fix!: reject an empty prefix or suffix expectation - #1258
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`StartsWith("")` and `EndsWith("")` throw because an empty expectation is almost always a mistake - an accidentally empty variable - but the same expectation written as `IsEqualTo("").AsPrefix()` or `.AsSuffix()` silently passed. One expectation had two spellings that disagreed, and the silent one is the dangerous one, so the guard now also applies to the match type, wherever it is reached: `IsEqualTo`, `Contains`, `HasItem` and the string collection overloads.
Just like a regex or wildcard pattern, a prefix is only known when the expectation is verified, so the guard sits next to them in `StringEqualityOptions` and fires after the normalization: a prefix that only becomes empty through `IgnoringIndentation()` says just as little as a literal empty one. A `null` prefix or suffix is not rejected, because unlike a null regex it still expresses that the subject is null, an expectation that can fail in either polarity, and `IsEqualTo` deliberately accepts a nullable argument.
FluentAssertions follows the BCL, where every string starts with the empty string, so migrating users can hit the new exception.
vbreuss
enabled auto-merge (squash)
September 19, 2026 17:06
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Test Results 6 files ±0 6 suites ±0 1m 41s ⏱️ -3s Results for commit cac677a. ± Comparison against base commit 08dca50. This pull request removes 126 and adds 126 tests. Note that renamed tests count towards both. |
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StartsWith("")andEndsWith("")throw because an empty expectation is almost always a mistake - an accidentally empty variable - but the same expectation written asIsEqualTo("").AsPrefix()or.AsSuffix()silently passed. One expectation had two spellings that disagreed, and the silent one is the dangerous one, so the guard now also applies to the match type, wherever it is reached:IsEqualTo,Contains,HasItemand the string collection overloads.Just like a regex or wildcard pattern, a prefix is only known when the expectation is verified, so the guard sits next to them in
StringEqualityOptionsand fires after the normalization: a prefix that only becomes empty throughIgnoringIndentation()says just as little as a literal empty one. Anullprefix or suffix is not rejected, because unlike a null regex it still expresses that the subject is null, an expectation that can fail in either polarity, andIsEqualTodeliberately accepts a nullable argument.