c# - 如何告诉编译器变量将存在

Question

您如何告诉 Visual Studio 相信某个变量会在需要时存在。这是我的代码:

            string line;
        bool fileExists = false;
        System.IO.StreamReader fileIn;
        System.IO.StreamWriter fileOut;

        fileIn = new System.IO.StreamReader(inputFile);

        while ((line = fileIn.ReadLine()) != null)
        {
            if (line.Length > 0 && line.Substring(0, 1) == "O")
            {
                _parent.StatusOutput(line);
                if (fileExists)
                {
                    // close the existing file ...
                    fileOut.WriteLine();
                    fileOut.WriteLine();
                    fileOut.WriteLine("%");
                    fileOut.Close();
                }
                // create a new file ...
                fileOut = new System.IO.StreamWriter(outputFolder + @"\" + line);
                fileExists = true;
                fileOut.WriteLine();
                fileOut.WriteLine("%");
                fileOut.WriteLine(line);
            }
            else if (line.Length == 0)
            {
                _parent.StatusOutput("Blank line.");
                if (fileExists) fileOut.WriteLine();
            }
            else if (line.Length > 0 && line == "%")
            {
                _parent.StatusOutput("BO/EOF found.");
                // do nothing ...
            }
            else
            {
                // everything else goes to the new file ...
                if (fileExists) fileOut.WriteLine();
            }
        }    // while ...
        
        // close the last file ...
        fileOut.WriteLine();
        fileOut.WriteLine();
        fileOut.WriteLine("%");
        fileOut.Close();
        fileIn.Close();

错误是在我尝试写入 fileOut 时出现的。我知道 fileOut 写入会及时存在，但编译器不存在。

我怎样才能让编译器忽略它？

Answer 1

您可以在声明时将 null 赋值给 fileOut，然后检查它是否不为 null:

    string line;
    System.IO.StreamReader fileIn;
    System.IO.StreamWriter fileOut = null;

    fileIn = new System.IO.StreamReader(inputFile);

    while ((line = fileIn.ReadLine()) != null)
    {
        if (line.Length > 0 && line.Substring(0, 1) == "O")
        {
            _parent.StatusOutput(line);
            if (fileOut != null)
            {
                // close the existing file ...
                fileOut.WriteLine();
                fileOut.WriteLine();
                fileOut.WriteLine("%");
                fileOut.Close();
            }

            // create a new file ...
            fileOut = new System.IO.StreamWriter(outputFolder + @"\" + line);
            fileOut.WriteLine();
            fileOut.WriteLine("%");
            fileOut.WriteLine(line);