How do I generate an http ETag in c#?

Question

Given a file being returned as part of a http request. What is the correct method of creating an ETag for that file?

http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.19

I have seen it done several ways

• Using the last write time of the file converted to ticks. Which is what NancyFX does https://github.com/NancyFx/Nancy/blob/master/src/Nancy/Responses/GenericFileResponse.cs
• Using a hash of the file http://hartzer.wordpress.com/2009/10/04/client-side-caching/

Answer 1

To generate a static file's ETag which will be compatible with Nginx:

// Generate ETag from file's size and last modification time as unix timestamp in seconds from 1970
public static string MakeEtag(long lastMod, long size)
{
    string etag = '"' + lastMod.ToString("x") + '-' + size.ToString("x") + '"';
    return etag;
}

public static void Main(string[] args)
{
    long lastMod = 1578315296;
    long size = 1047;
    string etag = MakeEtag(lastMod, size);
    Console.WriteLine("ETag: " + etag);
    //=> ETag: "5e132e20-417"
}

See my comment about different ETag schemas

Answer 2

Semantically the ETag should change when the content changes:

So the Hash seems appropriate... but the ETag must also be unique on different URLs and/or different timestamps of duplicate files... so to be on the safe side hash the file, concatenate that with the timestamp of the last modification and the url and hash that again...