Question

I have a class like the following:

public class DropDownControl<T, Key, Value> : BaseControl     where Key: IComparable {     private IEnumerable<T> mEnumerator;     private Func<T, Key> mGetKey;     private Func<T, Value> mGetValue;     private Func<Key, bool> mIsKeyInCollection;      public DropDownControl(string name, IEnumerable<T> enumerator, Func<T, Key> getKey, Func<T, Value> getValue, Func<Key, bool> isKeyInCollection)         : base(name)     {         mEnumerator = enumerator;         mGetKey = getKey;         mGetValue = getValue;          mIsKeyInCollection = isKeyInCollection;     } 

And I want to add a convenience function for Dictionaries (because they support all operations efficiently on their own).

But the problem is that such a constructor would only specify Key and Value but not T directly, but T is just KeyValuePair. Is there a way to tell the compiler for this constructor T is KeyValuePair, like:

public DropDownControl<KeyValuePair<Key, Value>>(string name, IDictionary<Key, Value> dict) { ... } 

Currently I use a static Create function as workaround, but I would like a direct constructor better.

public static DropDownControl<KeyValuePair<DKey, DValue>, DKey, DValue> Create<DKey, DValue>(string name, IDictionary<DKey, DValue> dictionary)             where DKey: IComparable         {             return new DropDownControl<KeyValuePair<DKey, DValue>, DKey, DValue>(name, dictionary, kvp => kvp.Key, kvp => kvp.Value, key => dictionary.ContainsKey(key));         } 

Answer 1

No, basically. The static method in a non-generic class (such as DropDownControl [no <>]) is the best approach, as you should be able to use type-inference when you call Create() – i.e.

var control = DropDownControl.Create(name, dictionary); 

C# 3.0 helps here both via ‘var’ (very welcome here) and by the much-improved generic type inference rules. In some (more general) case, another similar option is an extension method, but an extension method to create a very specific control from a dictionary doesn’t feel very natural – I’d use a non-extension method.

Something like:

public static class DropDownControl {     public static DropDownControl<KeyValuePair<TKey,TValue>, TKey, TValue>             Create<TKey,TValue>(IDictionary<TKey, TValue> value, string name)     where TKey : IComparable     {         return new DropDownControl<KeyValuePair<TKey, TValue>, TKey, TValue>             (name, value, pair => pair.Key, pair => pair.Value,             key => value.ContainsKey(key)         );     } } 

Another option is inheritance, but I don’t like it much…

public class DropDownControl<TKey, TValue> :     DropDownControl<KeyValuePair<TKey, TValue>, TKey, TValue>     where TKey : IComparable {     public DropDownControl(IDictionary<TKey, TValue> lookup, string name)         : base(name, lookup, pair => pair.Key, pair => pair.Value,             key => lookup.ContainsKey(key)) { } } 

This adds complexity and reduces your flexibility… I wouldn’t do this…

Overall, it sounds like you want to be working with just IDictionary<,> – I wonder if you can’t simplify your control to just use this, and force non-dictionary callers to wrap themselves in an IDictionary<,> facade?