Special dictionary for aggregating values

Aggregator.cs

using System;
using System.Collections;
using System.Collections.Generic;

/// <summary>
/// Special dictionary for aggregating values
/// </summary>
public class Aggregator<TKey, TValue> : IEnumerable<KeyValuePair<TKey, TValue>>
{
  private Dictionary<TKey, TValue> _values = new Dictionary<TKey, TValue>();

  public TValue this[TKey key]
  {
    get
    {
      TValue existing;
      if (_values.TryGetValue(key, out existing))
        return existing;
      return default(TValue);
    }
    set
    {
      _values[key] = value;
    }
  }

  public bool ContainsKey(TKey key)
  {
    return _values.ContainsKey(key);
  }

  public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
  {
    return _values.GetEnumerator();
  }

  public void Remove(TKey key)
  {
    _values.Remove(key);
  }

  public void Update(TKey key, Func<TValue, TValue> func)
  {
    TValue existing;
    if (_values.TryGetValue(key, out existing))
      _values[key] = func(existing);
    else
      _values[key] = func(default(TValue));
  }

  IEnumerator IEnumerable.GetEnumerator()
  {
    return GetEnumerator();
  }
}