主题：[求助]网络编程的自动编号问题

Dim bh As String
Dim dr As DataRow = e.DataRow
Dim pf As String
Dim max As String
Dim cmd As New SQLCommand
Dim dt As DataTable

pf = Format(dr("Riqi"),"yyMMdd") '获得编号的前缀

cmd.C
cmd.commandText = "Select top 1 Danhao,* From Bill_Shou Where Danhao Like  '" & pf & "*'"
dt = cmd.ExecuteReader
dr = dt.DataRows(0)
If dr("Danhao") Is Nothing Then
    bh = Format(dr("Danhao"),"yyMMdd001")
Else
    max = dr("Danhao")
    max = cint(max.Substring(7,3)) + 1
    bh = pf & format(max,"000")
End If

e.DataRow("Danhao") = bh

以上代码目的是如果是今天的第一单，单号就是121019001，第二单是121019002，依次类推。

运行报错，错误代码为

Exception has been thrown by the target of an invocation.