C# Count Dictionary

You want to compute the number of keys and values in your Dictionary using the C# programming language. With the Count property, you can quickly gauge how much data you are storing in the Dictionary, providing useful metrics about your C# program.

Example

The Count property on Dictionary returns an integer that tells you how many keys are in the Dictionary. This count integer is not equal to the Dictionary fill rate or its internal bucket count. Instead, Count returns the exact number of key/value pairs you have added and not removed.

This C# program demonstrates the Count property on the Dictionary type.

Program that counts keys in Dictionary [C#]

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
	//
	// Create new Dictionary with four keys.
	//
	Dictionary<string, int> dictionary = new Dictionary<string, int>();
	dictionary.Add("carrot", 1);
	dictionary.Add("pear", 4);
	dictionary.Add("apple", 6);
	dictionary.Add("kiwi", 3);
	//
	// Count the keys.
	//
	int count1 = dictionary.Count;
	//
	// Remove one key.
	//
	dictionary.Remove("pear");
	//
	// Count the keys again.
	//
	int count2 = dictionary.Count;
	//
	// Clear the Dictionary contents.
	//
	dictionary.Clear();
	//
	// Count the keys again.
	//
	int count3 = dictionary.Count;
	//
	// Write the counts of the Dictionary.
	//
	Console.WriteLine(count1);
	Console.WriteLine(count2);
	Console.WriteLine(count3);
    }
}

Output

4
3
0

Count property usage. The program above first adds four keys to the Dictionary. Its count at this point is equal to 4. Next, one key is removed, resulting in a count of 3. Third, the Dictionary is cleared, which puts its count value at zero. The Count property is readonly, so don't try to assign it.

Limitations of count. The Count property gives you no insight into the internal implementation state of the Dictionary. Internally, the Dictionary has many fields, such as a "int[] buckets" field, along with freeList and freeCount fields. Unfortunately, you cannot easily access these members outside of the Visual Studio debugger. This could make performance analysis more difficult.

Count keys based on condition

The Count property cannot be used to filter or selectively count keys. To count only keys that match some condition, it is easiest to use a foreach loop over the Keys property, with an if conditional in the loop. The Count() extension from LINQ likely has much worse performance.

Implementation

The Count member on Dictionary is implemented as a property accessor. This means it is not a direct field access. Instead, the call to Count actually does a very simple and fast calculation each time you call it. The "freeCount" field is subtracted from the "count" field, resulting in one "sub" IL instruction.

Implementation of Count [C#]

public int Count
{
    get
    {
	return (this.count - this.freeCount);
    }
}

Summary

We looked at how you can count the number of keys and values in your Dictionary. Because the Dictionary enforces unique keys, you can use Count to compute the number of unique keys in a collection. We reviewed some related issues and looked into the IL instructions generated by Count.