Loop in Dictionary

Question

I use this:

foreach(KeyValuePair<String,String> entry in MyDic)
  {
      // do something with entry.Value or entry.Key

  }

The problem is that I can't change the value of entry.Value or entry.Key

My question is that how can i change the value or key when looping through a dictionary? And, does dictionary allow duplicated key? And if yes, how can we avoid ? Thank you

Answer 1

You cannot change the value of a dictionary entry while looping through the items in the dictionary, although you can modify a property on the value if it's an instance of a reference type.

For example,

public class MyClass 
{
    public int SomeNumber { get; set;}
}

foreach(KeyValuePair<string, MyClass> entry in myDict)
{
    entry.Value.SomeNumber = 3; // is okay
    myDict[entry.Key] = new MyClass(); // is not okay
}

Trying to modify a dictionary (or any collection) while looping through its elements will result in an InvalidOperationException saying the collection was modified.

To answer your specific questions,

My question is that how can i change the value or key when looping through a dictionary?

The approach to both will be pretty much the same. You can either loop over a copy of the dictionary as Anthony Pengram said in his answer, or you can loop once through all the items to figure out which ones you need to modify and then loop again through a list of those items:

List<string> keysToChange = new List<string>();
foreach(KeyValuePair<string, string> entry in myDict)
{
    if(...) // some check to see if it's an item you want to act on
    {
        keysToChange.Add(entry.Key);
    }
}

foreach(string key in keysToChange)
{
   myDict[key] = "new value";

   // or "rename" a key
   myDict["new key"] = myDict[key];
   myDict.Remove(key);
}

And, does dictionary allow duplicated key? And if yes, how can we avoid ?

A dictionary does not allow duplicate keys. If you want a collection of <string, string> pairs that does, check out NameValueCollection.

Answer 2

Updating the dictionary in the loop is going to be a problem, as you cannot modify the dictionary as it is being enumerated. However, you can work around this pretty easily by converting the dictionary to a list of KeyValuePair<> objects. You enumerate that list, and then you can modify the dictionary.

foreach (var pair in dictionary.ToList())
{
    // to update the value
    dictionary[pair.Key] = "Some New Value";

    // or to change the key => remove it and add something new
    dictionary.Remove(pair.Key);
    dictionary.Add("Some New Key", pair.Value);
}

For the second part, the key in a dictionary must be unique.