What's the fastest way to copy the values and keys from one dictionary into another in C#?

Question

There doesn't seem to be a dictionary.AddRange() method. Does anyone know a better way to copy the items to another dictionary without using a foreach loop.

I'm using the System.Collections.Generic.Dictionary. This is for .NET 2.0.

Stepparent

Answer 1

There's nothing wrong with a for/foreach loop. That's all a hypothetical AddRange method would do anyway.

The only extra concern I'd have is with memory allocation behaviour, because adding a large number of entries could cause multiple reallocations and re-hashes. There's no way to increase the capacity of an existing Dictionary by a given amount. You might be better off allocating a new Dictionary with sufficient capacity for both current ones, but you'd still need a loop to load at least one of them.

Menagerie

that's all a hypothetical AddRange method would do anyway No, it's very different. Under the hood, Array.Copy is used, which does copies on large blocks of contiguous bytes like a native memcpy, not one element at a time. While for(each) is O(n), Array.Copy/memcpy is more like O( n / ( blockSize * vectorWidth) ) where blockSize might be the width of a wide register and vectorWidth depends on your AVX/SSE implementation, for example. But yes, foreach is all we have. – Allyn

@Allyn You're looking at List<T>.AddRange. The question is about Dictionary. The Dictionary's underlying storage is an array, but the position of an item depends on the hash code, the size of the array, and the number of other items that ended up in the same hash bucket. See Dictionary.Insert: referencesource.microsoft.com/#mscorlib/system/collections/…. It can't just copy blocks of elements out of the other dictionary, because they won't be in the right place for this one. – Menagerie

"That's all a hypothetical AddRange method would do anyway". I hear you, yet you're reasoning it one way and I'm reasoning it another. List<T>.AddRange() is specifically meant to do blockwise copies IMO, hence -Range and its under-the-hood implementation. As I recently stated in my answer below, the same type of (contiguous, unfragemented, blockwise) copy is not possible with dictionaries. So it comes down semantics: your interpretation of AddRange() vs mine. Mine says that to use that name at all, you should be sure of contiguity. Perhaps you see this differently. – Allyn

That is to say, I would rather in that case use a name like AddMultiple() or AddEntries(). I definitely would not use AddRange() which later might confuse me as to what underlying implementation is being used, since a very different approach is needed from that of List<T>. One is fast, and the other (foreach) is a lot slower. In my world, that matters a lot. – Allyn

There's no way to increase the capacity of an existing Dictionary by a given amount. what about destination.EnsureCapacity(destination.Count + source.Count);? This should lead to a maximum of one re-hash – Palomino

Answer 2

There's the Dictionary constructor that takes another Dictionary.

You'll have to cast it IDictionary, but there is an Add() overload that takes KeyValuePair<TKey, TValue>. You're still using foreach, though.

Clem

Thanks for the answer, but both dictionaries have items in them at the point I need to do the copy. – Stepparent

Answer 3

var Animal = new Dictionary<string, string>();

one can pass existing animal Dictionary to the constructor.

Dictionary<string, string> NewAnimals = new Dictionary<string, string>(Animal);

Cither

Answer 4

For fun, I created this extension method to dictionary. This should do a deep copy wherever possible.

public static Dictionary<TKey, TValue> DeepCopy<TKey,TValue>(this Dictionary<TKey, TValue> dictionary)
        {
            Dictionary<TKey, TValue> d2 = new Dictionary<TKey, TValue>();

            bool keyIsCloneable = default(TKey) is ICloneable;
            bool valueIsCloneable = default(TValue) is ICloneable;

            foreach (KeyValuePair<TKey, TValue> kvp in dictionary)
            {
                TKey key = default(TKey);
                TValue value = default(TValue);
                if (keyIsCloneable)
                {
                    key = (TKey)((ICloneable)(kvp.Key)).Clone();
                }

                else
                {
                    key = kvp.Key;
                }

                if (valueIsCloneable)
                {
                    value = (TValue)((ICloneable)(kvp.Value)).Clone();
                }

                else
                {
                    value = kvp.Value;
                }

                d2.Add(key, value);
            }

            return d2;
        }

Mediaeval

Answer 5

If you're dealing with two existing objects, you might get some mileage with the CopyTo method: http://msdn.microsoft.com/en-us/library/cc645053.aspx

Use the Add method of the other collection (receiver) to absorb them.

Mint

I couldn't find an Add method for Dictionary that would take an Array? – Stepparent

Answer 6

I don't understand, why not using the Dictionary( Dictionary ) (as suggested by ageektrapped ).

Do you want to perform a Shallow Copy or a Deep Copy? (that is, both Dictionaries pointing to the same references or new copies of every object inside the new dictionary?)

If you want to create a new Dictionary pointing to new objects, I think that the only way is through a foreach.

Selaginella

Shallow copy is fine. I have two dictionaries that I'm populating in a method and I want to copy the second smaller dictionary into the first one at the end of the method. I need to keep them separate during the life of the method because they mean different things. – Stepparent

Answer 7

For a primitive type dictionary:

public void runIntDictionary()
{
    Dictionary<int, int> myIntegerDict = new Dictionary<int, int>() { { 0, 0 }, { 1, 1 }, { 2, 2 } };
    Dictionary<int, int> cloneIntegerDict = new Dictionary<int, int>();
    cloneIntegerDict = myIntegerDict.Select(x => x.Key).ToList().ToDictionary<int, int>(x => x, y => myIntegerDict[y]);
}

or with an Object that implement ICloneable:

public void runObjectDictionary()
{
    Dictionary<int, number> myDict = new Dictionary<int, number>() { { 3, new number(3) }, { 4, new number(4) }, { 5, new number(5) } };
    Dictionary<int, number> cloneDict = new Dictionary<int, number>();
    cloneDict = myDict.Select(x => x.Key).ToList().ToDictionary<int, number>(x => x, y => myDict[y].Clone());
}

public class number : ICloneable
{
    public number()
    {
    }
    public number(int newNumber)
    {
        nr = newnumber;
    }
    public int nr;

    public object Clone()
    {
        return new number() { nr = nr };
    }
    public override string ToString()
    {
        return nr.ToString();
    }
}

Draughts

Answer 8

The reason AddRange is not implemented on Dictionary is due to the way in which a hashtable (i.e. Dictionary) stores its entries: They're not contiguous in memory as we see in an array or a List, instead they're fragmented across multiple hash buckets, so you cannot block-copy the whole range into a List or you'll get a bunch of empty entries which the Dictionary usually hides from you, the user, through its interface. AddRange assumes a single contiguous range of valid data and can therefore use a fast copy implementation e.g.Array.Copy (like C's memcpy).

Due to this fragmentation, we are left no choice but to iterate through the Dictionary's entries manually in order to extract valid keys and values into a single contiguous List or array. This can be confirmed in Microsoft's reference implementation, where CopyTo is implemented using for.

Allyn