Dictionary<StudentType, List<Student>> to IDictionary<StudentType, IList<Student>>?

Question

Please consider the following code:

class Student
{
}

enum StudentType
{
}

static void foo(IDictionary<StudentType, IList<Student>> students)
{   
}

static void Main(string[] args)
{
    Dictionary<StudentType, List<Student>> studentDict = 
                     new Dictionary<StudentType, List<Student>>();

    foo(studentDict);

    ...
}

There is the error:

error CS1503: Argument '1': cannot convert from 'System.Collections.Generic.Dictionary>' to 'System.Collections.Generic.IDictionary>'

Is there any way to call foo function?

Answer 1

You could use the Linq ToDictionary method to create a new dictionary where the value has the correct type:

static void Main(string[] args)
{
  Dictionary<StudentType, List<Student>> studentDict = new Dictionary<StudentType, List<Student>>();
  var dicTwo = studentDict.ToDictionary(item => item.Key, item => (IList<Student>)item.Value);
  foo(dicTwo);
}

Answer 2

You will have build a new dictionary with the right types, copying the data from the old one into the new one.

Or, you could change the original dictionary to be of the right type to begin with.

Either way, no, you can't cast the dictionary.

The reason for this limitation is as follows:

1. The dictionary contains values of type Student
2. You could have many types that implement IStudent
3. The method you're giving the cast'ed dictionary to could potentially try to stuff another IStudent into the dictionary, even if it isn't Student