Web services and Silverlight 1.1 C# gotchas

As anyone must know by now, Silverlight is Microsoft's cross-browser, cross-platform RIA (Rich Internet Application) technology – like Adobe's Flash. With Silverlight, they say the sky is the limit, and it's only up to yourself what to invent. Silverlight offers you a big chunck of the .NET CLR in the browser, when using Silverlight 1.1 Alpha (Refresh). As always, there are do's and don'ts when you work with new technology and in this not-known-yet-part series, I'm going to recap my experience using Silverlight 1.1 Alpha and later Silverlight 1.1 Alpha Refresh to develop some sophisticated charting components.

Before you begin – general info

For the current release of Silverlight (Silverlight 1.1 Alpha Refresh) you don't have the System.Data namespace (that means no DataSets nor DataTables). Also the System.Xml namespace is very small, you don't get the XmlDocument which would have been useful. I haven't heard anything reliable about whether or not those namespaces and classes will make it into the final release of Silverlight, but it doesn't seem like it.

Maybe 3rd party components will come to help here, but that's a drawback because then the users have to install something new on their machines to be able to use your app.

Mark your webservice as a ScriptService

If you're going to use any kind of data in your Silverlight apps, and you probably are, then you have to use a webservice. Calling a webservice from Silverlight is a bit different from calling a webservice from another ASP.NET page.

First of all, you have to mark your webservice as Scriptable. The Scriptable attribute excists in the System.Web.Script.Services namespace, and is part of the ASP.NET AJAX framework (also included in the .NET Framework 3.5). If you are developing on ASP.NET 2.0, you have to make sure you have ASP.NET AJAX installed on the server, and reference the correct assemblies in your ASP.NET 2.0 application.

This is how your class definition will look like:

[WebService(Namespace = http://tempuri.org/)] [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)] [System.Web.Script.Services.ScriptService] publicclass SilverchartDataService : System.Web.Services.WebService{


Now your webservice can be called from Silverlight – and also JavaScript / AJAX.

Don't call a webservice on another domain

The security model of Silverlight, and also other client-side technologies, deny calling a webservice on another domain. That is also called cross-domain webservice calls, and if you do, you will get this exception when #debugging your Silverlight app using Visual Studio 2008:


I've seen ways to get around this problem, but it is beyond the purpose of this blog post. If you need info on this topic, try this: Using a proxy to access remote APIs (The article is for AJAX – should work for Silverlight)

JSON Serialization compatibillity with .NET

Whenever you use a webservice, all the objects transfered gets serialized. (Remember to mark your own classes as Serializable if you want to use them in your webservice.) And Silverlight uses the JSON Serialization 'engine'. This can result in some nasty errors, yet difficult to debug. I've found out, that some .NET classes cannot be used – these include:

  • decimal
  • enum (So don't use enum's with Silverlight – at least not until it is supported)

    And there are probably more than those two…

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