DateTime values cannot be summed. Instead you can get the hours and minutes from the field and sum them. Note that Sum() aggregate function skips the NULL values, so you don't need to include Is Null
check in the expression.
As for the expression checking if the field is NULL, you are correct. I also made some tests and it seems that it will only returns True if the entire property is missing. For example, check this screenshot: https://www.screencast.com/t/JUP9rsB8
. However, this is not an expected behavior and we consider this as bug. I have logged it on your behalf on our Feedback Portal as Bug Report (JsonDataSource evaluates NULL values incorrectly
) and also vote for it. This way you can follow the status of the item and when will be released. As a token of gratitude, I have updated your Telerik points. Thank you for bringing this to our attention.
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