Get max value from specific column in datagridvied

[kareninstructor]

Try this

<Runtime.CompilerServices.Extension()> _
Public Function GetMinValueIdentifier(
    ByVal sender As DataGridView,
    ByVal ColumnName As String, ByVal IdentifierColumn As String) _
As Long

    Dim MinValue =
       ( 
          From T In sender.Rows.Cast(Of DataGridViewRow)()
          Where Not T.IsNewRow
          Select CLng(T.Cells(ColumnName).Value)
       ).Min

    Dim Identifier =
       ( 
          From T In sender.Rows.Cast(Of DataGridViewRow)()
          Where Not T.IsNewRow AndAlso
                    T.Cells(ColumnName).Value.ToString = CStr(MinValue)
          Select CLng(T.Cells(IdentifierColumn).Value)
       ).FirstOrDefault

    Return Identifier

End Function