Question

问候。

因为，我是vb.net的新手，我正在尝试使用本地sql databsae创建一个简单的登录表单，并且当我将查询字符串传递给我时，对象引用未设置为对象的实例时出现错误命令文本如下所示：

Public con As SqlConnection = Nothing
Public cmd, cmd1 As SqlCommand
Public sqlrdr As SqlDataReader = Nothing
Public sqladp As SqlDataAdapter
Public ds As DataSet
Public dtbl As DataTable
Public query As String

我在模块中定义了这些，下面的代码是登录形式

 Dim frmHomesMarine As New HomesMarine
Dim connectionString As String = ConfigurationManager.ConnectionStrings("MyConn").ConnectionString
 Dim txtUserId = Trim(UsernameTextBox.Text)
 Dim txtUserPwd = Trim(PasswordTextBox.Text)
 Try
        MessageBox.Show("Hello", "", MessageBoxButtons.OK, MessageBoxIcon.Warning)
        con = New SqlConnection(connectionString)
        con.Open()
        query = "SELECT UserId, UserPassword FROM TBLUsers WHERE UserId ='" & txtUserId & "';"
        cmd.CommandText = query
        cmd.Connection = con
        sqladp.SelectCommand = cmd
        sqlrdr = cmd.ExecuteReader()
        While sqlrdr.Read
            If sqlrdr.HasRows Then
                MessageBox.Show("Table has some rows", "", MessageBoxButtons.OK, MessageBoxIcon.Warning)
            End If
        End While

    Catch ex As Exception
        MessageBox.Show(ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End Try

我从app.config文件中获取了连接字符串

<?xml version="1.0" encoding="utf-8" ?>
 <configuration>
<configSections>
</configSections>
<connectionStrings>
    <add name="MyConn"
        connectionString="Data Source=(LocalDB)\v11.0;AttachDbFilename=|DataDirectory|\dbHomesMarine.mdf;Integrated Security=True;Connect Timeout=30"
        providerName="System.Data.SqlClient" />
</connectionStrings>
<startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
</startup>

我用用户友好名称“MyConn”更改了名称任何人都知道错误的原因

vb.net throws对象引用未设置为对象的实例作为错误

0 个答案: