how to update the values while iterating dictionary items?

Question

I have a dictionary:

Dictionary<string, long> Reps = new Dictionary<string, long>();

and I want to update the values while iterating through all items, like this:

foreach (string key in Reps.keys)
{
    Reps[key] = 0;
}

it is giving me an error saying:

"Collection was modified; enumeration operation may not execute"

can anyone tell me why it is giving me this error, because I have one more function that adds the value, and it is called when button is clicked:

public static void Increment(string RepId, int amount)
{
     long _value = Convert.ToInt64(Reps[RepId]);
     _value = _value + amount;
     Reps[RepId] = _value;
}

and this function is working fine. so whats the problem when updating all the values? And whats the solution for this?

Answer 1

more simplified, do this:

foreach (string key in Reps.keys.ToList())
{
    Reps[key] = 0;
}

and the reason for the error is you are trying to edit the actual object which is in use and if you make a copy of it and then use it like this:

var repscopy = Reps;
foreach (string key in repscopy.keys)
    {
        Reps[key] = 0;
    }

it'll give the same error as it also pointing to the original object, and when the ToList() is added it created a new object of List