Is using Single as an Assert a bad practice?

Question

I'm testing a method that manipulates a collection. Given a set of parameters it should contain exactly one element that matches a condition. Edit: The collection might have several other elements not matching the condition aswell.

I'm using Single to test that behaviour, which works fine, as it will fail the test by throwing an exception if there is no match at all or more than one match. But there is no actual assert, which somehow violates arrange, act, assert. So I'm wondering if this is a bad practice and if there's a better way to do this.

Following pseudo code to demonstrate my question:

[TestMethod]
public void TestMethod()
{
    List list = MethodToTest(param1, param2);

    list.Single(s => s.Matches(condition));

    //No actual Assert
}

Answer 1

I'm wondering if this is a bad practice and if there's a better way to do this.

Yes and yes.

it will fail the test by throwing an exception if there is no match at all or more than one match.

Don't fail the test by throwing an exception. Fail the test by failing the test. Your test framework has a mechanism for asserting the condition being tested by the test. You bought this test framework, and now you are resisting using its features. Use the test framework as it was designed to be used, or, if you don't like it, abandon it and choose a framework you like better. But don't do end-runs around its mechanisms.

Unexpected exceptions are not necessarily test failures; they could be buggy tests. You need to be able to tell the difference. The way you tell is: if the exception originates in the code under test, its a bug in the code. If it originates in the test code, its a bug in the test. And now you go and detect a bug in the code under test by making the testing code throw. And now it is harder to tell where the bug is. Don't make your future self think that hard; don't write surprising code that deliberately avoids the conventions of the testing platform.