when I want to change parameters in column I have to write the index of the column and not the name

for example
this code work good


Dim dtchild As DataTable = New DataTable
dtchild.Columns.Add("cID", GetType(System.Int32))
dtfather.Columns(1).Unique = True

but here I get error


Dim dtchild As DataTable = New DataTable
dtchild.Columns.Add("cID", GetType(System.Int32))
dtfather.Columns("cID").Unique = True

the Same problem with dataTable

this code is OK

For Each ro In ds.Tables(0).Rows

and here I got error

For Each ro In ds.Tables("dtfather").Rows

how can I use the name and not the indexes?

thanks alot