Large stream writing. Try use StreamWriter

I read some sample code:

IWorkbook workbook = new XSSFWorkbook();
using (FileStream stream = new FileStream("outfile.xlsx", FileMode.Create, FileAccess.Write))
{
  workbook.Write(stream);
}  

There is only a Write method to usage. In this case, we need write bytes to file. that would consume disk I/O. Think about scenario when user generate excel data and return excel file. The normal way to achieve that is like below:

var stream = new MemoryStream();
workbook.Write(stream);
return File(stream.ToArray(), contentType);

If we use MemoryStream instead, disk I/O wouldn’t be used. But the problem would be occurs when excel data too large. if we use MemoryStream, and use ToArray() to get bytes[]. This would allocate too much memory from application.

Can you please consider to use StreamWriter to write stream async? C# MVC action return File() would wait StreamWriter finished. There is a sample below:

In any controller:

        private static StreamWriter sw;
        public async Task<IActionResult> Excel()
        {
            var ms = new MemoryStream();
            if (sw == null)
            {
                sw = new StreamWriter(ms);
            }
            sw.WriteLine("Sync Write");
            await sw.WriteLineAsync("Async Write");
            await sw.FlushAsync();
            await WriteAsync(sw, ms);
            ms.Position = 0;
            var extensionProvider = new FileExtensionContentTypeProvider();
            if (!extensionProvider.TryGetContentType(".txt", out string contentType))
            {
                contentType = "application/octet-stream";
            }
            return File(ms, contentType);
        }

        private async Task WriteAsync(StreamWriter sw, MemoryStream ms)
        {
            
            Thread.Sleep(5000);
            await sw.WriteLineAsync("Large bytes to write");
            await sw.FlushAsync();
        }

You can define StreamWriter inside of XSSFWorkbook. and provide WriteAsync(Stream ms). User only need to pass MemorySteam in and send to File(Stream fileStream, string contentType). This way can save memory and won’t easily to emit garbage collection.

[tonyqus]

This sounds to be a good suggestion. I will consider that. Thank you!

[tonyqus]

@goldenc What the fuck are you saying? Shame? I hope it’s because English is not your native language.

I don’t think WriteAsync really makes much difference although I totally understand what’s .NET async is. I’m a very senior dev on .NET for many years. .NET has been WITHOUT async for the past 10 years when .NET core didn’t exist.

Anyway, looks you are not a very satisfied user, I suggest you use commercial library instead. NPOI doesn’t welcome you anymore.