How to Merge Two Dictionaries in C# Easily

In C#, you can merge two dictionaries using the Concat method combined with ToDictionary to create a new dictionary. Alternatively, you can use a foreach loop to add or update entries from one dictionary into another.

Syntax

To merge two dictionaries, you can use LINQ's Concat method to combine key-value pairs and then convert them back to a dictionary with ToDictionary. This creates a new dictionary containing all entries.

Alternatively, use a foreach loop to add or update entries from the second dictionary into the first one.

csharp

var merged = dict1.Concat(dict2).ToDictionary(pair => pair.Key, pair => pair.Value);

// Or update dict1 with dict2 entries
foreach (var pair in dict2)
{
    dict1[pair.Key] = pair.Value; // Adds or updates
}

Example

This example shows how to merge two dictionaries into a new one using LINQ, and how to update the first dictionary with entries from the second.

csharp

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main()
    {
        var dict1 = new Dictionary<string, int>
        {
            {"apple", 1},
            {"banana", 2}
        };

        var dict2 = new Dictionary<string, int>
        {
            {"banana", 3},
            {"cherry", 4}
        };

        // Merge into new dictionary
        var merged = dict1.Concat(dict2)
                          .ToDictionary(pair => pair.Key, pair => pair.Value);

        Console.WriteLine("Merged dictionary:");
        foreach (var item in merged)
        {
            Console.WriteLine($"{item.Key}: {item.Value}");
        }

        // Update dict1 with dict2 entries
        foreach (var pair in dict2)
        {
            dict1[pair.Key] = pair.Value;
        }

        Console.WriteLine("\nUpdated dict1:");
        foreach (var item in dict1)
        {
            Console.WriteLine($"{item.Key}: {item.Value}");
        }
    }
}

Output

Merged dictionary:
apple: 1
banana: 3
cherry: 4

Updated dict1:
apple: 1
banana: 3
cherry: 4

Common Pitfalls

When merging dictionaries, a common mistake is not handling duplicate keys, which causes an exception with ToDictionary. Using Concat blindly merges all pairs, so if keys repeat, you must decide which value to keep.

Another pitfall is modifying the original dictionary unintentionally when you want a new merged dictionary.

csharp

try
{
    // This throws if duplicate keys exist
    var merged = dict1.Concat(dict2).ToDictionary(pair => pair.Key, pair => pair.Value);
}
catch (ArgumentException)
{
    // Handle duplicates by choosing dict2's value
    var mergedSafe = dict1.Concat(dict2)
                          .GroupBy(pair => pair.Key)
                          .ToDictionary(g => g.Key, g => g.Last().Value);
}

Quick Reference

• Concat + ToDictionary: Creates a new merged dictionary but throws on duplicate keys.
• GroupBy + ToDictionary: Handles duplicates by selecting which value to keep.
• foreach loop: Updates or adds entries directly to an existing dictionary.

Key Takeaways

Use LINQ's Concat and ToDictionary to merge dictionaries into a new one.

Handle duplicate keys carefully to avoid exceptions during merging.

Use a foreach loop to update an existing dictionary with another's entries.

Decide which dictionary's values to keep when keys overlap.

Merging creates a new dictionary unless you update an existing one explicitly.