What is BenchmarkDotNet, and why is it preferred over manual Stopwatch-based timing for measuring code performance?

BenchmarkDotNet is a statistically rigorous micro-benchmarking library that handles JIT warm-up, runs many iterations to produce statistically meaningful results, accounts for GC behavior, and reports detailed statistics (mean, error margin, standard deviation, memory allocated) -- manual Stopwatch timing is unreliable for benchmarking because it doesn't account for JIT warm-up effects, GC pauses, or measurement noise, often producing misleading results especially for very fast operations measured in nanoseconds.

[MemoryDiagnoser]
public class StringConcatBenchmarks {
    [Benchmark]
    public string StringConcat() => "a" + "b" + "c";

    [Benchmark]
    public string StringBuilder() => new StringBuilder().Append("a").Append("b").Append("c").ToString();
}

// Run via: BenchmarkRunner.Run<StringConcatBenchmarks>();