Does LINQ work with IEnumerable?

You can use Cast<T>() or OfType<T> to get a generic version of an IEnumerable that fully supports LINQ.

Eg.

IEnumerable objects = ...;
IEnumerable<string> strings = objects.Cast<string>();

Or if you don't know what type it contains you can always do:

IEnumerable<object> e = objects.Cast<object>();

If your non-generic IEnumerable contains objects of various types and you are only interested in eg. the strings you can do:

IEnumerable<string> strings = objects.OfType<string>();

Yes it can. You just need to use the Cast<T> function to get it converted to a typed IEnumerable<T>. For example:

IEnumerable e = ...;
IEnumerable<object> e2 = e.Cast<object>();

Now e2 is an IEnumerable<T> and can work with all LINQ functions.

You can also use LINQ's query comprehension syntax, which casts to the type of the range variable (item in this example) if a type is specified:

IEnumerable list = new ArrayList { "dog", "cat" };

IEnumerable<string> result =
  from string item in list
  select item;

foreach (string s in result)
{
    // InvalidCastException at runtime if element is not a string

    Console.WriteLine(s);
}

The effect is identical to @JaredPar's solution; see 7.16.2.2: Explicit Range Variable Types in the C# language specification for details.