Dictionary<K,V> in depth
Learn Dictionary<K,V>'s hashing and collision behavior.
Dictionary<K,V> in depth
A Dictionary<K,V> places keys into a bucket array. Each key's GetHashCode() yields a bucket index; keys landing in the same bucket form a chained list.
When two different keys fall into the same bucket it is called a collision. Collisions are normal; inside the chain the key is compared with Equals. A good hash function reduces collisions and keeps lookups fast.
Add, indexer and TryGetValue
The Add method inserts a new key-value pair; if the key exists it throws ArgumentException. Writing prices["apple"] = 5 overwrites an existing value without error.
To check whether a key exists use ContainsKey or TryGetValue. TryGetValue checks presence and returns the value in one pass; it does not search twice.
Exercise Look up a value with TryGetValue
The prices dictionary below is ready. Use TryGetValue to look up "apple"; if found, print the value with Console.WriteLine.
Show example solution
Declare int value; call TryGetValue, and when it returns true print value.
Example solution:
using System;
using System.Collections.Generic;
class Program
{
static void Main()
{
var prices = new Dictionary<string, int>();
prices.Add("apple", 5);
prices.Add("pear", 3);
int value;
if (prices.TryGetValue("apple", out value))
{
Console.WriteLine(value);
}
}
}Test yourself
-
Complete the TryGetValue call
prices.TryGetValue("elma", out____);Show answer
prices.TryGetValue("elma", outint value); -
What if you Add an existing key?
- Value is updated
- An exception is thrown
- It is ignored
- It is overwritten
Show answer
An exception is thrown — Correct. Add throws ArgumentException on a duplicate key.
-
Match the method to its behavior
- TryGetValue
- Returns value if found
- ContainsKey
- Checks key presence
- Add
- Throws on duplicate
- [] indexer
- Overwrites value