Private Sub Search_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Search.Click

        Dim a As String = search1.Text
        Dim b As String = search2.Text
        Dim c As String = search3.Text
        Dim d As String = search4.Text

        If a = "" And b = "" And c = "" And d = "" Then
        
        '************************
        'как вариант, более подходящий к топику
        'If a = "" Or b = "" Or c = "" Or d = "" Then
        'MsgBox("Fill all search text box")
        '************************
         
            MsgBox("Insert Criteria")
            Exit Sub
        Else
            If a <> "" Then a = " AND (osnova_table.col1 LIKE '" & search1.Text & "%')"
            If b <> "" Then b = " AND (osnova_table.col2 LIKE '" & search2.Text & "%')"
            If c <> "" Then c = " AND (osnova_table.col3 = '" & search3.Text & "')"
            If d <> "" Then d = " AND (osnova_table.col4 = '" & search4.Text & "')"

            Dim allunit As String = a & b & c & d
            If allunit.StartsWith(" AND") Then allunit = allunit.Remove( 0 ,  4 )

            OleDbDataAdapter1.SelectCommand.CommandText = "SELECT * FROM osnova_table WHERE " & allunit

        DataSet1.Clear()
        OleDbDataAdapter1.Fill(DataSet1.osnova_table)
        DataGrid1.CaptionText = "Search Table Count - " & DataSet1.osnova_table.Count

        End If

    End Sub