Reputation: 17509
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
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:
Upvotes: 32
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
Reputation: 9682
Method 2 is better:
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
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
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
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
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