IEnumerable<T>, IComparable<T> and a few more are now type-variant. IList<T>, ICollection<T> and many others aren't. Why?
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254answers:
3
+11
A:
.NET Framework 4.0 introduces safe co/contra-variance. IList<T> and ICollection<T> have T both in input and output positions while IEnumerable<T> has T only in output positions and IComparable<T> has T only in input positions.
Assume IList<T> supported type variance:
static void FailingMethod(IList<object> list) {
list[0] = 5;
}
static void Test() {
var a = new List<string>();
a[0] = "hello";
FailingMethod(a); // if it was variant, this method call would be unsafe
}
Mehrdad Afshari
2009-05-31 19:16:41
Thanks! Looks like using indexers makes any class non-variant.But ICollection<> has T only in input parameters.
UserControl
2009-05-31 19:27:25
No, it's not related to indexers in general. It depends on the parameter and return type of indexers (and existence of get/set accessors), just like any other method or property. You might have an indexer like: T this[int index] { get { return x[index]; } } and without a set accessor, it could be variant.
Mehrdad Afshari
2009-05-31 19:31:06
+1
A:
Anders Hejlseberg has a brief, but illuminating discussion that describes co/contravariance in his talk, "The Future of C#." His discussion on covariance and contravariance starts at 50 minutes and 17 seconds into the presentation.
Robert Harvey
2009-05-31 19:22:23