Question

为什么只有else条件运行？ Postalcode列为float，City列为nvarchar。我认为失败的是字符串可能是错误的。

private void txt_st_postalcode_Leave(object sender, EventArgs e)
{        
    using (var connection = new SqlConnection("Data Source=mypublicip\\SQLEXPRESS2017;Initial Catalog=studentreg; User = myusername; Password=mypassword;"))
    {
        connection.Open();

        using (var command = new SqlCommand("SELECT City FROM Cities WHERE Postcode=@Postcode", connection))
        {
            command.Parameters.AddWithValue("@Postcode", "10101");

            using (var reader = command.ExecuteReader())
            {
                string txt_st_postalcode = reader.Read() ?
                reader[1] as string : ("City");

                if (reader.Read())
                {
                    txt_st_city.Text = reader.GetString(reader.GetOrdinal("City"));
                }
                else
                {
                    MessageBox.Show("Sh*t!");
                }
            }
        }
    }
}

Answer 1

不太确定这些邮政编码是如何工作的（英国不同），但鉴于您使用ExecuteReader，您似乎期待多个结果。正如评论所指出的那样，您目前正在阅读两次结果;但是，你应该有某种形式的循环;例如：

using (var command = new SqlCommand("SELECT City FROM Cities WHERE Postcode=@Postcode", connection))
{
    command.Parameters.AddWithValue("@Postcode", "10101");
    using (var reader = command.ExecuteReader())
    {
        while (reader.Read())
        {
            string txt_st_postalcode = reader[0] as string;

            //txt_st_city.Text = reader.GetString(reader.GetOrdinal("City"));
            // Depends what you're doing here?
        }
    }
}

如果您只期望单个结果，请尝试使用ExecuteScalar()，这将返回单个结果;例如：

using (var command = new SqlCommand("SELECT City FROM Cities WHERE Postcode=@Postcode", connection))
{
    command.Parameters.AddWithValue("@Postcode", "10101");
    txt_st_city.Text = command.ExecuteScalar();
}

Answer 2

首先，如果您需要从阅读器阅读PostCode，您必须先选择。因此，请更改您的查询。

using (var command = new SqlCommand("SELECT City, PostCode FROM Cities WHERE Postcode=@Postcode", connection))

其次，在获取数据之前每行调用一次read。

if (reader.Read())
{
    txt_st_postalcode .Text = reader.GetString(reader.GetOrdinal("PostCode"));
    txt_st_city.Text = reader.GetString(reader.GetOrdinal("City"));
}
else
{
    MessageBox.Show("Sh*t!");
}

如果条件，C＃ExecuteReader将失败

2 个答案: