Question

to check if a string exists in some column i use something like

mydatatable.AsEnumerable().Any(Function(r) r.Field(Of String)("somecolumn") = "somestring")

but how can i find the row index of "somestring"? considering its allowed to exist only once in mydatatable , and what if it existed more than once?

Answer 1

You can use the overload that passes the index:

Dim rows = myDataTable.AsEnumerable().
    Select(Function(r, i) New With {.Row = r, .Index = i}).
    Where(Function(x) x.Row.Field(Of String)("somecolumn") = "somestring")
If rows.Any() Then
    Dim firstIndex As Int32 = rows.First.Index
End If