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Is a Silverlight 4 assembly binary compatible with Silverlight 5. Is a SL4 assembly "run in the SL5 runtime"? If it is compatible, is it 100% compatible. You should be able to use a 3rd party SL4 assembly in your SL5 project?

Or is it more like .NET framework 2 and .NET framework 4, that runs side by side?

If I remember correctly SL2 and SL3 was not compatible.

Morten Frederiksen
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    http://stackoverflow.com/questions/8474485/silverlight-5-backward-compatibility – keyboardP Jan 02 '12 at 13:38
  • But it does not answer my question exactly. I want to mix SL5 and SL4 assemblies in one XAP file (My projects are SL5, but I uses 3rd party SL4 build assemblies). My question is not how does a SL4 run in a SL5 environment... – Morten Frederiksen Jan 02 '12 at 15:43
  • I have a silverlight solution with mixed assemblies of SL5 and SL4. In SL5 Beta/RC it had some issues compiling. In SL5 RTM it compiles without issue. Just try and see if it works. More than likely it will. – Josh C. Jan 05 '12 at 14:47
  • Thank you for the answer. However i use 3rd party SL4 assemblies in binary form, I don't have the source code... – Morten Frederiksen Jan 05 '12 at 15:35
  • I also didn't get the answer. Please have a look in the following use case - I'm using 3-rd party component which has Silverlight 4 target version. - My application has Silverlight 5 target version. - Bob (client of my application) has only Silverlight 5 installed on his PC. Will it work for him or he will have to install Silverlight 4 in that case as well? – Access Denied Jan 18 '12 at 10:27
  • A related piece of trivia: Since your SL4 app is run in the SL5 runtime you can even sometimes use SL5 features that aren't there in SL4. In particular, some properties on system classes that aren't there in SL4 can be accessed via reflection from an SL4 app. – John Sep 18 '12 at 17:55

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You can reference silverlight 4 dll from Silverlight 5 project You CANNOT reference silverlight 5 dll from Silverlight 4 project

Leon K
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