In order to make the usage of embedded Telerik skins even more transparent than before and reduce the size of the main Telerik AJAX assembly (Telerik.Web.UI.dll), we decided to introduce a separate dll for the AJAX skins (Telerik.Web.UI.Skins.dll) to attain both goals. This assembly will be commenced with the official Q2 2011 release and some of you may have already noticed it in the AJAX Q2 2011 BETA pack posted several days ago.
Telerik.Web.UI.Skins.dll will accommodate the new set of skins shipped with the AJAX Q2 2011 drop (Transparent + Office2010 Blue/Silver/Black) while the rest of the skins will remain wrapped in the Telerik.Web.UI.dll. You will not need to register this additional dll explicitly on your pages but simply copy and reference it either from the /bin folder of your project (see the screenshot below) or the Global Assembly Cache (GAC), which ensures seamless backward compatibility with previous versions of the Telerik AJAX controls. Thus the Telerik.Web.UI.Skins assembly will be automatically referenced/loaded by triggering mechanism initiated when one of the newly introduced skins mentioned above is specified for a Telerik AJAX controls, either from design-time or at runtime. Note that in case you do not intend to apply Transparent or Office2010 type of skin, you do not need to copy and reference the Telerik.Web.UI.Skins assembly at all and solely the Telerik.Web.UI.dll will be loaded in the memory.
To make the transition from pre- Q2 2011 versions as smooth as possible, the predefined Telerik skins are divided both in the Telerik.Web.UI and Telerik.Web.UI.Skins assemblies for one quarter, starting Q2 2011 until Q3 2011. After that, all embedded skins (except the Default one, which will remain plugged in the Telerik.Web.UI.dll) will be shifted to the Telerik.Web.UI.Skins.dll and to utilize any embedded skin other than the Default skin, you will have to have a valid reference to the new AJAX skins dll in your project.
Note: You will be able to expend the four new skins (Transparent + Office2010 variations) with the Q2 2011 release of the AJAX controls only (older versions are not supported). This is due to the fact that the Telerik.Web.UI.Skins assembly has a version which is mandatory to match the Telerik.Web.UI assembly version. If you try to reference Telerik.Web.UI.Skins.dll with version different than that of the Telerik.Web.UI.dll you have, the first simply will not be recognized and referenced/loaded. The same holds true for future versions of both assemblies.