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C# Clear Dictionary: Remove All Keys

Test the Clear method on the Dictionary type. Benchmark the performance of Clear.
Clear, Dictionary. Clear() removes all existing Dictionary keys (and their values). In most cases, testing a Dictionary before using it is the best strategy.Dictionary
An experiment. We see whether you should test for an empty Dictionary before clearing one. This helps us learn how best to handle Dictionaries.
First example. This program shows how the Clear method works. After we call Clear, there are always zero elements in a Dictionary. It erases the internals.
C# program that clears Dictionary contents using System.Collections.Generic; using static System.Console; class Program { static void Main() { // Create a small dictionary. // ... Add 2 keys. var lookup = new Dictionary<string, int>(); lookup["cat"] = 10; lookup["dog"] = 20; // Write the count. WriteLine(lookup.Count); // Clear the dictionary and write its new count. lookup.Clear(); WriteLine(lookup.Count); } } Output 2 0
Benchmark, clear. A Dictionary is complex. Some actions may be slower than you think. The Count and Clear members both seem simple. But the Count property is much faster than Clear.Count DictionaryProperty

Version 1: This version of the code calls Clear each time through the inner loop. We compute the average amount of time.

BenchmarkStopwatch

Version 2: This code uses Clear but only when there are 1 or more elements in the Dictionary. So it can avoid calling Clear.

Result: Version 2 is faster. If you may have an empty Dictionary, it is best to call Count before calling Clear.

C# program that times Clear using System; using System.Collections.Generic; using System.Diagnostics; class Program { const int _max = 100000000; static void Main() { var dict = new Dictionary<string, string>(); var s1 = Stopwatch.StartNew(); // Version 1: use Clear. for (int i = 0; i < _max; i++) { dict.Clear(); } s1.Stop(); var s2 = Stopwatch.StartNew(); // Version 2: use Clear if needed. for (int i = 0; i < _max; i++) { if (dict.Count > 0) { dict.Clear(); } } s2.Stop(); Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns")); Console.Read(); } } Output 3.17 ns Always call Clear 0.64 ns Call Clear if Count > 0
It is faster to check for zero elements before clearing it if this case is common. Overall it seems faster to check for possible cases where no action should be taken on the Dictionary.

And: This is also true with the indexer to add a key: it is faster to call TryGetValue or ContainsKey rather than call the indexer.

IndexerTryGetValueContainsKey
Using the Dictionary efficiently is an important optimization strategy. By storing data in memory, you can avoid a lot of computations, disk accesses, or even network loads.
For performance, Clear is a good choice. The Clear method too can be used in a more efficient way. This avoids unneeded computations.
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