How to iterate through Dictionary and change values?

Question

Dictionary<string,double> myDict = new Dictionary();
//...
foreach (KeyValuePair<string,double> kvp in myDict)
 {
     kvp.Value = Math.Round(kvp.Value, 3);
}

I get an error: "Property or indexer 'System.Collections.Generic.KeyValuePair.Value' cannot be assigned to -- it is read only."
How can I iterate through myDict and change values?

Answer 1

You shouldn't change the dictionary while iterating it, otherwise you get an exception.

So first copy the key-value pairs to a temp list and then iterate through this temp list and then change your dictionary:

Dictionary<string, double> myDict = new Dictionary<string, double>();

// a few values to play with
myDict["a"] = 2.200001;
myDict["b"] = 77777.3333;
myDict["c"] = 2.3459999999;

// prepare the temp list
List<KeyValuePair<string, double>> list = new List<KeyValuePair<string, double>>(myDict);

// iterate through the list and then change the dictionary object
foreach (KeyValuePair<string, double> kvp in list)
{
    myDict[kvp.Key] = Math.Round(kvp.Value, 3);
}


// print the output
foreach (var pair in myDict)
{
    Console.WriteLine(pair.Key + " = " + pair.Value);
}

// uncomment if needed
// Console.ReadLine();

output (on my machine):

a = 2.2
b = 77777.333
c = 2.346

Note: in terms of performance, this solution is a bit better than currently posted solutions, since the value is already assigned with the key, and there's no need to fetch it again from the dictionary object.

Answer 2

Look through the keys in the dictionary, not the KeyValuePairs.

Dictionary<string, double> myDict = new Dictionary<string, double>();
//...
foreach (string key in myDict.Keys)
{
    myDict[key] = Math.Round(myDict[key], 3);
}

Answer 3

According to MSDN:

The foreach statement is a wrapper around the enumerator, which allows only reading from the collection, not writing to it.

Use this:

Dictionary<string, double> dictionary = new Dictionary<string, double>();
// populate it
foreach (string key in dictionary.Keys)
{
   dictionary[key] = Math.Round(dictionary[key], 3);
}