Hi Setu,
By looking your code snippet - I can say that you are using two different API:
1. CRM Api (CRM web services)
2. Telerik Api
Now come to EntityCollection Class & its object -
EntityCollection class present in both the API:
1.
CRM EntityCollection
2.
Telerik EntityCollection
And this might be an issue - You code might setting EntityCollection of Telerik insteed of CRM in your code first line.
EntityCollection entityCollection = serviceProxy.Re....
//update this to:
Microsoft.Xrm.Sdk.EntityCollection entityCollection = serviceProxy.Re....
It will force it to use the EntityCollection from Microsoft CRM sdk, which having Count method.