Hello,

Thank you for contacting us and for the provided sample.

In the future could I ask you to be more specific with your question, this will help us to save time and efforts to find and investigate  the issue. This will be highly appreciated.

Now about it, I have noticed that you are using the Columns collection without referring it from GridView. My suggestion is to add GridView and now on rebind the ViewState error will not be thrown.

You can find the modified sample attached to my reply.

Code snippet:

this.WebHierarchicalDataGrid1.GridView.Columns.Add(col2);

.....

this.WebHierarchicalDataGrid1.GridView.Columns.Clear();

....

this.WebHierarchicalDataGrid1.DataKeyFields = this.WebHierarchicalDataGrid1.GridView.Columns["UniqueID"].Key;

...