Testing Edge Cases — Boundary Values, Null Handling and Concurrency

Edge cases are where bugs live. Happy path tests verify that the system works correctly with ideal input. Edge case tests verify the boundaries — the off-by-one errors, the null inputs, the exact boundary of a validation rule, the concurrent modification scenario. Most production bugs are edge cases that were never tested because “that can’t happen” — until it does.

Edge Case Tests #

// ── File size boundary tests ──────────────────────────────────────────────
public class FileUploadValidationTests
{
    private readonly FileUploadValidator _sut = new(maxSizeBytes: 5_000_000);

    [Theory]
    [InlineData(0,         false)]           // empty file
    [InlineData(1,         true)]            // minimum valid
    [InlineData(4_999_999, true)]            // one byte under limit
    [InlineData(5_000_000, true)]            // exactly at limit (boundary)
    [InlineData(5_000_001, false)]           // one byte over limit
    [InlineData(10_000_000, false)]          // well over limit
    public void Validate_SizeCheck_ReturnsExpectedResult(long fileSize, bool isValid)
    {
        var result = _sut.ValidateSize(fileSize);
        result.IsValid.Should().Be(isValid,
            because: $"file size {fileSize} bytes should be {(isValid ? "valid" : "invalid")}");
    }
}

// ── Pagination boundary tests ─────────────────────────────────────────────
public class PaginationTests
{
    [Theory]
    [InlineData(100, 1, 10, true,  false)]  // first page of 10
    [InlineData(100, 10, 10, false, true)]  // last page of 10
    [InlineData(100, 5, 10, true,  true)]   // middle page
    [InlineData(0,   1, 10, false, false)]  // empty result
    [InlineData(1,   1, 10, false, false)]  // single item, fits on one page
    [InlineData(10,  1, 10, false, false)]  // exactly one full page
    [InlineData(11,  1, 10, true,  false)]  // one item spills to page 2
    public void Create_ReturnsCorrectHasNextHasPrev(
        int total, int page, int pageSize, bool hasNext, bool hasPrev)
    {
        var result = PagedResult<string>.Create([], total, page, pageSize);
        result.HasNextPage.Should().Be(hasNext);
        result.HasPrevPage.Should().Be(hasPrev);
    }
}

// ── ViewCount never-negative test ─────────────────────────────────────────
public class ViewCountTests
{
    private readonly Mock<IPostsRepository> _repo = new();
    private readonly PostsService _sut;

    [Fact]
    public async Task DecrementViewCount_WhenAlreadyZero_StaysAtZero()
    {
        var post = new Post { Id = 1, ViewCount = 0 };
        _repo.Setup(r => r.GetByIdAsync(1, default)).ReturnsAsync(post);
        _repo.Setup(r => r.UpdateAsync(It.IsAny<Post>(), default))
             .Callback<Post, CancellationToken>((p, _) => post = p)
             .Returns(Task.CompletedTask);

        await _sut.DecrementViewCountAsync(1, default);

        post.ViewCount.Should().Be(0, because:
            "view count must never go negative; minimum is 0");
    }
}

// ── Unicode slug generation ───────────────────────────────────────────────
public class SlugEdgeCaseTests
{
    private readonly SlugGenerator _sut = new();

    [Fact]
    public void Generate_WithEmDash_ReplacesWithHyphen()
    {
        var result = _sut.Generate("API Design—Best Practices");
        result.Should().Be("api-design-best-practices");
    }

    [Fact]
    public void Generate_WithConsecutiveSpecialChars_CollapsesHyphens()
    {
        var result = _sut.Generate("C# & .NET — Together");
        result.Should().NotContain("--");  // no double hyphens
    }

    [Fact]
    public void Generate_WithVeryLongTitle_TruncatesAt200Chars()
    {
        var longTitle = new string('a', 250);  // 250 chars
        var result    = _sut.Generate(longTitle);
        result.Length.Should().BeLessThanOrEqualTo(200);
    }

    [Fact]
    public void Generate_WithAllNonAscii_ReturnsEmpty()
    {
        var result = _sut.Generate("日本語のタイトル");
        result.Should().BeEmpty();  // or a fallback UUID-based slug
    }
}

Common Mistakes #

Mistake 1 — Testing only happy path boundaries (missing off-by-one bugs) #

❌ Wrong — test with 4MB file (passes) and 6MB file (fails); never test 5MB exactly or 5MB+1 byte.

✅ Correct — test at, just under, and just over every boundary; catches strict vs non-strict comparison bugs.

Mistake 2 — Not testing empty collections separately from null #

❌ Wrong — test returns correct data but never test what happens when the repository returns an empty list vs null.

✅ Correct — test both: ReturnsAsync(new List<Post>()) (empty) and ReturnsAsync((List<Post>?)null) (null).