StringBuilder 实例中的运行时错误

Question

请帮助我理解这段代码有什么问题。 （我正在尝试构建一个字符串，从文本文件中逐行获取其中的一部分）。

我在 strbuild.Append(str); 行上收到运行时错误“在对象引用的实例中未设置为对象”

        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

看一下这些行：

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

当您调用 strbuild.Append(...) 时，您期望会发生什么？为什么要将 strbuild 设置为 null？

您似乎喜欢两行变量初始化 - 这是另一个示例：

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

这将更容易阅读，因为：

string str = reader.ReadLine();

(ReadLine 已经返回一个字符串，因此您不需要调用 ToString() 结果。）

但是，我确实建议您对 StreamReader 使用 using 语句 - 否则当抛出异常，您将使读者保持开放状态。

TextReader.ReadLine() 的一个好处是，完成后它会返回 null。您不需要先查看然后再阅读。

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来获取字符。所以，你可以：

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

您在初始化后将 stringbuilder 设置为 null。

更改

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

为

StringBuilder strbuild = new StringBuilder(); 

省略该行

strbuild = null;

Answer 3

改为

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

or

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

，可以防止此类错误：

  StringBuilder strbuild = new StringBuilder();

Answer 4

您的示例中有很多错误，这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());