如何在vb中正确连接到数据库

Question

我需要有关我的代码和/或连接的帮助。我收到一条错误消息：

System.Data.dll 中出现“System.InvalidOperationException”类型的未处理异常

附加信息：ExecuteReader 需要一个打开且可用的连接。连接的当前状态为关闭。

在此代码中：（第 4 行）

cmd.Connection = cn
    cmd.CommandText = "SELECT ID, LastName, FirstName FROM tblMembers WHERE ID = @ID"
    cmd.Parameters.Add(New SqlClient.SqlParameter("@ID", SqlDbType.Int)).Value = dg1.Item(0, e.RowIndex).Value
    dr = cmd.ExecuteReader
    If dr.HasRows() Then
        With dr
            .Read()
            txtLname.Text = dr("LastName")
            txtFname.Text = dr("FirstName")
        End With
    End If

但是如果我尝试打开连接。会发生这种情况。

System.Data.dll 中出现“System.InvalidOperationException”类型的未处理异常

附加信息：连接未关闭。连接的当前状态为打开。

使用此代码：（第 1 行）

 cn.Open()
    cmd.Connection = cn
    cmd.CommandText = "SELECT ID, LastName, FirstName FROM tblMembers WHERE ID = @ID"
    cmd.Parameters.Add(New SqlClient.SqlParameter("@ID", SqlDbType.Int)).Value = dg1.Item(0, e.RowIndex).Value
    dr = cmd.ExecuteReader
    If dr.HasRows() Then
        With dr
            .Read()
            txtLname.Text = dr("LastName")
            txtFname.Text = dr("FirstName")
        End With
    End If

Answer 1

    Dim cn As SqlConnection
    Dim cmd As SqlCommand
    Dim dr As SqlDataReader
    Dim connetionString As String
    Dim Process_Id As String
    Dim Processname As String

    connetionString = "Data Source=ServerName;Initial Catalog=DataBaseName;User ID=UserName;Password=PassWord"

    cn = New SqlConnection(connetionString)

    cn.Open()

    cmd = New SqlCommand("SELECT Process_Id,Processname FROM  dbo.Process_Master where Process_Id = @ID", cn)

    cmd.Parameters.Add(New SqlClient.SqlParameter("@ID", SqlDbType.Int)).Value = "21"

    dr = cmd.ExecuteReader

    If dr.HasRows() Then

        With dr.Read()

            Process_Id = dr("Process_Id")
            Processname = dr("Processname")

        End With
    End If