为什么要向上转换IDictionary <TKey, TValue> 到IEnumerable <object> 失败？

Question

See the following code snippet:

(IEnumerable<object>)new Dictionary<string, string>()

The above cast will throw an invalid cast exception.

Actually, IDictionary<TKey, TValue> also indirectly implements IEnumerable<out T> , because it also implements ICollection<T> . That is, the whole cast should be valid.

In fact, for me it is even more strange that if I run the whole cast on a debugger watch slot, it works !

What's going on?

Answer 1

That dictionary does implement IEnumerable<KeyValuePair<TKey, TValue>> and IEnumerable , but IEnumerable of a struct is not the same as IEnumerable of an object. Variance only works for reference-types .

This does work at my end and should be the logical choice:

var x = (IEnumerable)new Dictionary<string, string>();

As a sample, this does work:

List<string> l = new List<string>();
var x = (IEnumerable<object>)l;

But this one doesn't:

List<DateTime> l2 = new List<DateTime>();
var x = (IEnumerable<object>)l2;

Clearly indicating the struct is the problem.

(Why it works in your Watch windows, I don't know)

Answer 2

I think this is because KeyValuePair is a Value-Type.

This would fail:

List<int> ints = new List<int>();
var objs = (IEnumerable<object>)ints;

This would work:

List<string> ints = new List<string>();
var objs = (IEnumerable<object>)ints;

same goes for the dictionary.

Answer 3

如果你真的不能使用普通的IEnumerable ，试试这个：

new Dictionary<string, string>().Cast<object>();

Answer 4

This is strange. Your code snippet works fine with .NET 4.0. I suggest you cast to IEnumerable .

Read: IDictionary Interface (MSDN)