Reference for FluentAssertions - the canonical .NET fluent-assertion library pairable with xUnit / NUnit / MSTest; provides `.Should()` extension API (`.Should().Be()`, `.Should().BeOfType()`, `.Should().Throw()`, `.Should().BeEquivalentTo()` for deep equality, `.Should().Satisfy()` for predicates, `.Should().BeApproximately()` for floats); rich failure messages with object structure visualization. Covers the v8 license change: v8+ is free for open-source and non-commercial use but requires a paid license for commercial use, while v7 remains fully open-source. Use when a .NET test project needs deep object comparison or better failure output than `Assert.X` gives, when assertions must survive a move between xUnit / NUnit / MSTest, or when picking between v7 and v8+ on license grounds.