How can I read a file which will be upload from a form in .Net Core API?

In you Controller :

1. Check if IFormFile file contains something
2. Check if the file's extension is the one you are looking for (.dat)
3. Check if the file's Mime type is correct to avoid attacks

Then, if it is all right, call a Service class to read your file.

In your Service, you can do something like that :

var result = new StringBuilder();
using (var reader = new StreamReader(file.OpenReadStream()))
{
    while (reader.Peek() >= 0)
        result.AppendLine(await reader.ReadLineAsync()); 
}
return result.ToString();

Hope it helps.

The file will be bound to your IFormFile param. You can access the stream via:

using (var stream = file.OpenReadStream())
{
    // do something with stream
}

If you want to read it as a string, you'll need an instance of StreamReader:

string fileContents;
using (var stream = file.OpenReadStream())
using (var reader = new StreamReader(stream))
{
    fileContents = await reader.ReadToEndAsync();
}