如何在c#中最好地管理一个循环中的多个流
本文关键字：一个 循环 管理 | 更新日期: 2023-09-27 18:02:36
我正在按类型顺序遍历电影列表，并且我正在尝试创建和写入文件，因为我这样做。我想创建一个文件，写一堆，然后当我移动到下一个类型时关闭它，并创建一个新文件。
现在我得到"使用未分配的局部变量'GenStream'"，当我试图关闭流。但是，如果我将其注释掉，并且不关闭流，则在ELSE语句中会得到相同的错误。我不清楚为什么我得到这些错误，并将感谢帮助解决它们。我注意到我在下面的代码中得到错误的地方。
/// <summary>
/// Creates a file for each Genre, and writes movie info to each for the cooresponding movies
/// </summary>
/// <param name="cPath">Path to create HTML files in</param>
/// <param name="mList">List of Movies to generate Genre and Movie info from</param>
public static void WriteGenreHTML(string cPath, List<Movie> mList)
{
int lineID = 0;
string tmpGen = null;
string strHeader, strMovie, strGenre, tmpGenre = null;
StreamWriter genStream;
// Gets a list of unique Genres from the MovieList
var distinctGenres = from m in mList
from genre in m.Genres
group genre by genre into genres
select genres.First();
// Gets a list of Movies with the associated Genres
var moviesWithGenre = from g in distinctGenres
from m in mList
where m.Genres.Contains(g)
orderby g, m.Title
select new { Genre = g, Movie = m };
// Traverses list of movies creating new HTML Genre files, and writing movie info to the HTML genre files
foreach (var m in moviesWithGenre)
{
// Creates new HTML file if new Genre is detected
if (m.Genre != tmpGen)
{
tmpGen = m.Genre;
// Closes previously open Stream
genStream.Close(); // ERROR: "Use of unassigned local variable 'genStream'
// initiates streamwriter for catalog output file
FileStream fs = new FileStream(cPath + Path.DirectorySeparatorChar + m.Genre, FileMode.Create);
genStream = new StreamWriter(fs);
// Generates header info for new file, and new Genre
strHeader = "<style type='"text/css'">'r'n" + "<!--'r'n" + "tr#odd {'r'n" + " background-color:#e2e2e2;'r'n" + " vertical-align:top;'r'n" + "}'r'n" + "'r'n" + "tr#even {'r'n" + " vertical-align:top;'r'n" + "}'r'n" + "div#title {'r'n" + " font-size:16px;'r'n" + " font-weight:bold;'r'n" + "}'r'n" + "'r'n" + "div#mpaa {'r'n" + " font-size:10px;'r'n" + "}'r'n" + "'r'n" + "div#genre {'r'n" + " font-size:12px;'r'n" + " font-style:italic;'r'n" + "}'r'n" + "'r'n" + "div#plot {'r'n" + " height: 63px;'r'n" + " font-size:12px;'r'n" + " overflow:hidden;'r'n" + "}'r'n" + "'r'n" + "div#genre_heading {'r'n" + " height: 50px;'r'n" + " font-size: 24px;'r'n" + " font-weight: bold;'r'n" + " text-align: center;'r'n" + " text-decoration: underline;'r'n" + "}'r'n" + "-->'r'n" + "</style>'r'n" + "'r'n" + "<html>'r'n" + " <body>'r'n" + " <table>'r'n";
strHeader += " <tr>'r'n" + " <td colspan=2>'r'n" + " <div id='"genre_heading'">" + m.Genre + "</div>'r'n" + " </td>'r'n" + " </tr>'r'n" + "'r'n";
// Writes header HTML to stream
genStream.WriteLine(strHeader);
Console.WriteLine();
Console.WriteLine("Now Processing " + m.Genre);
}
// Otherwise creates and writes HTML code for the Movie
else
{
// Creates string of links to the Genre HTML pages
foreach (string genre in m.Movie.Genres)
tmpGenre += ", <a href='"" + genre + ".html'" target='"_blank'">" + genre + "</a>";
strGenre = tmpGenre != null ? tmpGenre.Substring(2) : null;
// Generates the HTML for the Movie
strMovie = lineID == 0 ? " <tr id='"odd'" style='"page-break-inside:avoid'">'r'n" : " <tr id='"even'" style='"page-break-inside:avoid'">'r'n";
strMovie += " <td>'r'n" + " <img src='".''images''" + m.Movie.ImageFile + "'" width='"75'" height='"110'">'r'n" + " </td>'r'n" + " <td>'r'n" + " <div id='"title'">" + m.Movie.Title + "</div>'r'n" + " <div id='"mpaa'">" + m.Movie.Certification + " " + m.Movie.MPAA + "</div>'r'n" + " <div id='"genre'">" + strGenre + "</div>'r'n" + " <div id='"plot'">" + m.Movie.Plot + "</div>'r'n" + " </td>'r'n" + " </tr>'r'n";
// Writes the HTML to the stream
genStream.WriteLine(strMovie); // ERROR: "Use of unassigned local variable 'genStream'
lineID = lineID == 0 ? 1 : 0;
}
}
string closingHTML = " </table>'r'n" + " </body>'r'n" + "</html>";
genStream.WriteLine(closingHTML);
genStream.Close();
}
您可以使用下面的代码来验证流是否已经初始化
如果流已初始化，则可以关闭它
if (genStream == null )
{
genStream.Close()
}
希望这对你有用…
你得到"使用未分配的局部变量'GenStream'"错误的原因是因为你已经声明了它，当你在if块中调用GenStream . close()时，它没有被分配任何东西-它只是一个声明(将其视为占位符)。
当你注释掉genStream.Close()时，你会在else块中再次得到错误，因为(从else块的作用域的角度来看)它再次没有被分配任何东西。
下面是我在评论你之前的文章时建议的一个例子:
foreach (var g in DistinctGenres)
{
FileStream fs = new FileStream(cPath + Path.DirectorySeparatorChar + g.Genre, FileMode.Create);
StreamWriter genStream = new StreamWriter(fs);
// Write your header here
foreach (var m in Genres)
{
// Generates the HTML for the Movie
// Writes the HTML to the stream
genStream.WriteLine(strMovie);
lineID = lineID == 0 ? 1 : 0;
}
string closingHTML = " </table>'r'n" + " </body>'r'n" + "</html>";
genStream.WriteLine(closingHTML);
genStream.Close();
}
这消除了if检查的需要，并且通常简化了你想要完成的任务。
你可能需要调整你的两个查询，或者保留你的第一个查询，并根据你正在构建的网页的当前类型做第二个查询。
另一个想法-将第二个查询移动到第一个foreach循环中，将当前类型分配给一个变量，并像这样调整第二个查询:
var moviesWithGenre = from g in distinctGenres
from m in mList
where m.Genres.Contains(currentGenre) // currentGenre is assigned a value in the first (outer) foreach loop
orderby g, m.Title
select new { Genre = g, Movie = m };
也许你也可以添加，除了这里的答案，是使用"using"关键字，因为它会注入到最后的IL try/finally符号，这将确保你的流是关闭的，即使在异常引发的情况下。简而言之，伪代码可以是这样的:
using(StreamWriter genStream = new StreamWriter(…)){
//use stream here
genStream.Close();
}//退出"using"时将自动调用genStream的dispose。
希望对你有帮助。
问候。