Hello Andrew Powrie,
Thank you for writing.
This is the proper behavior in MultiExtended mode because it was designed like this. MultiExtended at its core is the same as One but provides different modes depending on the modifier keys (CTRL and SHIFT). When setting an item to Selected and adding it to a collection it makes no sense to take into accound the CTRL and SHIFT keys since adding the item is not caused by mouse selection, and therefore behaves like One.
Please share your thoughts on why should all the items be selected. If another vendor provides this behavior or if this is a particular requirement of yours we will consider adding a mode that will allow the adding of items in MultiExtended mode to behave as in MultiSimple.
Also thank you for reporting the MultiSimple issue, it will be fixed for the upcoming Q2 2010 SP1 release. Your Telerik points have been updated.
Please write again if you have other questions or if you need assistance with our controls.
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