HttpClient ETag based client cache handler.

HttpETagCacheHandler.cs

class HttpETagCacheHandler : HttpClientHandler
{
	private static readonly ConcurrentDictionary<string, string> ETags = new ConcurrentDictionary<string, string>();
	private static readonly ConcurrentDictionary<string, byte[]> MemoryCache = new ConcurrentDictionary<string, byte[]>();

	protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
	{
		var pathAndQuery = request.RequestUri.PathAndQuery;

		if (ETags.TryGetValue(pathAndQuery, out var ifNonMatch))
		{
			request.Headers.Add("If-None-Match", ifNonMatch);
		}

		var response = await base.SendAsync(request, cancellationToken);

		if (response.StatusCode == HttpStatusCode.OK && !string.IsNullOrEmpty(ifNonMatch))
		{
			MemoryCache.TryRemove(ifNonMatch, out _);
			ETags.TryRemove(pathAndQuery, out _);
		}

		var responseETag = response.Headers?.ETag?.Tag;
		if (response.StatusCode == HttpStatusCode.OK && !string.IsNullOrEmpty(responseETag) && !ETags.ContainsKey(responseETag) && response.Content != null)
		{
			ETags.TryAdd(pathAndQuery, responseETag);
			var bytes = await response.Content.ReadAsByteArrayAsync();
			MemoryCache.TryAdd(responseETag, bytes);
		}

		if (response.StatusCode == HttpStatusCode.NotModified && !string.IsNullOrEmpty(responseETag) && MemoryCache.TryGetValue(responseETag, out var data))
		{
			response.Content = new ByteArrayContent(data);
			response.StatusCode = HttpStatusCode.OK;
		}

		return response;
	}
}