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为什么环境提示cmd.ExecuteNonQuery()----提示将参数值从 WindowsPrincipal 转换到 String 失败。

<%@ Page Language="VB" %>
<%@ Import Namespace="System.Data" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
Dim usern As String = Trim(txt1.Text)
Dim mima As String = Trim(txt2.Text)
Dim sconstr As String = System.Configuration.ConfigurationManager.AppSettings("scon")
Dim con As SqlClient.SqlConnection = _
New SqlClient.SqlConnection(sconstr)
Dim sql As String = " insert into test(usern,mima) values(" + usern + "," + mima + ")"
Dim cmd As SqlClient.SqlCommand = _
New SqlClient.SqlCommand(sql, con)
con.Open()
cmd.Parameters.Add(New SqlClient.SqlParameter("@usern", SqlDbType.VarChar, 50)).Value = User
cmd.Parameters.Add(New SqlClient.SqlParameter("@mima", SqlDbType.VarChar, 50)).Value = mima
cmd.ExecuteNonQuery()----为什么这里提示将参数值从 WindowsPrincipal 转换到 String 失败。
lab2.Text = "注册成功"
con.Close()
End Sub