CODEWITHSUNDEEP
c#dictionarycontainskey
Manish Basantani
Manish Basantani

Reputation: 17509

Method to add new or update existing item in C# Dictionary

In some legacy code I have seen the following extension method to facilitate adding a new key-value item or updating an existing value:

Method-1 (legacy code):

public static void CreateNewOrUpdateExisting<TKey, TValue>(
    this IDictionary<TKey, TValue> map, TKey key, TValue value)
{            
    if (map.ContainsKey(key))
    {
        map[key] = value;
    }
    else
    {
        map.Add(key, value);
    }
}

Though, I have checked that map[key] = value does exactly the same job. That is, Method-1 could be replace with Method-2 below.

Method-2:

public static void CreateNewOrUpdateExisting<TKey, TValue>(
    this IDictionary<TKey, TValue> map, TKey key, TValue value)
{
    map[key] = value;
}

Now, my question is...
Could there be any problem if I replace Method-1 by Method-2?
Will it break in any possible scenario?

Also, I think this used to be the difference between HashTable and Dictionary. HashTable allows updating an item, or adding a new item by using indexer while Dictionary does not!
Has this difference been eliminated in C# > 3.0 versions?

The objective of this method is too not throw an exception if the user sends the same key-value again. The method should, if the key is:

Upvotes: 322

Views: 279199

Answers (7)

Luis Filipe
Luis Filipe

Reputation: 8718

Old question but I feel I should add the following, even more because .NET 4.0 had already launched at the time the question was written.

Starting with .NET 4.0 there is the namespace System.Collections.Concurrent which includes collections that are thread-safe.

The collection System.Collections.Concurrent.ConcurrentDictionary<> does exactly what you want. It has the AddOrUpdate() method with the added advantage of being thread-safe.

If you're in a high-performance scenario and not handling multiple threads the already given answers of map[key] = value are faster.

In most scenarios this performance benefit is insignificant. If so I'd advise to use the ConcurrentDictionary because:

  1. It is in the framework - It is more tested and you are not the one who has to maintain the code
  2. It is scalable: if you switch to multithreading your code is already prepared for it

Upvotes: 32

Shiran1136460
Shiran1136460

Reputation: 171

The only problem could be if one day

map[key] = value

will transform to -

map[key]++;

and that will cause a KeyNotFoundException.

Upvotes: 15

Misha Zaslavsky
Misha Zaslavsky

Reputation: 9682

Method 2 is better:

  • Less code
  • Does the exact same (except for some edge cases where it is not possible)
  • Faster

Here is a benchmark sample to demonstrate the performance advantage of method 2.

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

public class Program
{
    public static void Main()
    {
        BenchmarkRunner.Run<DictionaryBenchmarks>();
    }
}

[MemoryDiagnoser]
public class DictionaryBenchmarks
{
    private readonly IDictionary<int, bool> _map = new Dictionary<int, bool>();
    private readonly int[] _arr = new int[20]
    {
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19
    };

    [Benchmark]
    public void CreateNewOrUpdateExistingWithContainsKeyCheck()
    {
        foreach (int item in _arr)
        {
            if (_map.ContainsKey(item))
            {
                _map[item] = true;
            }
            else
            {
                _map.Add(item, true);
            }
        }
    }

    [Benchmark]
    public void CreateNewOrUpdateExistingWithoutContainsKeyCheck()
    {
        foreach (int item in _arr)
        {
            _map[item] = true;
        }
    }
}
Method Mean Error StdDev Allocated
CreateNewOrUpdateExistingWithContainsKeyCheck 232.0 ns 4.19 ns 10.52 ns -
CreateNewOrUpdateExistingWithoutContainsKeyCheck 131.9 ns 2.12 ns 4.83 ns -

Upvotes: 2

Steven
Steven

Reputation: 172825

There's no problem. I would even remove the CreateNewOrUpdateExisting from the source and use map[key] = value directly in your code, because this this idiomatic C#; C# developers would typically know that map[key] = value means add or update.

Upvotes: 67

StepUp
StepUp

Reputation: 38199

I know it is not Dictionary<TKey, TValue> class, however you can avoid KeyNotFoundException while incrementing a value like:

dictionary[key]++; // throws `KeyNotFoundException` if there is no such key

by using ConcurrentDictionary<TKey, TValue> and its really nice method AddOrUpdate()..

Let me show an example:

var str = "Hellooo!!!";
var characters = new ConcurrentDictionary<char, int>();
foreach (var ch in str)
    characters.AddOrUpdate(ch, 1, (k, v) => v + 1);

Upvotes: 3

ulrichb
ulrichb

Reputation: 20054

Could there be any problem if i replace Method-1 by Method-2?

No, just use map[key] = value. The two options are equivalent.


Regarding Dictionary<> vs. Hashtable: When you start Reflector, you see that the indexer setters of both classes call this.Insert(key, value, add: false); and the add parameter is responsible for throwing an exception, when inserting a duplicate key. So the behavior is the same for both classes.

Upvotes: 340

ya23
ya23

Reputation: 14526

Functionally, they are equivalent.

Performance-wise map[key] = value would be quicker, as you are only making single lookup instead of two.

Style-wise, the shorter the better :)

The code will in most cases seem to work fine in multi-threaded context. It however is not thread-safe without extra synchronization.

Upvotes: 10

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