How to read into memory the lines of a text file from an IFormFile in ASP.NET Core? - parsing

Question

Say you have this Action:
public List<string> Index(IFormFile file){
//extract list of strings from the file
return new List<string>();
}
I've found plenty of examples of saving the file to the drive, but what if I instead want to skip this and just read the lines of text into an array in memory, directly from the IFormFile?

Answer 1

The abstraction for the IFormFile has an .OpenReadStream method.
To prevent a ton of undesirable and potentially large allocations, we should read a single line at a time and build up our list from each line that we read. Additionally, we could encapsulate this logic in an extension method. The Index action ends up looking like this:
public List<string> Index(IFormFile file) => file.ReadAsList();
The corresponding extension method looks like this:
public static List<string> ReadAsList(this IFormFile file)
{
var result = new StringBuilder();
using (var reader = new StreamReader(file.OpenReadStream()))
{
while (reader.Peek() >= 0)
result.AppendLine(reader.ReadLine());
}
return result;
}
Likewise you could have an async version as well:
public static async Task<string> ReadAsStringAsync(this IFormFile file)
{
var result = new StringBuilder();
using (var reader = new StreamReader(file.OpenReadStream()))
{
while (reader.Peek() >= 0)
result.AppendLine(await reader.ReadLineAsync());
}
return result.ToString();
}
Alternatively, you could you use an ObjectPool<StringBuilder> and modern C# 8 features.
public static async Task<string> ReadAsStringAsync(
this IFormFile file, Object<StringBuilder> pool)
{
var builder = pool.Get();
try
{
using var reader = new StreamReader(file.OpenReadStream());
while (reader.Peek() >= 0)
{
builder.AppendLine(await reader.ReadLineAsync());
}
return builder.ToString();
}
finally
{
pool.Return(builder);
}
}
Then you could use this version this way:
public Task<List<string>> Index(
IFormFile file, [FromServices] ObjectPool<StringBuilder> pool) =>
file.ReadAsListAsync(pool);

Answer 2

ASP.NET Core 3.0 - Reading a form file's content into string
public static async Task<string> ReadFormFileAsync(IFormFile file)
{
if (file == null || file.Length == 0)
{
return await Task.FromResult((string)null);
}
using (var reader = new StreamReader(file.OpenReadStream()))
{
return await reader.ReadToEndAsync();
}
}