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

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);