RadFilter can be integrated with databound controls as shown in this demo or with any datasource controls (such as SqlDataSource, EntityDataSource, LinqDataSource etc.) as described in this help topic. For additional custom scenarios you will need to define the expressions manually as shown here.
If you are using any of the predefined approaches above and you still get an error let us know how exactly you have integrated the RadFilter with the entity framework so we can check it on our side.
Also note that there are many different filter expressions depending on what type of data you are trying to filter and in order for the control to automatically generate the correct expression you have set its DataSourceControlID property to the proper value.
I hope this helps.
the Telerik team
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