Why Does `Enumerable.Cast` Throw an `InvalidCastException` When Casting from `IEnumerable`?

Puzzling Cast Exception in Enumerable.Cast

An InvalidCastException is encountered when attempting to cast an IEnumerable of integers (ints) to an IEnumerable of long integers (longs) using the Cast operator, as seen below:

using System.Collections.Generic;
using System.Linq;

namespace InvalidCast
{
    class Program
    {
        static void Main(string[] args)
        {
            // Initialize a list of ints
            IEnumerable<int> list = new List<int>() { 1 };

            // Attempt to cast the list to longs
            IEnumerable<long> castedList = list.Cast<long>();

            // Attempt to write the first element of the casted list
            Console.WriteLine(castedList.First());
        }
    }
}

Why does this exception occur?

This behavior is unexpected because the Cast operator is intended to perform a safe and reliable conversion. However, this particular case demonstrates a peculiar issue that arose with a modification in the behavior of Cast between .NET 3.5 and .NET 3.5 SP1.

The Root of the Problem

The Cast operator is an extension method defined for IEnumerable (the base interface for collections), not specifically for IEnumerable. This means that when the values are being cast, they have been boxed into objects of type System.Object.

As a result, the casting process mimics the following:

int i = 1;
object o = i;
long l = (long)o;

This code throws the InvalidCastException as it attempts to cast a boxed int to a long, instead of directly casting the int.

Workaround

To resolve this issue, it's necessary to perform the cast explicitly using a lambda expression or a select method, as follows:

// Cast using a lambda expression
var castedList = list.Select(i => (long)i);

// Cast using a select method
var castedList = from long l in list select l;

These approaches explicitly convert each int value to a long, avoiding the boxing/unboxing process and circumventing the InvalidCastException.

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