in ASP net 2.0
a function that query/add item to cache:
public function1
... ... ..
If HttpContext.Current.Cache(myKey) Is Nothing Then
     Dim ds As DataSet = functionToCreateDataset.... ...
      HttpContext.Current.Cache.Insert(myKey, ds, Nothing)
End If
Return HttpContext.Current.Cache

in the web form code behind, I have:
Dim ds As DataSet = Cache("myKey")
dim ds as dataset
ds.Tables(0).Columns.Add("customCol", GetType(String), "col1+'- '+col2")

the problem is, when "function1"  create the cache the first time,  the
statement:
ds.Tables(0).Columns.Add("col1", GetType(String), "col1+'- '+col2")
works fine. But when "function1" returned the cached item, the same
statement display the error:
"A column named 'customCol' already belongs to this DataTable."

it looks like  the cache get changed too when the dataset derived from it is
changed.
how can I fix that ?