SortedDictionary in C# – Beginner to Advanced Guide

1. Default constructor (uses default sorting)

Creates an empty dictionary using the default comparer, which uses IComparable<TKey> or Comparer<TKey>.Default

public SortedDictionary ();

var dict1 = new SortedDictionary<int, string>();

dict1.Add(3, "C#");
dict1.Add(1, "Java");
dict1.Add(2, "Python");

foreach (var kv in dict1)
    Console.WriteLine($"Key: {kv.Key}, Value: {kv.Value}");
// Output sorted by key: 1 → Java, 2 → Python, 3 → C#

This example uses the default comparer, so the numeric keys sort in ascending order.

2. Custom comparer

Creates an empty dictionary using a custom comparer to define key order

public SortedDictionary (System.Collections.Generic.IComparer<TKey> comparer);

class DescendingComparer<T> : IComparer<T> where T : IComparable<T>
{
    public int Compare(T x, T y) => y.CompareTo(x);
}

var dict2 = new SortedDictionary<int, string>(new DescendingComparer<int>());
dict2.Add(3, "Three");
dict2.Add(1, "One");
dict2.Add(2, "Two");

foreach (var kv in dict2)
    Console.WriteLine($"{kv.Key}: {kv.Value}");
// Output: 3 → Three, 2 → Two, 1 → One

Here the custom comparer reverses numeric order.

3. Construct from existing dictionary

Copies elements from an existing dictionary and uses the default comparer.

public SortedDictionary (System.Collections.Generic.IDictionary<TKey,TValue> dictionary);

var init = new Dictionary<string, int>
{
    ["apple"] = 5,
    ["banana"] = 3,
    ["cherry"] = 7
};

var dict3 = new SortedDictionary<string, int>(init);
foreach (var kv in dict3)
    Console.WriteLine($"{kv.Key}: {kv.Value}");
// Output: apple → 5, banana → 3, cherry → 7 (alphabetic order)

This copies from an ordinary dictionary and orders by key naturally.

4. Construct from dictionary with custom comparer

Copies elements from an existing dictionary and uses a custom comparer.

public SortedDictionary (System.Collections.Generic.IDictionary<TKey,TValue> dictionary, 
        System.Collections.Generic.IComparer<TKey>? comparer);

var original = new Dictionary<int, string>
        {
            [10] = "Ten",
            [5]  = "Five",
            [20] = "Twenty",
            [15] = "Fifteen"
        };
var comparer = Comparer<int>.Create((a, b) => b.CompareTo(a));
 var sorted = new SortedDictionary<int, string>(original, comparer);
foreach (var kv in sorted)
    Console.WriteLine($"Key = {kv.Key}, Value = {kv.Value}");

Key = 20, Value = Twenty
Key = 15, Value = Fifteen
Key = 10, Value = Ten
Key = 5, Value = Five

1. ArgumentNullException

Keys in SortedDictionary cannot be null. This exception is thrown early because null keys break the comparer logic.

So, Passing null as the key, either in Add(key, value), dict[key] = value, dict.Remove(null) or retrieving via indexer will throw ArgumentNullException.

var dict = new SortedDictionary<string, int>();

// Throws ArgumentNullException because key is null
dict.Add(null!, 5);

2. ArgumentException

SortedDictionary enforces unique keys. If a key already exists (per comparer logic), Add() throws ArgumentException.

var dict = new SortedDictionary<string, int>();
dict.Add("apple", 1);
dict.Add("banana", 2);

try {
    dict.Add("apple", 10);
} catch (ArgumentException ex) {
    Console.WriteLine("ArgumentException: " + ex.Message);
}

ArgumentException: An item with the same key has already been added. Key: [apple, 10]

3. KeyNotFoundException

If the key is not present, accessing a non-existent key via the indexer: var v = dict[key]; throws KeyNotFoundException exception

var dict = new SortedDictionary<string, int> { ["apple"] = 1 };

try {
    var val = dict["banana"];
} catch (KeyNotFoundException ex) {
    Console.WriteLine("KeyNotFoundException: " + ex.Message);
}

KeyNotFoundException: The given key 'banana' was not present in the dictionary.