Question

我正在尝试根据变量创建DataReader。我需要根据选择TextBoxes中的哪个节点填充一系列TreeView，并且父节点数据与子节点数据不同。所以，我写了这段代码：

using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["PBRConnectionString"].ConnectionString))
{
    try
    {
        conn.Open();
        if (TreeViewAccts.SelectedNode.Parent == null)
        {
            // At the parent level, we filter by Account Group ID
            SqlCommand cmd = new SqlCommand(@"Select [ACCT_GRP_PK], [ACCT_GRP], 'X' as [DOS], [ACTIVE_FLG], [LOAD_BY], [LOAD_TIMESTAMP], [UPDT_BY], [UPDT_TIMESTAMP], [A_FLG_UPDT_BY], [A_FLG_UPDT_TIMESTAMP] from [ACCT_GRP_LIST] where [ACCT_GRP_PK] = @AcctID ORDER BY [ACCT_GRP] ASC", conn);

            cmd.Parameters.AddWithValue("@AcctID", Convert.ToInt32(TreeViewAccts.SelectedValue.ToString()));
        }
        else
        {
            // At the child level, we filter by Account ID
            SqlCommand cmd = new SqlCommand(@"Select [ACCT_PK], [ACCT_NUM], [DOS], [ACTIVE_FLG], [LOAD_BY], [LOAD_TIMESTAMP], [UPDT_BY], [UPDT_TIMESTAMP], [A_FLG_UPDT_BY], [A_FLG_UPDT_TIMESTAMP] from [ACCT_LIST] where [ACCT_PK] = @AcctID ORDER BY [ACCT_NUM] ASC", conn);

            cmd.Parameters.AddWithValue("@AcctID", Convert.ToInt32(TreeViewAccts.SelectedValue.ToString()));
        }

        //Here is the trouble
        using (SqlDataReader reader = cmd.ExecuteReader())
        while (reader.Read())
        {
            txtID.Text = reader.GetByte(0).ToString();
            txtName.Text = reader.GetByte(1).ToString();
            txtDOS.Text = reader.GetByte(2).ToString();
            txtFlag.Text = reader.GetByte(3).ToString();
            txtLoadedBy.Text = reader.GetByte(4).ToString();
            txtLoadedOn.Text = reader.GetByte(5).ToString();
        }

问题是，在这一行：

using (SqlDataReader reader = cmd.ExecuteReader())

它告诉我

'cmd'在当前上下文中不存在。

我假设是因为它位于定义if的{{1}} / else块之外。

我该如何做到这一点？

Answer 1

您需要在if语句之外设置命令，否则该对象指针只在if块和else块中具有范围。然后，您可以在块中分配它。

using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["PBRConnectionString"].ConnectionString))
{
    try
    {
        conn.Open();
        SqlCommand cmd = null; // declare it here
        if (TreeViewAccts.SelectedNode.Parent == null)
        {
            // At the parent level, we filter by Account Group ID
            cmd = new SqlCommand(@"Select [ACCT_GRP_PK], [ACCT_GRP], 'X' as [DOS], [ACTIVE_FLG], [LOAD_BY], [LOAD_TIMESTAMP], [UPDT_BY], [UPDT_TIMESTAMP], [A_FLG_UPDT_BY], [A_FLG_UPDT_TIMESTAMP] from [ACCT_GRP_LIST] where [ACCT_GRP_PK] = @AcctID ORDER BY [ACCT_GRP] ASC", conn);

            cmd.Parameters.AddWithValue("@AcctID", Convert.ToInt32(TreeViewAccts.SelectedValue.ToString()));
        }
        else
        {
            // At the child level, we filter by Account ID
            cmd = new SqlCommand(@"Select [ACCT_PK], [ACCT_NUM], [DOS], [ACTIVE_FLG], [LOAD_BY], [LOAD_TIMESTAMP], [UPDT_BY], [UPDT_TIMESTAMP], [A_FLG_UPDT_BY], [A_FLG_UPDT_TIMESTAMP] from [ACCT_LIST] where [ACCT_PK] = @AcctID ORDER BY [ACCT_NUM] ASC", conn);

            cmd.Parameters.AddWithValue("@AcctID", Convert.ToInt32(TreeViewAccts.SelectedValue.ToString()));
        }

Answer 2

来自MSDN：

如果在块构造（如If语句）中声明变量，则该变量的作用域仅在块结束之前。生命周期一直持续到程序结束。

问题是您在cmd块的范围内定义了if-else变量，因此在这些块之外不知道它。解决方案是在cmd块中声明if-else。像这样：

try
{
    SqlCommand cmd = null;
    ....
    if (TreeViewAccts.SelectedNode.Parent == null)
    { 
        cmd = new SqlCommand(@"Select [ACCT_GRP_PK], [ACCT_GRP], 'X' as [DOS], [ACTIVE_FLG], [LOAD_BY], [LOAD_TIMESTAMP], [UPDT_BY], [UPDT_TIMESTAMP], [A_FLG_UPDT_BY], [A_FLG_UPDT_TIMESTAMP] from [ACCT_GRP_LIST] where [ACCT_GRP_PK] = @AcctID ORDER BY [ACCT_GRP] ASC", conn);
        ....
    }
    else
    {
         // At the child level, we filter by Account ID
         cmd = new SqlCommand(@"Select [ACCT_PK], [ACCT_NUM], [DOS], [ACTIVE_FLG], [LOAD_BY], [LOAD_TIMESTAMP], [UPDT_BY], [UPDT_TIMESTAMP], [A_FLG_UPDT_BY], [A_FLG_UPDT_TIMESTAMP] from [ACCT_LIST] where [ACCT_PK] = @AcctID ORDER BY [ACCT_NUM] ASC", conn);
         ....
    }
}

基于变量创建DataReader

2 个答案: