I suppose that our previous reply was not precise enough - please excuse us for this.
At first let me explain why the simultaneous usage of Load On Demand and CheckBoxes is not supported
: RadComboBox items loaded via Load On Demand are not accessible at server-side - this is made for better performance when the control deals with large amount of data.
However CheckBoxes feature need to access items at server-side and that is why by design these two features are not intended to work together.
In case you use only Load On Demand and you want to prevent user typing in RadComboBox, please set the input readOnly property in this way:
"Select a Company"
All the best,
the Telerik team
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