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Clear DictionaryTest the Clear method on the Dictionary type. Benchmark the performance of Clear.
C#
This page was last reviewed on May 25, 2021.
Clear, Dictionary. Clear() removes all existing Dictionary keys (and their values). In most cases, testing a Dictionary before using it is the best strategy.
Shows a 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.
Dictionary
Array.Clear
List Clear
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.
Shows a dictionary
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); } }
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 Dictionary
Property
Version 1 This version of the code calls Clear each time through the inner loop. We compute the average amount of time.
Benchmark
Stopwatch
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 Tested in .NET 5, version 2 is faster. It is best to call Count before calling 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")); } }
1.88 ns Always call Clear 1.04 ns Call Clear if Count > 0
A summary. Often we need to erase all the elements in a C# Dictionary. This allows us to reuse an existing Dictionary. The Clear() method helps here.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on May 25, 2021 (simplify).
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