Looping Over Dictionary in C#

Question

I realize that you cannot iterate over a Dictionary in C# and edit the underlying Dictionary as in the following example:

Dictionary<Resource, double> totalCost = new Dictionary<Resource, double>();
// Populate the Dictionary in here - (not showing code).    
foreach (Resource resource in totalCost.Keys)
{
     totalCost[resource] = 5;
}

One way I see to fix this is to make a List backed by the Dictionary's keys, like this:

Dictionary<Resource, double> totalCost = new Dictionary<Resource, double>();
// Populate the Dictionary in here - (not showing code).    
foreach (Resource resource in new List(totalCost.Keys))
{
     totalCost[resource] = 5;
}

Because I'm not editing the keys themselves, is there any reason that this should not be done or that it's bad to choose this as a solution. (I realize if I was editing those keys, this could cause a lot of problems.)

Thank you.

Edit: Fixed my code example. Sorry about that.

Answer 1

Your first piece of code looks fine to me - you're not editing the dictionary at all.

Answer 2

Hi Jason, in your examples it doesn't look to me like you're editing the dictionary values (or keys)?

In general your solution looks fine, you could do it with a bit less code like this:

List<double> total = new List<double>();
foreach (AKeyObject key in aDictionary.Keys.ToList())
{
   for (int i = 0; i < aDictionary[key].Count; i++)
   {
      total[i] += aDictionary[key][i];
   }
}

Answer 3

You can loop over dictionaries with using KeyValuePair class.

Dictionary<string, string> d1 = new Dictionary<string, string>();
foreach (KeyValuePair<string, string> val in d1)
{ 
    ...
}

Answer 4

Here is another more LINQ-esque version:

totalCost = totalCost
     .ToDictionary( kvp => kvp.Key, 5 );

Or, if 5 is not quite what you want :)

totalCost = totalCost
     .ToDictionary( kvp => kvp.Key, CalculateSomething(kvp.Value) );

(Note: this doesn't edit the underlying dictionary, instead it replaces it with a new one)