Question

我有以下c＃代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace StringTest
{
    class Program
    {
        static void Main(string[] args)
        {

               String strSQLCode;
            strSQLCode = " select rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * "
                      += " from view_dg_game_details gd (nolock) "
                      += " where gd.gametypeid = {0} " 
                      += " and gd.numberofrounds = {1} "
                      += " and gd.gamevalues = '{2}' ";
        }
    }
}

由于某种原因，我收到错误“作业的左侧必须是变量，属性或索引器”。

我无法看到错误告诉我的错误。我已经注释掉了违规行，但错误只是向上移动了一行。

我可以使用此方法使字符串concation工作：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace StringTest
{
    class Program
    {
        static void Main(string[] args)
        {

            String strSQLCode;
            strSQLCode = " select rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * ";
            strSQLCode = strSQLCode +  " from view_dg_game_details gd (nolock) ";
            strSQLCode = strSQLCode + " where gd.gametypeid = {0} ";
            strSQLCode = strSQLCode + " and gd.numberofrounds = {1} ";
            strSQLCode = strSQLCode + " and gd.gamevalues = '{2}' ";            
        }
    }
}

有人可以向我解释这个错误是什么吗？

由于

肯

Answer 1

因为你不能在不重复你正在操作的变量的情况下将+=运算符串在一起：

strSQLCode = @"select rank() over (order by percentagecorrect desc, 
                                            totalmilliseconds asc) as rank, * ";
strSQLCode += " from view_dg_game_details gd (nolock) ";
strSQLCode += " where gd.gametypeid = {0} ";
strSQLCode += " and gd.numberofrounds = {1} ";
strSQLCode += " and gd.gamevalues = '{2}' ";

如果您想将其声明为“长”单行，请使用+

strSQLCode = @"select rank() over (order by percentagecorrect desc, 
                                            totalmilliseconds asc) as rank, * "
           + " from view_dg_game_details gd (nolock) "
           + " where gd.gametypeid = {0} " 
           + " and gd.numberofrounds = {1} "
           + " and gd.gamevalues = '{2}' ";

或者，如果你不想要任何一个，你可以只使用一个字符串文字：

strSQLCode = 
    @"select rank() over (order by percentagecorrect desc, 
                                totalmilliseconds asc) as rank, *
      from view_dg_game_details gd (nolock)
      where gd.gametypeid = {0}
          and gd.numberofrounds = {1}
          and gd.gamevalues = '{2}' ";

Answer 2

这是一个单一陈述，因此您应该使用以下内容：

        strSQLCode = " select rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * "
                  + " from view_dg_game_details gd (nolock) "
                  + " where gd.gametypeid = {0} " 
                  + " and gd.numberofrounds = {1} "
                  + " and gd.gamevalues = '{2}' ";

Answer 3

对于您的第一个代码段，您想要的是+，而不是+=。

您只想分配一次变量，然后以正常方式将所有部分连接在一起后执行此操作。这是+。

Answer 4

在您的第一个代码段中，您不应该使用+=一个简单的代码+

来自MSDN：

使用+ =赋值运算符的表达式，例如

x += y

相当于

x = x + y

，但x只评估一次。

这意味着你不能使用+=来连接一串字符串文字或两个以上的变量。

Answer 5

你正在写

something += "a" += "b";

这没有意义。

Answer 6

只需按照这种方式使用

strSQLCode = " select rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * "
            + " from view_dg_game_details gd (nolock) "
            + " where gd.gametypeid = {0} "
            + " and gd.numberofrounds = {1} "
            + " and gd.gamevalues = '{2}' ";

或

strSQLCode = 
          @"select rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * 
            from view_dg_game_details gd (nolock) 
            where gd.gametypeid = {0} 
            and gd.numberofrounds = {1} 
            and gd.gamevalues = '{2}' ";

Answer 7

您的语法略有错误。

应该是：

namespace StringTest
{
    class Program
    {
        static void Main(string[] args)
        {

               String strSQLCode;
            strSQLCode = @" select rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * "
                      + @" from view_dg_game_details gd (nolock) "
                      + @" where gd.gametypeid = {0} " 
                      + @" and gd.numberofrounds = {1} "
                      + @" and gd.gamevalues = '{2}' ";
        }
    }
}

Answer 8

你正在做的是：

string variable = "string" += "another string";

这基本上与：

相同

string variable;
(variable = "string") += "another string";

因为括号表达式的结果是一个字符串（特别是已分配的值），所以您现在可以有效地执行此操作：

string variable;
variable = "string";
"string" += "another string;

编译器第三行出了问题。

具体来说，编译器告诉你的是，为了执行赋值，你必须要分配给它。

像这样写：

strSQLCode = @" select rank() over (order by percentagecorrect desc, totalmilliseconds   asc) as rank, *
    from view_dg_game_details gd (nolock) 
    where gd.gametypeid = @gameType 
    and gd.numberofrounds = @numberOfRounds
    and gd.gamevalues = @gameValues ";

并使用参数化查询。

Answer 9

像其他人一样提到+ =应该是+。如果构建SQL至少参数化它，SQL注入是一个严重的问题。我可以从您的数据库中的控制台或winapp文本框删除表。从第一个变量你可以做到

 1 ; drop table dg_game_details --

示例：

conDatabase =
new SqlConnection("Data Source=(local);" +
"Database='projectGames';" +
"Integrated Security=true");
SqlCommand cmdDatabase =
new SqlCommand("SELECT rank() over (order by percentagecorrect desc, totalmilliseconds asc) as rank, * FROM view_dg_game_details gd (nolock)" +
"WHERE gd.gametypeid= @GameId;", conDatabase);

cmdDatabase.Parameters.Add("@GameId", SqlDbType.Int);
cmdDatabase.Parameters["@GameId"].Value = 1;

Answer 10

使用+代替+ =。

另外，我强烈建议不要存储和连接这样的SQL查询，因为这样会因为SQL注入而非常不安全。

在此处阅读：SQL injection

C＃String Concatenation问题为什么不是+ =在这里工作？

10 个答案: