使用getstring方法的问题（c＃） [英] Problem in using the getstring method (c#)

问题描述

我在项目中有下一行。



rdr = scomm.ExecuteReader（）;

log.Debug（log 1+ rdr.Read（）++ rdr.GetString（0）++ String.ReferenceEquals（rdr.GetString（0），SUCCES））;



这是日志文件中的返回log 1True SUCCES False。



我不明白为什么String.ReferenceEquals（rdr.GetString（0），SUCCES）不是真的。



提前谢谢！



我的尝试：



我试图验证sql命令执行的结果。

解决方案

参见Object.ReferenceEquals方法（对象，对象）（系统） [ ^ ] 。

还看看这个CodeProject文章：比较.NET中的平等价值：身份和对等 [ ^ ]。

不要做那样的事情！

问题是你不能控制执行顺序：编译器（更糟糕的是，优化器）可以自由评估该字符串的元素按其选择的顺序排列 - 并且它不知道需要首先调用Read方法。

尝试明确订购操作：

 rdr = scomm.ExecuteReader（）; 
  bool  isData = rdr.Read（）; 
  string  s1 = rdr.GetString（ 0 ）; 
  string  s2 =  String  .ReferenceEquals（rdr.GetString（ 0 ），  SUCCES）; 
 log.Debug（  log 1 + isData +   + s1 +   + s2）; 

至于为什么它们不是同一个引用，字符串的实习池只适用于字符串文字 - 而不是在编译时未定义的字符串。试试这个：

  private   string  GC （ string  s1， string  s2）
 {
  return  s1 + s2; 
} 
 ... 
  string  s1 = GC（  hello，  there）; 
  string  s2 = GC（  hello ，  there）; 
  string  s3 =  你好那里< /跨度>; 
  string  s4 =  你好那里< /跨度>; 
 
 Console.WriteLine（  {0}：{1}， s1 == s2， string  .ReferenceEquals（s1，s2））; 
 Console.WriteLine（  {0}：{1}，s1 == s3 ， string  .ReferenceEquals（s1，s3））; 
 Console.WriteLine（  {0}：{1}，s3 == s4 ， string  .ReferenceEquals（s3，s4））; 

你会得到：

 True：False 
 True：False 
 True：True 

I have in the project the next line.

rdr = scomm.ExecuteReader();
log.Debug("log 1" + rdr.Read() + " " + rdr.GetString(0) + " " + String.ReferenceEquals(rdr.GetString(0), "SUCCES"));

and this is return in the logfile log 1True SUCCES False.

I do not understand why "String.ReferenceEquals(rdr.GetString(0), "SUCCES")" is not true.

Thank you in advance!

What I have tried:

I tried to verify the result of the sql command execution.

解决方案

See Object.ReferenceEquals Method (Object, Object) (System)[^].

Have also a look at this CodeProject article: Comparing Values for Equality in .NET: Identity and Equivalence[^].

Don't do things like that!
The problem is that you don't control the execution order: the compiler (and worse the optimiser) is at liberty to evaluate the "elements" of that string in any order it chooses - and it doesn't know that the Read method needs to be called first.
Try explicitly ordering the operations:

rdr = scomm.ExecuteReader();
bool isData = rdr.Read();
string s1 = rdr.GetString(0);
string s2 = String.ReferenceEquals(rdr.GetString(0), "SUCCES");
log.Debug("log 1" + isData + " " + s1 + " " + s2);

As to why they aren't the same reference, the intern pool for strings is only applied to string literals - not to strings that aren't defined at compile time. Try this:

        private string GC(string s1, string s2)
            {
            return s1 + s2;
            }
...
            string s1 = GC("hello", " there");
            string s2 = GC("hello", " there");
            string s3 = "hello there";
            string s4 = "hello there";

            Console.WriteLine("{0}:{1}", s1 == s2, string.ReferenceEquals(s1, s2));
            Console.WriteLine("{0}:{1}", s1 == s3, string.ReferenceEquals(s1, s3));
            Console.WriteLine("{0}:{1}", s3 == s4, string.ReferenceEquals(s3, s4));

And you will get:

True:False
True:False
True:True

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