C# 字符串连接问题为什么 += 不能在这里工作？

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}' ";
        }
    }
}

出于某种原因，我收到错误消息“赋值的左侧必须是变量、属性或索引器”。

我看不出错误试图告诉我什么。我已经注释掉了有问题的行，但错误只是向上移动了一行。

我可以使用此方法使字符串连接正常工作:

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}' ";            
        }
    }
}

谁能给我解释一下这个错误是怎么回事？

谢谢

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}' ";

如果你想将它声明为“长”单行，只需使用 +

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，如果您不需要这些，您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}' ";