Linq on DataTable

i have to write linq query on a datatable.

i need to write code like below

Dim lobjDataRows = From lobjDataRow In lobjRelationsTable.Rows
                   Where String.Equals(CType(lobjDataRow, DataRow).Item("RevName"), "DocRev1")
                   Select CType(lobjDataRow, DataRow)

i am not able to write code like below

Dim lobjDataRows = From lobjDataRow As DataRow In lobjRelationsTable.Rows
                   Where String.Equals(lobjDataRow.Item("RevName"), "DocRev1")
                   Select lobjDataRow

Why we cant specify data type after lobjDataRow ?

so what to do to avoid many ctype in fisrt linq queries??