Appending list of values in dictionary on same key

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Hello,

need help in adding objects in dictionary, so when there is new object i am adding key & object but struggling with object because i cant cast it or am doing something wrong ? & it already has the key am appending value to list on same key

C#

Dictionary<string, list<object="">> uniqueComp = new Dictionary<string, list<object="">>();

if(!uniqueComp.ContainsKey( cmp1[i].name))
  {
      
      uniqueComp.Add(cmp1[i].name,(System.Collections.Generic.List) cmp1[i]);
  }
  if (uniqueComp.ContainsKey(cmp1[i].name))
  {
// append new object in exisitng key
  }

Please help

What I have tried:

List<object>>();

if(!uniqueComp.ContainsKey( cmp1[i].name))
  {
      
      uniqueComp.Add(cmp1[i].name,(System.Collections.Generic.List) cmp1[i]);
  }
  if (uniqueComp.ContainsKey(cmp1[i].name))
  {
// append new object in exisitng key
  }

Posted 12-Apr-24 1:14am

Member 14224038

Updated 13-Apr-24 7:49am

Andre Oosthuizen

v2

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2 solutions

Solution 1

Your code should look something like this:

C#

if (!uniqueComp.ContainsKey(cmp1[i].name))
{
  uniqueComp.Add(cmp[i].name, new List<object>());
}
uniqueComp[cmp[i].name].Add(... whatever the value is ...);

Basically, you are testing to see whether you have the key or not and, if not, you add a list. At this point, you know that you have the list present, so you can simply call the Add method. You don't need to test whether the key is present again.

Posted 12-Apr-24 1:30am

Pete O'Hanlon

Comments

Richard Deeming 12-Apr-24 7:51am

You can use TryGetValue to reduce the key lookups:

if (!uniqueComp.TryGetValue(cmp1[i].name, out List<object> list))
{
    list = new List<object>();
    uniqueComp.Add(cmp1[i].name, list);
}

list.Add(...);

Pete O'Hanlon 12-Apr-24 8:26am

This is true and it's such a great reply, you should add it to the question so I can award it as many points as possible and recommend it as the accepted answer.

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Richard Deeming · Accepted Answer · 2024-04-12T03:18:00

Solution 2

Further to Pete's excellent answer, you can use the TryGetValue method[^] to avoid the double key lookup of ContainsKey followed by the indexer:

C#

if (!uniqueComp.TryGetValue(cmp1[i].name, out List<object> list))
{
    list = new List<object>();
    uniqueComp.Add(cmp1[i].name, list);
}

list.Add(...);

You could even turn this pattern into an extension method, inspired by the ConcurrentDictionary<TKey, TValue>'s GetOrAdd method[^]:

C#

public static class DictionaryExtensions
{
    public static TValue GetOrAdd<TKey, TValue>(
        this IDictionary<TKey, TValue> dictionary,
        TKey key,
        Func<TKey, TValue> valueFactory)
    {
        if (!dictionary.TryGetValue(key, out TValue value))
        {
            value = valueFactory(key);
            dictionary.Add(key, value);
        }
        
        return value;
    }
}

which would then make your code:

C#

List<object> list = uniqueComp.GetOrAdd(cmp1[i].name, _ => new List<object>());
list.Add(...);

Posted 12-Apr-24 3:18am

Richard Deeming

Comments

Pete O'Hanlon 12-Apr-24 9:32am

I know it's only a measly 5 from me, but I wish I could award this 50 on style points alone. Very elegant.

Pete O'Hanlon 12-Apr-24 9:33am

If an answer is going to be marked as the accepted answer, it should be this one.

Member 14224038 12-Apr-24 10:13am

thanks for the answers guys but i think it wont work when i have multiple objects of same type

So my primary array is like below

(cmp1.name, nut)
(cmp2.name, bolt)
(cmp1.name, nut)
(cmp3.name, washer)
(cmp1.name, nut)
(cmp1.name,nut)
(cmp3.name, washer)

I can have the key & I can have mulitple values in arraylist again & when i say below
if (!uniqueComp.TryGetValue(cmp1[i].name, out List<object> list))
{
list = new List<object>();
uniqueComp.Add(cmp1[i].name, list);
}
there will be new list & delete already old entry for cmp1.name (key) ?
SO i am looking to group nut, bolt, washer in above case.
So the output would be like
(key(cmp1.name, list of(nut)x4))
(key(cmp2.name, list of(bolt)x1))
(key(cmp3.name,list of(washer)x2))

Richard Deeming 12-Apr-24 10:15am

No. If the dictionary already contains the specified key, then TryGetValue will return true, and the list variable will contain the existing value for that key.

The only time it creates a new list is if TryGetValue returns false, meaning that the key does not already exist in the dictionary.

George Swan 12-Apr-24 17:51pm

Is the extension method example correct? Will it accept a new List as a Func that takes a key and returns a value?

Richard Deeming 15-Apr-24 3:27am

It is correct. The code _ => new List<object>() is a lambda method which ignores the input parameter, and returns a new empty List<object>.

The compiler has enough information to infer all of the type parameters used in the lambda, so there's nothing more required. :)

George Swan 15-Apr-24 3:31am

The compiler is smarter than I thought. Excellent, +5 from me