字典与键和值列表

Question

I can't figure out how to add to dictionary _collection new value from registeredValues list if the key already exists in _collection . I want to add to the same key another object from registeredValues . Or maybe I'm doing it wrong?

private readonly List<ThreePropertyHolder<string, string, string>> _registeredValues = new List<ThreePropertyHolder<string, string, string>>();

private Dictionary<string, List<object>> _collection = new Dictionary<string, List<object>>();

public Dictionary<string, List<object>> BlaBlaMethod()
{
    foreach (var reValues in _registeredValues)
    {
        _collection.Add(reValues.Value2, new List<object>{reValues.Value3});
    }
}

Answer 1

You need to have a different action if it is adding or it is updating. Use TryGetValue to see if the value exists and update the dictionary if it does.

public Dictionary<string, List<object>> BlaBlaMethod()
{
    foreach (var reValues in _registeredValues)
    {
        List<object> list;
        if(!_collection.TryGetValue(reValues.Value2, out list))
        {
            //The key was not there, add a new empty list to the dictionary.
            list = new List<object>();
            _collection.Add(reValues.Value2, list);
        }

        //Now, if we are adding or updating, we just need to add on to our list.
        list.Add(reValues.Value3);
    }
    return _collection;
}

Answer 2

A LINQ version in case anyone's interested:

public Dictionary<string, List<object>> BlaBlaMethod()
{
    _collection = _collection
        .SelectMany(x => x.Value, (x, y) => new { x.Key, Value = y })
        .Concat(_registeredValues.Select(x => new { Key = x.Value2, Value = (object)x.Value3 }))
        .GroupBy(x => x.Key, x => x.Value)
        .ToDictionary(x => x.Key, x => new List<object>(x));
    return _collection;
}

What this does:

1. Break _collection into key/value pairs.
2. Concatenate that with _registeredValues converted into key/value pairs.
3. Group by Key .
4. Convert it back to a dictionary.

This would be a bit simpler if _collection were defined as Dictionary<string, List<string>> rather than Dictionary<string, List<object>> . It would eliminate the need to cast to object , and you could just do x.ToList() instead of new List<object>(x) .

Answer 3

By checking if the key already exists you can chose whether to add or updated an entry. Based on your sample it should work like this.

private readonly List<ThreePropertyHolder<string, string, string>> 
            _registeredValues = new List<ThreePropertyHolder<string, string, string>>();

private Dictionary<string, List<object>> _collection = new 
            Dictionary<string, List<object>>();

public Dictionary<string, List<object>> BlaBlaMethod()
{
    foreach (var reValues in _registeredValues)
    {
        if (_collection.ContainsKey(reValues.Value2))
        {
           _collection[reValues.Value2].Add(someOtherObject);
        }
        else
        {
           _collection.Add(
               reValues.Value2, 
               new List<object> { reValues.Value3 }
           );
        }
    }

    return _collection;
}