Writing to file asynchronously results in incorrect number of lines and incorrect contents

Question

I have a small logger class that just contains some static helper methods for easily writing to a log file. I wanted to test how robust it was, so I attempted to run it asynchronously 100 times at once.

[TestMethod]
public void Diagnostics_AsyncWritePass()
{
    int linesToWrite = 100;
    String logMsg = "Async test log ";
    List<Task> asyncTaskList = new List<Task>();

    for (int i = 0; i < linesToWrite; i++)
    {
        Task task = new Task(() => Logger.Info(logMsg + i));
        asyncTaskList.Add(task);
        task.Start();
    }

    Task.WaitAll(asyncTaskList.ToArray());

    List<String> lines = GetLastLogLines(linesToWrite);

    foreach (String line in lines)
    {
        Assert.IsTrue(line.Contains(logMsg));
    }
}

The actual logging (inside the Logger.Info method) is performed using:

File.AppendAllText(LogPath, logMsg + Environment.NewLine);

The GetLastLogLines method is:

private List<String> GetLastLogLines(int numLines)
{
    List<String> lines = File.ReadLines(Logger.LogPath)
        .Reverse()
        .Take(numLines)
        .ToList();
    return lines;
}

However, I have two issues with this.

The first is that it is not writing 100 lines, it is writing around 90.
The second is that every line is Async test log 100 - the number does not increment every line. I'm guessing that the i counter is being passed by reference, not by value.

Why might these problems occur?

EDIT: The incorrect number of lines issue was solved by replacing the contents of GetLastLogLines. Previously, it was using File.ReadLines then using LINQ to take only x number of lines. Instead, I used this answer and it returned the correct number of lines.

score 0 · Accepted Answer · answered Jul 31 '17 at 16:32

Your second problem is standard issue when a lambda expression captures an iteration variable.

Make the following change.

for (int i = 0; i < linesToWrite; i++)
{
    var t = i;
    Task task = new Task(() => Logger.Info(logMsg + t));
    asyncTaskList.Add(task);
    task.Start();
}

Can you show us what GetLastLogLines() does?

Writing to file asynchronously results in incorrect number of lines and incorrect contents

1 Answers1