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部分字符串。）

我收到运行时错误 “在对象引用的实例中未设置为对象” 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

看看以下几行：

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

当您致电strbuild.Append(...)时，您期待会发生什么？为什么要将strbuild设置为空？

你似乎喜欢两行变量初始化 - 这是另一个例子：

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

这更容易阅读：

string str = reader.ReadLine();

（ReadLine已经返回一个字符串，因此您无需在结果上调用ToString()。）

但是，我做建议您对using使用StreamReader语句 - 否则当抛出异常时，您将打开阅读器。< / p>

关于TextReader.ReadLine()的一个好处是它在你完成后返回null。你不需要偷看，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

初始化后，您将stringbuilder设置为null。

更改

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

到

StringBuilder strbuild = new StringBuilder();

忽略该行

strbuild = null;

Answer 3

更改

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

到

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

或者，为了防止这种错误：

  StringBuilder strbuild = new StringBuilder();

Answer 4

您的示例中有一些错误，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());

StringBuilder实例中的运行时错误

4 个答案: