Question

类型“String”的值无法转换为“System.Data.SqlClient.SqlConnection”

谁能帮我解决？
类型“String”的值无法转换为“System.Data.SqlClient.SqlConnection”
Imports System.Data.SqlClient

Public Class Form2

Dim cnn As SqlConnection '声明这些SQL的类

Private Property SqlConnection As SqlConnection

Private Sub Form2_Load(sender As Object, e As EventArgs) Handles Me.Load

End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

''cnn = New SqlConnection("Initial Catalog=学生成绩管理 ;User ID=sa;password=sasa")
' ''cnn.Open()
Dim cnn As String = "Server=.\SQLEXPRESS;AttachDbFiename=E:\学生成绩管理\学生成绩管理\学生成绩管理.mdf;Integrated Security=Ture;User Instance=Ture;"
Using connection As New SqlConnection(cnn)
connection.Open()

Dim mystry As String = "Select 密码 from 用户 Where 用户名='" & TextBox1.Text & "'"

Dim myadapter As New SqlCommand(mystry, cnn)//这块有错误

Dim myReader As SqlDataReader = myadapter.ExecuteReader

While (myReader.Read = True)
If Val(myReader(0)) = Val(TextBox2.Text) Then

If RadioButton1.Checked = True Then

If Val(myReader(0)) = Val(TextBox2.Text.Substring(0, 3)) Then

Answer 1

代码中，你一共定义了两个cnn
1、Dim cnn As SqlConnection '声明这些SQL的类
2、Dim cnn As String = "Server=.\SQLEXPRESS;AttachDbFiename=E:\学生成绩管理\学生成绩管理\学生成绩管理.mdf;Integrated Security=Ture;User Instance=Ture;"
在出错的语句中第二个cnn定义生效，是String类型，非SqlConnection，故报错。

改成connection试试。

Answer 2

Dim cnn As String 最后这个Stringj是不是string呢，C#里是小写的。