Casting string to IEnumerable

Question

I have propertyValue which is an object passed into my function.

I can assume that this is an IEnumerable of some kind, however due to the interface contract I must accept it as an object rather than directly as an IEnumerable.

When I try to cast my value to an IEnumerable<object>, so that I can get the length, I get an InvalidCastException.

var length = ((IEnumerable<object>) propertyValue).Cast<object>().Count();

I get the following exception:

System.InvalidCastException: Unable to cast object of type 'System.String' to type 'System.Collections.Generic.IEnumerable`1[System.Object]'.

Jakub Lortz · Answer 1 · 2015-12-22T12:42:15.230

The IEnumerable<T> interface is covariant, but covariance doesn't work with value types. Casting a value type to reference type changes representation - the value needs to be boxed and the reference to the boxed value is returned. Casting reference types doesn't change representation - it's still the same reference, referencing the same object.

string is an IEnumerable<char>, so it cannot be cast to IEnumerable<object>.

As mentioned in other answers, the solution is to cast to non-generic IEnumerable.

var length = ((IEnumerable) propertyValue).Cast<object>().Count();

score 3 · Accepted Answer · answered Dec 22 '15 at 12:32

3

I have fixed the issue by simply casting to IEnumerable rather than casting to IEnumerable<object>.

var length = ((IEnumerable) propertyValue).Cast<object>().Count();

answered Dec 22 '15 at 12:32

James Monger

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Why do you need to use Cast – Vadim Martynov Dec 22 '15 at 12:34
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@VadimMartynov To use the `Count()` extension method, which is only available for generic `IEnumerable` – Jakub Lortz Dec 22 '15 at 12:38
@VadimMartynov On the non-generic class `IEnumerable`, there is no extension method for `Count()`. To use the method, I must cast it to `IEnumerable` of some type. – James Monger Dec 22 '15 at 12:39
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You must not. Actually you should not because it causes boxing for char type. – Vadim Martynov Dec 22 '15 at 13:02

score 1 · Answer 3 · edited May 23 '17 at 12:23

1

String implements the IEnumerable<char> interface, you can cast it to this interface

var length = ((IEnumerable<char>) propertyValue).Count();

For unknown types of collection you can cast it to IEnumerable and next enumerate values manually with your own extension method.

static class EnumerableExtensions
{
    public static int Count(this IEnumerable source)
    {
        int res = 0;

        foreach (var item in source)
            res++;

        return res;
    }
}

var collection = propertyValue as IEnumerable;
if(collection != null)
{
    var length = collection.Count();
}

Using Cast() method will cause to boxing of value types and it may have an performance issues for big collections.

edited May 23 '17 at 12:23

Community

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answered Dec 22 '15 at 12:33

Vadim Martynov

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Since the author does not know of which type the Enumerable is this won't necessary work always. – dryman Dec 22 '15 at 12:37
@dryman you are right, it's my fail. But Cast will result in boxing for value types. I'll update my answer. – Vadim Martynov Dec 22 '15 at 12:48

score 0 · Answer 4 · answered Dec 22 '15 at 12:41

0

The following code should actually do what you need

var length = (propertyValue as IEnumerable<object>).Count();

answered Dec 22 '15 at 12:41

dontbyteme

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4 Answers4