How to update the value stored in Dictionary in C#?

Just point to the dictionary at given key and assign a new value:

myDictionary[myKey] = myNewValue;

It's possible by accessing the key as index

for example:

Dictionary<string, int> dictionary = new Dictionary<string, int>();
dictionary["test"] = 1;
dictionary["test"] += 1;
Console.WriteLine (dictionary["test"]); // will print 2

This simple check will do an upsert i.e update or create.

if(!dictionary.TryAdd(key, val))
{
    dictionary[key] = val;
}

1. update - modify existent only. To avoid side effect of indexer use:

   int val;
   if (dic.TryGetValue(key, out val))
   {
       // key exist
       dic[key] = val;
   }
   

2. update or (add new if value doesn't exist in dic)

   dic[key] = val;
   

   for instance:

   d["Two"] = 2; // adds to dictionary because "two" not already present
   d["Two"] = 22; // updates dictionary because "two" is now present
   

This may work for you:

Scenario 1: primitive types

string keyToMatchInDict = "x";
int newValToAdd = 1;
Dictionary<string,int> dictToUpdate = new Dictionary<string,int>{"x",1};

if(!dictToUpdate.ContainsKey(keyToMatchInDict))
   dictToUpdate.Add(keyToMatchInDict ,newValToAdd );
else
   dictToUpdate[keyToMatchInDict] = newValToAdd; //or you can do operations such as ...dictToUpdate[keyToMatchInDict] += newValToAdd;

Scenario 2: The approach I used for a List as Value

int keyToMatch = 1;
AnyObject objInValueListToAdd = new AnyObject("something for the Ctor")
Dictionary<int,List<AnyObject> dictToUpdate = new Dictionary<int,List<AnyObject>(); //imagine this dict got initialized before with valid Keys and Values...

if(!dictToUpdate.ContainsKey(keyToMatch))
   dictToUpdate.Add(keyToMatch,new List<AnyObject>{objInValueListToAdd});
else
   dictToUpdate[keyToMatch] = objInValueListToAdd;

Hope it's useful for someone in need of help.

This extension method allows a match predicate delegate as the dictionary key selector, and a separate delegate to perform the dictionary value replacement, so it's completely open as to the type of key/value pair being used:

public static void UpdateAll<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, Func<TKey, TValue, bool> matchPredicate, Func<TValue, TValue> updatePredicate)
{
  var keys = dictionary.Keys.Where(k => matchPredicate(k, dictionary[k])).ToList();
  foreach (var key in keys)
  {
    dictionary[key] = updatePredicate(dictionary[key]);
  }
}

Example usage:

    Dictionary<int, string> dict = new Dictionary<int, string>();
    dict.Add(1, "One");
    dict.Add(2, "Two");
    dict.Add(3, "Three");

    //Before
    foreach(var kvp in dict){
      Console.WriteLine(kvp.Value);
    }

    dict.UpdateAll(
        matchPredicate: (k, v) => k >= 2, //Update any dictionary value where the key is >= 2
        updatePredicate: (v) => v = v + " is greater than One"
      );

    //After
    foreach(var kvp in dict){
      Console.WriteLine(kvp.Value);
    }

You Can Also Use This Method :

Dictionary<int,int> myDic = new();
if (myDic.ContainsKey(1))
{
    myDic[1] = 1234; // or use += to update it 
}

Or By Value :

if (myDic.ContainsValue(1))
{
    //do something ... 
}