Why I can't add a class library project reference to a Silverlight project?

I am a newcomer to Silverlight and while I have been following some tutorials I wanted to create a new project with a class library so that I can use it from my page's xaml.cs code behind.

But when I tried to add my class library project I get a warning message saying:

You can only add p开发者_如何转开发roject references to other Silverlight projects in the solution

It is clear what it says. But what is the reason behind this? Why I cannot add a class library project reference to a Silverlight project?

Thanks

Instead of regular Class Library, you can use the "Silverlight Class Library" template. The reason why you can't use a regular Class Library is that the Common Language Runtime of Silverlight is different than the CLR of "regular" .NET Framework.

You can add a reference to a class library to a silverlight application but that class library needs to be a Silverlight Class Library.

When you use Add Project ont he solution select the "Silverlight" option under "Visual C#" ( or "Visual Basic" or even "Visual F#"). You will see a template for a "Silverlight Class Library". Your class library needs to have used this template in order to be eligable to be reference by a Silverlight application or other Silverlight Class libraries.