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

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 

You can follow this approach:

void addOrUpdate(Dictionary<int, int> dic, int key, int newValue) { int val; if (dic.TryGetValue(key, out val)) { // yay, value exists! dic[key] = val + newValue; } else { // darn, lets add the value dic.Add(key, newValue); } } 

The edge you get here is that you check and get the value of corresponding key in just 1 access to the dictionary. If you use ContainsKey to check the existance and update the value using dic[key] = val + newValue; then you are accessing the dictionary twice.

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

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

Use LINQ: Access to dictionary for the key and change the value

Dictionary<string, int> dict = new Dictionary<string, int>(); dict = dict.ToDictionary(kvp => kvp.Key, kvp => kvp.Value + 1); 

Here is a way to update by an index much like foo[x] = 9 where x is a key and 9 is the value

var views = new Dictionary<string, bool>(); foreach (var g in grantMasks) { string m = g.ToString(); for (int i = 0; i <= m.Length; i++) { views[views.ElementAt(i).Key] = m[i].Equals('1') ? true : false; } } 

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 

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 ... } 

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); }