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Dictionary Copy Example
Updated Nov 27, 2023
Dot Net Perls
Copy Dictionary. The C# Dictionary has a copy constructor. When you pass an existing Dictionary to its constructor, it is copied. This is an effective way to copy a Dictionary's data.
When the original Dictionary is modified, the copy is not affected. Once copied, a Dictionary has separate memory locations to the original.
Constructor
Dictionary
First example. We can use a custom loop over the keys, but this is prone to code duplication and errors. It makes no sense to write the loop yourself.
Part 1 We copy the Dictionary into the second Dictionary "copy" by using the copy constructor.
Part 2 The code adds a key to the first Dictionary. And we see that the key added after the copy was made is not in the copy.
Part 3 We print the contents of both collections. The example demonstrates how the internal structures are copied.
Console.WriteLine
using System;
using System.Collections.Generic;

Dictionary<string, int> dictionary = new Dictionary<string, int>();
dictionary.Add("cat", 1);
dictionary.Add("dog", 3);
dictionary.Add("iguana", 5);

// Part 1: copy the Dictionary to a second object.
Dictionary<string, int> copy = new Dictionary<string, int>(dictionary);

// Part 2: change the first Dictionary.
dictionary.Add("fish", 4);

// Part 3: display the 2 Dictionaries.
Console.WriteLine("--- Dictionary 1 ---");
foreach (var pair in dictionary)
{
    Console.WriteLine(pair);
}
Console.WriteLine("--- Dictionary 2 ---");
foreach (var pair in copy)
{
    Console.WriteLine(pair);
}
--- Dictionary 1 --- [cat, 1] [dog, 3] [iguana, 5] [fish, 4] --- Dictionary 2 --- [cat, 1] [dog, 3] [iguana, 5]
Internals. The Dictionary constructor that accepts IDictionary is implemented with this loop. My research established that the foreach-loop is fast and eliminates lookups.
foreach
IDictionary
Also We could easily adapt the loop to copy to a Hashtable or List of KeyValuePair structs.
KeyValuePair
foreach (KeyValuePair<TKey, TValue> pair in dictionary)
{
    this.Add(pair.Key, pair.Value);
}
A summary. In C# we copied an entire Dictionary to a new Dictionary. As Dictionary does not define a Copy or CopyTo method, the copy constructor is the easiest way.
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This page was last updated on Nov 27, 2023 (edit).
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