无法在if else中声明对象.if else子句以外不可见

Question

当我在if/else子句中实例化Textwriter对象时,它在子句外部是不可见的.我能在这做什么？我想要附加或写一个新文件.

这是代码:

     private static void eventfull_doing()
    {
        string path, line;
        int countLines;
        Console.Write("Enter the name(with extension) of the file in the \"data\" folder, or create a new name(with extension): ");
        path = Console.ReadLine();
        TextReader inFile = new StreamReader(@"C:\data\" + path);
        Console.Write("How many lines of text do you want to add to the file?: ");
        countLines = Convert.ToInt32(Console.ReadLine());
        if (inFile.Peek() == -1 || inFile == null)
        {
            TextWriter outFile = File.AppendText(@"C:\data\" + path);
        }
        else
        {
            TextWriter outFile = new StreamWriter(@"C:\data\" + path);
        }
        for (int i = 0; i < countLines; i++)
        {
            Console.Write("Enter line: ");
            line = Console.ReadLine();
            outFile.
    }

这就是我所做的:请注意,在第一个片段中,if条件不是预期的.

    string path, line;
        int countLines;
        Console.Write("Enter the name(with extension) of the file in the \"data\" folder, or create a new name(with extension): ");
        path = Console.ReadLine();
        string file = Path.Combine(@"c:\data", path);
        Console.Write("How many lines of text do you want to add to the file?: ");
        countLines = Convert.ToInt32(Console.ReadLine());
        TextWriter outFile = null;
        if (File.Exists(file))
        {
            using (outFile = File.AppendText(file))
            {
                for (int i = 0; i < countLines; i++)
                {
                    Console.Write("Enter line: ");
                    line = Console.ReadLine();
                    outFile.WriteLine(line);
                }
            }
        }
        else
        {
            using (outFile = new StreamWriter(file))
            {
                for (int i = 0; i < countLines; i++)
                {
                    Console.Write("Enter line: ");
                    line = Console.ReadLine();
                    outFile.WriteLine(line);
                }
            }
        }

    }

Answer 1

您可以在if语句之前声明它,但没有赋值 - 然后确保在两个分支中为它分配值:

TextWriter outFile;
if (inFile.Peek() == -1 || inFile == null)
{
    outFile = File.AppendText(@"C:\data\" + path);
}
else
{
    outFile = new StreamWriter(@"C:\data\" + path);
}

但是,如果文件存在,您仍然可以打开文件这一事实可能会导致您出现问题 - File.AppendText如果您发现它仍然是空的,那么就不清楚为什么要打电话.你真的只是想创造或追加？如果是这样,只需使用AppendText- 它会没事的.

您还应该使用using语句自动关闭编写器...如果您确实需要@"C:\data\" + path在多个位置使用,我会将该公共表达式提取到局部变量:

string file = Path.Combine(@"c:\data", path);
// Now use file everywhere

如果你不坚持使用File.AppendText或StreamWriter构造,可以考虑使用条件表达式:

TextWriter outFile = inFile.Peek() == -1 || inFile == null
    ? File.AppendText(file) : new StreamWriter(file);