I would recommend using the KendoUI Validator
for the client side validation. Basically you can initialize a Validator, define a custom rule that checks whether the MultiSelect has some items selected and if it doesn't - display an error message. For convenience I prepared this example
, illustrating the above approach.
The server side validation is not directly related to KendoUI, however more information on that topic can be found on various resources over the Internet.
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