When you perform filtering its overrides the FilterExpression set into the Page_Load event. To achieve the desired functionality you need to concatenate the current FilterExpression with the @" AND ([ParentCategoryId] = 0)". For example:
sender, EventArgs e)
rgCategories.MasterTableView.FilterExpression = @
"([ParentCategoryId] = 0)"
rgCategories.MasterTableView.FilterExpression += @
" AND ([ParentCategoryId] = 0)"
Also I am sending you a simple example.
Additionally I noticed that you have opened a duplicate post on the same matter. Please, refer to the other support ticket post for additional information. To avoid duplicate posts, I suggest you continue the communication there.
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