem item, bool isNewItem = false)
    {
        try
        {
            var todoItem = ToASMXServiceTodoItem(item);
            saveRequestComplete = new TaskCompletionSource<bool>();
            if (isNewItem)
            {
                todoService.CreateTodoItemAsync(todoItem);
            }
            else
            {
                todoService.EditTodoItemAsync(todoItem);
            }
            await saveRequestComplete.Task;
        }
        catch (SoapException se)
        {
            Debug.WriteLine("\t\t{0}", se.Message);
        }
        catch (Exception ex)
        {
            Debug.WriteLine("\t\tERROR {0}", ex.Message);
        }
    }

    private void TodoService_SaveTodoItemCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
    {
        saveRequestComplete?.TrySetResult(true);
    }

    ...
}

Delete data

Deleting data requires a similar implementation. Define a TaskCompletionSource, implement an event handler, and the ISoapService.DeleteTodoItemAsync method:

public class SoapService : ISoapService
{
    TaskCompletionSource<bool> deleteRequestComplete = null;
    ...

    public SoapService()
    {
        ...
        todoService.DeleteTodoItemCompleted += TodoService_DeleteTodoItemCompleted;
    }

    public async Task DeleteTodoItemAsync (string id)
    {
        try
        {
            deleteRequestComplete = new TaskCompletionSource<bool>();
            todoService.DeleteTodoItemAsync(id);
            await deleteRequestComplete.Task;
        }
        catch (SoapException se)
        {
            Debug.WriteLine("\t\t{0}", se.Message);
        }
        catch (Exception ex)
        {
            Debug.WriteLine("\t\tERROR {0}", ex.Message);
        }
    }

    private void TodoService_DeleteTodoItemCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e)
    {
        deleteRequestComplete?.TrySetResult(true);
    }

    ...
}

Test the web service

Testing physical or emulated devices with a locally-hosted service requires custom IIS Configuration, endpoint addresses, and firewall rules to be in place. For more detail on how to set up your environment for testing, see the Configure remote access to IIS Express. The only difference between testing WCF and ASMX is the port number of the TodoService.