Difference between SortedList and SortedDictionary in C#

Both SortedList and SortedDictionary in C# are generic collections that store key-value pairs in sorted order based on the key. However, they differ significantly in their internal implementation, memory usage, and performance characteristics.

Understanding these differences helps you choose the right collection for your specific use case based on performance requirements and usage patterns.

Syntax

Following is the syntax for declaring a SortedList −

SortedList<TKey, TValue> sortedList = new SortedList<TKey, TValue>();

Following is the syntax for declaring a SortedDictionary −

SortedDictionary<TKey, TValue> sortedDictionary = new SortedDictionary<TKey, TValue>();

Key Differences

Using SortedList

Example

using System;
using System.Collections.Generic;

class Program {
   public static void Main() {
      SortedList<string, int> sortedList = new SortedList<string, int>();
      
      // Adding elements
      sortedList.Add("Charlie", 30);
      sortedList.Add("Alice", 25);
      sortedList.Add("Bob", 35);
      
      Console.WriteLine("SortedList contents:");
      foreach (var kvp in sortedList) {
         Console.WriteLine($"{kvp.Key}: {kvp.Value}");
      }
      
      // Index-based access
      Console.WriteLine($"\nFirst item: {sortedList.Keys[0]} = {sortedList.Values[0]}");
      Console.WriteLine($"Count: {sortedList.Count}");
   }
}

The output of the above code is −

SortedList contents:
Alice: 25
Bob: 35
Charlie: 30

First item: Alice = 25
Count: 3

Using SortedDictionary

Example

using System;
using System.Collections.Generic;

class Program {
   public static void Main() {
      SortedDictionary<string, int> sortedDict = new SortedDictionary<string, int>();
      
      // Adding elements
      sortedDict.Add("Charlie", 30);
      sortedDict.Add("Alice", 25);
      sortedDict.Add("Bob", 35);
      
      Console.WriteLine("SortedDictionary contents:");
      foreach (var kvp in sortedDict) {
         Console.WriteLine($"{kvp.Key}: {kvp.Value}");
      }
      
      // Key-based access only
      Console.WriteLine($"\nAlice's age: {sortedDict["Alice"]}");
      Console.WriteLine($"Count: {sortedDict.Count}");
   }
}

The output of the above code is −

SortedDictionary contents:
Alice: 25
Bob: 35
Charlie: 30

Alice's age: 25
Count: 3

Performance Comparison Example

Example

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

class Program {
   public static void Main() {
      const int iterations = 100000;
      
      // SortedList performance
      var sortedList = new SortedList<int, string>();
      var sw = Stopwatch.StartNew();
      
      for (int i = 0; i < iterations; i++) {
         sortedList[i] = $"Value{i}";
      }
      sw.Stop();
      Console.WriteLine($"SortedList insertion: {sw.ElapsedMilliseconds} ms");
      
      // SortedDictionary performance
      var sortedDict = new SortedDictionary<int, string>();
      sw.Restart();
      
      for (int i = 0; i < iterations; i++) {
         sortedDict[i] = $"Value{i}";
      }
      sw.Stop();
      Console.WriteLine($"SortedDictionary insertion: {sw.ElapsedMilliseconds} ms");
      
      Console.WriteLine($"\nSortedList memory (approx): Lower");
      Console.WriteLine($"SortedDictionary memory (approx): Higher");
   }
}

The output of the above code is −

SortedList insertion: 156 ms
SortedDictionary insertion: 31 ms

SortedList memory (approx): Lower
SortedDictionary memory (approx): Higher

When to Use Which

Use SortedList when:

• You need index-based access to elements

• Memory usage is a concern

• You perform more lookups than insertions/deletions

• You need to access elements by position

Use SortedDictionary when:

• You frequently insert and delete elements

• You only need key-based access

• Insertion performance is more important than memory usage

• You're working with large datasets with frequent modifications

Conclusion

SortedList offers better memory efficiency and index-based access but slower insertions, while SortedDictionary provides faster insertions and deletions using a tree structure. Choose SortedList for read-heavy scenarios and SortedDictionary for write-heavy operations with frequent modifications.