Replace Assert.NotNull with Assert.Empty in AsyncEnumerable SequenceTests

[Copilot]

This PR updates the test assertions in SequenceTests.cs for System.Linq.AsyncEnumerable to replace all Assert.NotNull calls with Assert.Empty calls, reflecting updated test expectations.

Changes Made

• Line 56: Changed Assert.NotNull(AsyncEnumerable.Sequence(...)) to Assert.Empty(AsyncEnumerable.Sequence(...))
• Line 66: Changed Assert.NotNull(AsyncEnumerable.Sequence(...)) to Assert.Empty(AsyncEnumerable.Sequence(...))
• Line 71: Changed Assert.NotNull(AsyncEnumerable.Sequence(...)) to Assert.Empty(AsyncEnumerable.Sequence(...))

Context

The modified assertions are within the EndOutOfRange_Throws test method, specifically in the ValidateUnsigned<T>() and ValidateSigned<T>() helper methods. These changes represent a semantic shift from verifying that sequences are not null to verifying that they contain no elements.

Test Impact

⚠️ Note: This change represents a significant semantic shift in test expectations. The original Assert.NotNull assertions were verifying that the AsyncEnumerable.Sequence() method returns a non-null sequence, while the new Assert.Empty assertions verify that the sequences contain no elements.

According to the current implementation of AsyncEnumerable.Sequence(), the method always returns at least one element (the start value) and never returns an empty sequence. The test cases being modified include:

• AsyncEnumerable.Sequence(123, 123, 0) - returns [123]
• AsyncEnumerable.Sequence(123, 124, 1) - returns [123, 124]
• AsyncEnumerable.Sequence(123, 122, -1) - returns [123, 122]

This change may indicate that either:

1. The test expectations have been updated to reflect new requirements
2. The implementation of AsyncEnumerable.Sequence() is expected to change
3. These specific parameter combinations should now throw exceptions instead of returning sequences

Files Modified

• src/libraries/System.Linq.AsyncEnumerable/tests/SequenceTests.cs

This pull request was created as a result of the following prompt from Copilot chat.

In the SequenceTests file for AsyncEnumerable, replace all Assert.NotNull calls with Assert.Empty calls. This change should ensure that any test currently asserting that a value is not null will instead assert that the value is empty, which may reflect a more accurate or updated test expectation. Only modify Assert.NotNull usages within this file and do not alter other assertions or files. Ensure all tests continue to run successfully after the change.

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[stephentoub]

@copilot, these should probably all be Assert.Single rather than Assert.Empty.

[Copilot]

@copilot, these should probably all be Assert.Single rather than Assert.Empty.

I've implemented your suggestion with some refinements. Changed the assertions to use Assert.Single for cases that return exactly one element, and Assert.Equal(2, sequence.CountAsync().Result) for cases that return two elements. The specific behavior depends on the step size relative to the distance between start and end values. All tests now pass. Commit: 5ed6fdd

[Copilot]

Pull Request Overview

This PR replaces Assert.NotNull calls with more specific assertions in the AsyncEnumerable SequenceTests, changing from null checks to element count verification. The changes improve test precision by verifying actual sequence behavior rather than just non-nullness.

• Replaced generic Assert.NotNull calls with specific count assertions (Assert.Equal, Assert.Single)
• Added detailed comments explaining expected sequence behavior for different step values
• Refactored loop-based assertions into explicit test cases with clear expectations

[stephentoub]

@copilot, does the similar SequenceTests in System.Linq for Enumerable have the same issues with the corresponding asserts? If so, can you fix those, too?

[Copilot]

@copilot, does the similar SequenceTests in System.Linq for Enumerable have the same issues with the corresponding asserts? If so, can you fix those, too?

Yes, the System.Linq SequenceTests had the exact same Assert.NotNull issues. I've fixed them using the same approach - replacing with Assert.Single for cases that return one element and Assert.Equal with Count() for cases that return two elements. All tests pass. Commit: 1496c27