StringBuilder实例中的运行时错误

Question

请帮我理解，这段代码有什么问题。 （我试图建立一个字符串，从文本文件中逐行读取它的一部分）。

我得到一个运行时错误“在不设置到对象的对象引用的实例”上线strbuild.Append(str);

 StreamReader reader = new StreamReader("buf.txt", System.Text.Encoding.ASCII); 
     StringBuilder strbuild = new StringBuilder(); 
     strbuild = null; 

     while (reader.Peek() >= 0) 
     { 
      string str = null; 
      str = reader.ReadLine().ToString(); 

      string segment = str.Substring(0, 1); 

      if (segment == "A") 
      { 
       strbuild.Append(str); //here i get an error 
      } 
      else if (segment == "B") 
      { 
       strbuild.Append("BET"); 
      } 

     } 
     printstr = strbuild.ToString(); 
     reader.Close(); 

     MessageBox.Show(printstr); 

Answer 1

看看这些行：

StringBuilder strbuild = new StringBuilder(); 
strbuild = null; 

你是什么预计发生时，然后你打电话strbuild.Append(...)？为什么你将strbuild设置为空？

你似乎喜欢两行变量初始化 - 这里是另一个例子：

string str = null; 
str = reader.ReadLine().ToString(); 

这将是更容易阅读的只是：

string str = reader.ReadLine(); 

（ReadLine已经返回一个字符串，所以你不需要致电ToString()的结果。）

但是，我做建议您为StreamReader使用using声明 - 否则，当抛出异常时，您将打开阅读器。

TextReader.ReadLine()的一个好处是，它完成后返回null。你不需要偷看，然后然后阅读。

最后，如果您只测试单个字符，则不需要子字符串 - 只需使用字符串索引器即可获取字符。所以，你可以有：

StringBuilder builder = new StringBuilder(); 

// Consider using File.OpenText 
using (StreamReader reader = new StreamReader("buf.txt", Encoding.ASCII)) 
{ 
    string line; 
    // Normally side-effect + test is ugly, but this is a common and 
    // neat idiom 
    while ((line = reader.ReadLine()) != null) 
    { 
     // TODO: What do you want to happen for empty lines? 
     char segment = str[0]; 
     if (segment == 'A') 
     { 
      builder.Append(line); 
     } 
     else if (segment == 'B') 
     { 
      builder.Append("BET"); 
     } 
    } 
} 
MessageBox.Show(builder.ToString()); 

Answer 2

您初始化后的StringBuilder设置为NULL。

变化

StringBuilder strbuild = new StringBuilder(); 
strbuild = null; 

到

StringBuilder strbuild = new StringBuilder(); 

离开了行

strbuild = null; 

Answer 3

变化

StringBuilder strbuild = new StringBuilder(); 
    strbuild = null; 

到

StringBuilder strbuild = null; 
    strbuild = new StringBuilder(); 

，或者防止这种错误：

StringBuilder strbuild = new StringBuilder(); 

Answer 4

有一些很多错误在你的例子中，这里是第一个更正的版本：

StringBuilder strbuild = new StringBuilder(); 

// Put resource into using statements, for deterministic cleanup 
using (TextReader reader = new StreamReader("buf.txt", System.Text.Encoding.ASCII)) 
{ 
    string line; 

    //Maybe looks a little ugly the first time, but is commonly used to 
    //process all the lines of a file (.ReadToEnd() can cause memory problems 
    //on really big files) 
    while ((line = reader.ReadLine()) != null) 
    { 
     //Instead of if, else if, else if, etc just take a switch 
     //statement. Makes it much easier to read. 
     switch (line[0]) 
     { 
      //If you need case insensitivity put both versions of the character 
      //before your first line of code 
      case 'A': 
      case 'a': 
       strbuild.Append(line); 
       break; 
      //Otherwise just use the lower or upper case version you like 
      case 'B': 
       strbuild.Append("BET"); 
       break; 
     } 
    } 
} 

//Put the result of the StringBuilder directly into the Show() function 
MessageBox.Show(strbuild.ToString());