如何从SqlDataReader返回单个值? [英] How can I return a single value from an SqlDataReader?

问题描述

我忘了在单层应用程序中返回值.

I forget to return value in single tier application.

public  int Studentid()
    {
        try
        {
            SqlConnection con = new SqlConnection(connectionStr);
            SqlCommand cmd = new SqlCommand("SELECT s_id FROM student where name = + ('" + Request.QueryString.ToString() + "')", con);
            con.Open();
            SqlDataReader dr = null;
            con.Open();
            dr = cmd.ExecuteReader();
            if (dr.Read())
            {
                //Want help hear how I return value
            }

            con.Close();
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

推荐答案

这是实现您所追求的目标的方法的版本.

Here is a version of your method that achieves what you're after.

public int GetStudentId()
{
    var sql = string.Format("SELECT s_id FROM student where name = '{0}'", Request.QueryString);
    using (var con = new SqlConnection(connectionStr))
    using (var cmd = new SqlCommand(sql, con))
    {
        con.Open();
        var dr = cmd.ExecuteReader();
        return dr.Read() ? return dr.GetInt32(0) : -1;
    }
}

当您除了重新抛出(异常抛出)(实际上您通过使用 throw ex; 而不是只需 throw; .另外，C# using 语句可帮助您以更少的代码行为您清理资源.

There's no need to use try/catch when you don't do anything with the exception except re-throw (and in fact you were losing the original stack trace by using throw ex; instead of just throw;. Also, the C# using statement takes care of cleaning up your resources for you in fewer lines of code.

重要

IMPORTANT

像这样直接将查询字符串传递到SQL，意味着任何人都可以在您的数据库中执行随机SQL，从而有可能删除所有内容(或更糟的是删除所有内容).阅读 SQL注入.

Passing the query string directly into SQL like that means that anyone can execute random SQL into your database, potentially deleting everything (or worse). Read up on SQL Injection.

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