Add reference problem in silverlight: You can't add a reference to dll as it was not built against the silverlight runtime

I have a silverlight application which has a class library project in it. th class library is running in .Net framework 4 and the silverlight project is running in silverlight 3. when I want to add a reference in the silverlight proj to the class library I get this message:

"You can't add a reference to Classlibrary1.dll as it was not built against the silverlight runtime. silverlight project wi开发者_运维问答ll only work with silverlight asseblies."

I've tried to change the framework the classlibrary1 used to .net framework 3.5, 3, and 2 but it does not help. ( and my silverlight application can only run in silverlight 3)

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Apparently Silverlight project will only work with Silverlight asseblies. CLRs for .NET and Silverlight are different, therefore assemblies are not compatible. These frameworks provide different set of classes, assemblies have different versions, et cetera.

You need to change your Class Library to Silverlight Class Library.
This can either be done by changing project file manually or by creating Silverlight Class Library project and copying the sources in it. If you need the same library for a non-Silverlight project and don't want to maintain two versions, consider using Add as Link feature of Add Existing Item... dialog to only reference the original sources without copying them. But you'll need to make sure you only use functionality available on both platforms for both library versions to build.

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You have to add a new project to the solution using the "Silverlight Class Library" project template. Then move your files from the .NET Class Library project into the Silverlight Class Library project.

Silverlight applications can't reference .NET assemblies.

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Rebuild the library as a sliverlight library; this will mean a new csproj, with mostly the same files. You may (not always) find you also need to tweak the code to avoid any operations not available in the slimmer Silverlight BCL.