Scott Hanselman

Experiments in Open Source: Exploring vcr-sharp for Http record and playback

September 10, 2017 Comment on this post [8] Posted in DotNetCore | Open Source
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I've always said that reading source code is as important as write it - especially as part of the learning process. You learn a ton about how other coders think and solve problems, plus you learn about lots of new libraries and methods you may not be familiar with.

Last week I noticed this tweet from Brendan Forster about an experiment he's working on. He is interesting in your feedback on his experiment and if you would use it.

He's created a new library for .NET called vcr-sharp that is inspired by the vcr Ruby gem and the scotch .NET library.

Again, he's made it clear he's just experimenting but I think this has some interesting potential.

Vcr-sharp lets you record and playback HTTP requests! In this example, WithCassette is an extension method on HttpClientFactory. That extension method sets up a DelgatingHandler to a ReplayingHandler. That ReplayingHandler "loads the cassette" and returns it as a cached response.

using (var httpClient = HttpClientFactory.WithCassette("my-test-scenario"))
var request = new HttpRequestMessage(HttpMethod.Get, "");
var response = await httpClient.SendAsync(request);
var body = await response.Content.ReadAsStringAsync();
body.ShouldContain("Example domains");

Also worth noting is that within the VCR-Sharp library Brendan uses an assertion library for .NET called "Shouldly." Shouldly has some interesting extension methods that let you express how you Assert within your Tests.

They say - this is the old Assert way:

Assert.That(contestant.Points, Is.EqualTo(1337));

For your troubles, you get this message, when it fails:

Expected 1337 but was 0

They say - this is how it Should be:


Which is just syntax, so far, but check out the message when it fails:

contestant.Points should be 1337 but was 0

Another example:

Assert.That(map.IndexOfValue("boo"), Is.EqualTo(2));    // -> Expected 2 but was 1
map.IndexOfValue("boo").ShouldBe(2);                    // -> map.IndexOfValue("boo") should be 2 but was 1

It makes tests very easy to read. A nice bit of syntactic sugar:

public async Task AppendsNewRequestToCache()
Environment.SetEnvironmentVariable("VCR_MODE", "Cache");
var session = "append-second-request";

using (var httpClient = HttpClientFactory.WithCassette(session))
var request = new HttpRequestMessage(HttpMethod.Get, "");
var response = await httpClient.SendAsync(request);

var cassette = await ReadCassetteFile(session);

It also uses BenchmarkDotNet, which you may be familiar with. It allows you to mark methods as [Benchmark] methods and you'll get smart warming up, running, teardowns and statistics like this;

public async Task ReadFromCache()

using (var httpClient = HttpClientFactory.WithCassette("example-test"))
var request = new HttpRequestMessage(HttpMethod.Get, "");
var response = await httpClient.SendAsync(request);
} Output:
        Method |     Mean |    Error |   StdDev |
-------------- |---------:|---------:|---------:|
ReadFromCache | 684.1 us | 3.154 us | 2.796 us |

I'd encourage you to check vcr-sharp out over at, read the source code, and think about how you'd use it. I am sure Brendan would appreciate your thoughts and feedback in the GitHub Issues! Also check out how he uses Tests, Shouldly, and BenchmarkDotNet in his project and consider how you'd use them in yours!

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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September 10, 2017 11:17
I really like the name: vcr-sharp. It captures the tape-like nature of the caching provided.

This is certainly a good concept too. I've done this myself to speed up HTTP tests, so they don't have to make high latency requests over and over again.
September 10, 2017 11:31
Another play and record tool is which can return stubbed/mocked responses based on pattern matching on url, headers and body.

It can also be used in unit testing.

I'm also working on a standalone Docker image which can run in a Docker Linux container.
September 10, 2017 22:43
Cool library. I thought it would fun to see if this would play nicely with Flurl. If anyone's interested, here's an approach that would work well:
September 11, 2017 15:12
Shouldly looks very similar to Fluent Assertions (, which I've enjoyed using and seems more established.
September 11, 2017 15:17
Can you please explain why WithCassette is an extension method?
September 11, 2017 16:45
thank you so much
September 12, 2017 4:42
Great find with Shouldly. I've been doing assertions like this with JavaScript and the Chai assertion library and been missing it on the .NET side of things. Thanks Scott.
September 12, 2017 17:43
Thanks for the tip!

Small typo in the fifth paragraph:

That extension method sets up a DelgatingHandler to a ReplayingHandler.

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Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.