c# async filewrite System.IO.IOException at random times

Question

I have this function that is called every minute and sometimes it is triggered other ways in the code.

static async Task WriteFileAsync(string file, string content)
        {
            using (StreamWriter outputFile = new StreamWriter(file))
            {
                await outputFile.WriteAsync(content);
            }
        }

Sometimes I get this error, but not always, it is quite random.

 System.IO.IOException: 'The process cannot access the file 'hello.json' because it is being used by another process.'

Answer 1

You need to have a lock to prevent multiple threads from calling WriteFileAsync simultaneously. Since lock keyword cannot be used when there is an await statmeent in the block, SemaphoreSlim .

// Initialize semaphore (maximum threads that can concurrently access the file is 1)
private static readonly SemaphoreSlim _semaphoreSlim = new SemaphoreSlim(1, 1);

static async Task WriteFileAsync(string file, string content)
{
    // Acquire lock
    await _semaphoreSlim.WaitAsync();
    try
    {
        using (StreamWriter outputFile = new StreamWriter(file, true))
        {
            await outputFile.WriteAsync(content);
        }
    }
    finally
    {
        // Release lock in finally block so that lock is not kept if an exception occurs
        _semaphoreSlim.Release();
    }
}