Converting Generic Dictionnary<> to ICollection<> problem

Here is an exemple of the situation:

public class ScheduleArea : 开发者_运维知识库IArea<Schedule>
{
//....
private Dictionary<int, ScheduleArea> subArea;
//.... 

#region IArea<Schedule> Members

public ICollection<KeyValuePair<int, IArea<Schedule>>> SubArea
{
     get {
        return (Collection<KeyValuePair<int, IArea<Schedule>>>)this.subArea;//Error here
     }

}

#endregion

subArea is containing a ScheduleArea that is in fact an IArea. Why does the conversion doesnt work and how can I fix it?

It doesn't work because you're assuming generic variance, which doesn't work at all until .NET 4.0.

The simplest way forward is going to be to do it manually (given the "c#2.0" tag I assume you can't use LINQ):

List<KeyValuePair<int, IArea<Schedule>>> list = new
    List<KeyValuePair<int, IArea<Schedule>>>(subArea.Count);
foreach (KeyValuePair<int, ScheduleArea> pair in subArea)
{
    list.Add(new KeyValuePair<int, IArea<Schedule>>(pair.Key, pair.Value);
}
return list;

I would make this a method rather than a property though, given the amount of copying going on.

You're running into the very popular problem of generic co/contra-variance. You're basically trying to cast a Dictionary<int, ScheduleArea> to a Dictionary<int, IArea<Schedule>>. Generics in .NET aren't assignable like that

However, you could cast it to an IDictionary<int, ScheduleArea> or ICollection<KeyValuePair<int, ScheduleArea>>. To actually get an ICollection<KeyValuePair<int, IArea<Schedule>> you need to either change the subArea variable type or create a new dictionary:

Dictionary<int, IArea<Schedule>> dict = new Dictionary<int, IArea<Schedule>>(subArea.Count);

foreach (KeyValuePair<int, ScheduleArea> kvp in subArea) 
{
    dict.Add(kvp.Key, kvp.Value);
}

return dict;

Also, Dictionary doesn't inherit off Collection - you need to use ICollection instead

Ah, the daily question about covariance.

See: Why can't I pass List<Customer> as a parameter to a method that accepts List<object>?

and

Convert List<DerivedClass> to List<BaseClass>

To sum up: each item will have to be converted into a new KeyValuePair for the new collection.