Benchmark different ways to create dictionary in .NET Core

Original Stackoverflow Question from Oct 2011

Idea for this test stems from Stackoverflow question I asked 8 years ago (Oct 2011):

How to convert IEnumerable of KeyValuePair<x, y> to Dictionary?

Since I unsuccesful attempted to use .ToDictionary() extension method from System.Linq namespace,correct answer was to add lambda selectors for key and value:

.ToDictionary(kvp=>kvp.Key,kvp=>kvp.Value);

My assumption was that, since Dictionary is implementation of ICollection<KeyValuePair<TKey, TValue>>, there has to be "direct", most efficient way to convert IEnumerable<KeyValuePair<TKey, TValue>> to Dictionary.

Benchmark creation of dictionary in .NET Core 3.0 using BenchmarkNet Nuget package

Few days ago (Nov 2019) there were few upvotes/downvotes on the original question, so I have taken a look at the my quesiton and answers again. With the hindsight, I wondered why I simply did not use Dictionary<TKey, TValue> constructor which accepts IEnumerable<KeyValuePair<TKey, TValue>>.

There was also one answer proposing usage of IDictionary<TKey, TValue> interface, claming 20% improvement over System.Linq's ToDictionary(..) extension method. I started to wonder if Dictionary<TKey, TVAlue> constructor overload accepting IEnumerable<KeyValuePair<TKey, TValue>> is more efficient than both extension method and IDictionary casting.

Several months ago, while reading Stephen Toub's blog post Performance Improvements in .NET Core 3.0, I played with Benchmark.NET library, so I decided to use same methodology to measure performance of 3 methods above.

Benchmarks can be run using following command from the repository root folder:

> dotnet run -c Release

(Surprising) Benchmark Results

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateUsingDictionaryConstructor	3.396 ms	0.0300 ms	0.0281 ms	257.8125	257.8125	257.8125	2.9 MB
CreateUsingLinqExtension	2.891 ms	0.0271 ms	0.0254 ms	253.9063	253.9063	253.9063	2.9 MB
CreateUsingCodeCopiedFromLinqExtensionSourceCode	5.565 ms	0.0806 ms	0.0714 ms	601.5625	554.6875	554.6875	8.06 MB
CreateUsingForEachAndApend	3.917 ms	0.0204 ms	0.0171 ms	1906.2500	-	-	7.63 MB
CreateUsingForEachApendAndInitialSize	5.230 ms	0.1346 ms	0.1124 ms	2296.8750	390.6250	390.6250	10.53 MB
CreateUsingIDictionaryInterface	5.823 ms	0.0451 ms	0.0422 ms	578.1250	531.2500	531.2500	8.06 MB