[stax76]

How does this look then (besides from being basic code ;-) ?:

Case "dga"
    Using f = File.OpenText(p.SourceFile)
        If f.ReadLine().StartsWith("DGAVCIndexFile") Then
            p.AvsDoc.SetFilter("Source", "AVCSource", "AVCSource(""%source_file%"")")
        Else
            p.AvsDoc.SetFilter("Source", "DGMultiSource", "DGMultiSource(""%source_file%"")")
        End If
    End Using
Case "dgv", "dgm"
    p.AvsDoc.SetFilter("Source", "DGMultiSource", "DGMultiSource(""%source_file%"")")