Clear, Dictionary. Clear() removes all existing Dictionary keys (and their values). In most cases, testing a Dictionary before using it is the best strategy.

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.

using System.Collections.Generic;
using static System.Console;

// 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.

Dictionary Count

Property

Benchmark

Stopwatch

using System;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 100000000;
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

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.