Why does my for loop increment past where it should stop? - for-loop

Question

I am attempting to increase the speed at which files in my application download by downloading them in parallel. Previously I was downloading them sequentially and it worked fine but when I attempted to download them in parallel I ran into unexplained issues.
Here is my method in which I downloaded the files in sequence:
public IActionResult DownloadPartFiles([FromBody] FileRequestParameters parameters)
{
List<InMemoryFile> files = new List<InMemoryFile>();
for (int i = 0; i < parameters.FileNames.Length; i++)
{
InMemoryFile inMemoryFile = GetInMemoryFile(parameters.FileLocations[i], parameters.FileNames[i]).Result;
files.Add(inMemoryFile);
}
byte[] archiveFile = null;
using (MemoryStream archiveStream = new MemoryStream())
{
using (ZipArchive archive = new ZipArchive(archiveStream, ZipArchiveMode.Create, true))
{
foreach (InMemoryFile file in files)
{
ZipArchiveEntry zipArchiveEntry = archive.CreateEntry(file.FileName, CompressionLevel.Optimal);
using (MemoryStream originalFileStream = new MemoryStream(file.Content))
using (Stream zipStream = zipArchiveEntry.Open())
{
originalFileStream.CopyTo(zipStream);
}
}
}
archiveFile = archiveStream.ToArray();
}
return File(archiveFile, "application/octet-stream");
}
Here is the method changed to download the files in parallel:
public async Task<IActionResult> DownloadPartFiles([FromBody] FileRequestParameters parameters)
{
List<Task<InMemoryFile>> fileTasks = new List<Task<InMemoryFile>>();
for (int i = 0; i < parameters.FileNames.Length; i++)
{
if(i == parameters.FileNames.Length - 1)
{
int breakpoint = 0;
}
if(i == parameters.FileNames.Length)
{
int breakpoint = 0;
}
fileTasks.Add(Task.Run(() => GetInMemoryFile(parameters.FileLocations[i], parameters.FileNames[i])));
}
InMemoryFile[] fileResults = await Task.WhenAll(fileTasks);
byte[] archiveFile = null;
using (MemoryStream archiveStream = new MemoryStream())
{
using (ZipArchive archive = new ZipArchive(archiveStream, ZipArchiveMode.Create, true))
{
foreach (InMemoryFile file in fileResults)
{
ZipArchiveEntry zipArchiveEntry = archive.CreateEntry(file.FileName, CompressionLevel.Optimal);
using (MemoryStream originalFileStream = new MemoryStream(file.Content))
using (Stream zipStream = zipArchiveEntry.Open())
{
originalFileStream.CopyTo(zipStream);
}
}
}
archiveFile = archiveStream.ToArray();
}
return File(archiveFile, "application/octet-stream");
}
Here is the method that does the actual downloading:
private async Task<InMemoryFile> GetInMemoryFile(string fileLocation, string fileName)
{
InMemoryFile file;
using (HttpClient client = new HttpClient())
using (HttpResponseMessage response = await client.GetAsync(fileLocation))
{
byte[] fileContent = await response.Content.ReadAsByteArrayAsync();
file = new InMemoryFile(fileName, fileContent);
}
return file;
}
Now the issues I run into is after I changed DownloadPartFiles to get all the files in parallel my for loop is now seeming to go past its stop condition. For example, if parameters.FileNames.Length returns 12 the for loop should not run when i = 12 and it should exit the loop. However, in my testing it will continue to run when i = 12 and as one might expect I run into an out of bounds error. I tried to set breakpoints in my code to make sure that it was actually running past the stop condition and more weird behavior arose. In my for loop I included two if statements with breakpoint variables to break on. It will always break when i should be on its last loop but will never break when i is one after its expected last loop. It seems to skip that breakpoint when i is one past the expected stop condition. It will run fine if I step through the code while debugging but will out of bounds error when I let it run normally.
I'm not sure why this is happening but I am still new to asynchronous programming so maybe its just an oversight somewhere. Let me know if I need to explain anything further.

Answer 1

I make a critical mistake in that I tried to wrap an asynchronous method (my GetInMemoryFile method) in the Task.Run() method which is used to wrap synchronous methods to make them run asynchronously. This caused the weird behavior.
So in short I changed
fileTasks.Add(Task.Run(() => GetInMemoryFile(parameters.FileLocations[i], parameters.FileNames[i])));
To
fileTasks.Add(GetInMemoryFile(parameters.FileLocations[i], parameters.FileNames[i]));