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Quick Tip: When Testing with Moq, Try Lambdas for More Flexible Tests

seankilleen profile image Sean Killeen Originally published at on ・2 min read

Mocks are invaluable to the art of testing. For those who are unfamiliar, mocks are essentially a way to fake a component and have it do exactly what you want it to do (e.g. a validator that always fails, or a method that always returns 1 when given a certain parameter).

While I’m pretty library-agnostic1, I happen to be pretty familiar with Moq and so it’s the one that I use most often.

I showed a tip to a developer recently and it struck me that it might be worth sharing with a wider audience.

A big challenge for tests is to keep them specific enough that they accurately capture what you’re trying to do, but flexible enough that the smallest of changes won’t break them. I prefer readability of each test over re-usability, but nobody likes fixing 20 tests with one minor change.

I see a lot of folks do things along the lines of the following:

var mockLogger = new Mock<ILogger>();
var myClass = New MyClass(mockLogger.Object);

mockLogger.Verify(x=> x.Warning("Setting 'mySetting' has no value; using default of 1");

Now, this is all well and good. However, what if the default value changes? Or the name of the setting? The test is too brittle; it will fail easily.

However, when using Moq, it allows you to use a lambda to specify things about a string, rather than the whole string itself:

mockLogger.Verify(x=> x.Warning(It.Is<string>(str => 
     str.Contains("Setting") && 
     str.Contains("has no value") && 
     str.Contains("using default"))));

This maintains the essence of the test, while at the same time fortifying it against things that might reasonably change. It’s slightly more verbose, but I don’t think that takes away from it much.

Have a different way of approaching mocks, mocking, or unit tests? I’d love to hear from you in the comments.

I prefer concepts over libraries. If a developer understands what mocking is at is core, a library change should never phase them, though they should be able to express trade-offs. ↩

Quick Tip: When Testing with Moq, Try Lambdas for More Flexible Tests was originally published by Sean Killeen at on June 28, 2015.

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