C# generic type constraint depending on used constructor

Question

I want to mimic the Python collections.defaultdict in C#. The following works fine as long as the value type has a parameterless constructor:

public class DefaultDictionary<TKey, TValue> : Dictionary<TKey, TValue> where TValue : new()
{
    public new TValue this[TKey key]
    {
        get
        {
            TValue val;
            if (!TryGetValue(key, out val)) {
                val = new TValue();
                Add(key, val);
            }
            return val;
        }
        set { base[key] = value; }
    }
}

But what if I want to use a constructor that takes the key as argument? Or generally, a factory function that, given the key, returns an instance of the value type?

public class DefaultDictionary<TKey, TValue> : Dictionary<TKey, TValue> where TValue : new()
{
    private readonly Func<TKey, TValue> factory;

    public DefaultDictionary() : this(key => new TValue())
    {}

    public DefaultDictionary(Func<TKey, TValue> factory)
    {
        this.factory = factory;
    }

    public new TValue this[TKey key]
    {
        get
        {
            TValue val;
            if (!TryGetValue(key, out val)) {
                val = factory(key);
                Add(key, val);
            }
            return val;
        }
        set { base[key] = value; }
    }
}

Now the problem is that TValue is still required to have a parameterless constructor, even when a factory function is used. The following won't compile if Thingy does not have a parameterless constructor:

new DefaultDictionary<int, Thingy>(key => new Thingy(key, otherStuff, moreStuff));

However, removing the constraint will cause error CS0304 in the new TValue() statement.

Intuitively I would want to place the type constraint on the DefaultDictionary() constructor, but I doubt this is possible. Is there a proper way to solve this problem in C#?

You could go with "key = default(TValue)" or "val = default(TValue)". — Lukasz Nowakowski
– Lukasz Nowakowski, Commented Mar 23, 2021 at 13:30
Or default value for struct. I don't see other way to do it, but maybe it's just my imagination not being creative enough :-) One idea would be to remove a default constructor from DefaultDictionary and thus removing usage of default constructor of TValue — Lukasz Nowakowski
– Lukasz Nowakowski, Commented Mar 23, 2021 at 13:35
Yes, but having to explicitly write the initializer lambda in every creation of DefaultDictionary would be quite impractical... — Борат Сагдиев
– Борат Сагдиев, Commented Mar 23, 2021 at 13:36
you may create another class and static method for creation like this (also using new to hide method has obvious downsides) — Selvin
– Selvin, Commented Mar 23, 2021 at 13:46

Борат Сагдиев · Accepted Answer · 2021-04-26 13:02:50Z

2

One way is to keep only the constructor taking the value factory in DefaultDictionary, and create a subclass that does have the : new() constraint, and move the parameterless constructor there:

public class DefaultDictionaryParameterless<TKey, TValue> :
    DefaultDictionary<TKey, TValue> where TValue : new()
{
    public DefaultDictionaryParameterless()
        : base(x => new TValue())
    {
        
    }
}

public class DefaultDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
    private readonly Func<TKey, TValue> factory;

    public DefaultDictionary(Func<TKey, TValue> factory)
    {
        this.factory = factory;
    }
    ...
}

edited Apr 26, 2021 at 13:02

Борат Сагдиев

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answered Mar 23, 2021 at 13:53

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