Description: if you filter a text column with "Does not contains" operator, the produced FilterDescriptors.Expression is "ProductName NOT LIKE '%c%' OR ProductName IS NULL". However, if you try to programmatically add this expression to the RadGridView.FilterDescriptors.Expression property it doesn't filter the grid.

 Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load 
     Me.ProductsTableAdapter.Fill(Me.NwindDataSet.Products)
     Me.RadGridView1.EnableFiltering = True 
 End Sub
 
 Private Sub RadButton1_Click(sender As Object, e As EventArgs) Handles RadButton1.Click
     Me.RadGridView1.FilterDescriptors.Expression = "ProductName NOT LIKE '%c%' OR ProductName IS NULL"
 End Sub

Workaround: don't set expression but add a FilterDescriptor: http://docs.telerik.com/devtools/winforms/gridview/filtering/setting-filters-programmatically-(simple-descriptors)


     Dim filter1 As New FilterDescriptor()
     filter1.[Operator] = FilterOperator.NotContains
     filter1.Value = "c"
     filter1.IsFilterEditor = True
     filter1.PropertyName = "ProductName"
     Me.RadGridView1.FilterDescriptors.Add(filter1)