使用 Stream 的 API 修改 C# 中的文件内容

Question

I have a .json file who handles the user's roles and I have wrote a Repository who's responsible of adding/removing roles to users. The pb is that when I modify the file I want to be sure that no one access it except me.

Here's (roughly) the code I use:

using (var fileStream = new FileStream(_rolesFilePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
using (var streamReader = new StreamReader(fileStream))
using (var streamWriter = new StreamWriter(fileStream))
{
    var oldContent = streamReader.ReadToEnd();
    var contentObject = Deserialize(oldContent);

    Modify(contentObject)

    var newContent = Serialize(contentObject);

    fileStream.Seek(0, SeekOrigin.Begin);
    streamWriter.Write(newContent);
}

The pb with this solution is that if newContent is a string shorter that oldContent some characters will be remaining in the file.

A solution I found is to add the following code:

using (var fileStream = new FileStream(_rolesFilePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
using (var streamReader = new StreamReader(fileStream))
using (var streamWriter = new StreamWriter(fileStream))
{
    //...

    var newContent = Serialize(contentObject);
    var endPosition = fileStream.Position;

    fileStream.Seek(0, SeekOrigin.Begin);
    streamWriter.Write(newContent);
    streamWriter.Flush();

    while (fileStream.Position < endPosition)
    {
        streamWriter.WriteLine();
        streamWriter.Flush();
    }
}

It works well but does not look very clean to me. Are there any better solution who ensure that I keep the control of the file ?

Thanks in advance, Thomas

Answer 1

You can do fileStream.SetLength(fileStream.Position) to truncate the remaining part of the file. This assumes that the FileStream is left correctly positioned by the StreamWriter after use, but that's an assumption your current code seems to be making too.

(This is a safer assumption than the corresponding usage of StreamReader where internal buffering may mean that the underlying stream's position is further advanced than the latest data returned by a call to a Read method)