Please note that modifying the FilterExpression is quite tricky and it can be error-prone, therefore, it is not recommended.
Instead, you can use the FireCommandEvent method to achieve the requested functionality:
For a client-side approach, the method is called fireCommand:
For example, in case you want to apply an initial filter, you can use the following approach:
sender, EventArgs e)
GridFilteringItem filterItem = RadGrid1.MasterTableView.GetItems(
GridColumn col = RadGrid1.MasterTableView.GetColumn(
In addition, I'm attaching a sample to demonstrate how you can build your custom filter expression in case you need to filter on multiple columns at once.
Hope this helps.