Why Does `IEnumerable.Cast` Throw an `InvalidCastException`?

Understanding InvalidCastException with IEnumerable.Cast

Attempting to cast an IEnumerable<int> to an IEnumerable<long> using Cast<T> often results in a perplexing InvalidCastException. This seemingly simple operation fails due to a subtle mechanism within the Cast<T> method. Consider this example:

<code class="language-csharp">IEnumerable<int> list = new List<int>() { 1 };
IEnumerable<long> castedList = list.Cast<long>();
Console.WriteLine(castedList.First());</code>

The Root Cause: Boxing and Unboxing

The key to understanding this lies in how Cast<T> works. It's an extension method on IEnumerable, not IEnumerable<T>. This means each element in the IEnumerable<int> is first boxed into a System.Object before the cast to long is attempted. This is functionally equivalent to:

<code class="language-csharp">int i = 1;
object o = i;  // Boxing
long l = (long)o; // Unboxing and Cast - This throws the exception!</code>

The InvalidCastException occurs during the unboxing step. While a direct cast from int to long is valid, unboxing a boxed int to a long is not.

Resolution

To avoid this InvalidCastException, use a method that handles the cast within the strongly-typed IEnumerable<T> context, such as Select:

<code class="language-csharp">IEnumerable<int> list = new List<int>() { 1 };
IEnumerable<long> castedList = list.Select(x => (long)x);
Console.WriteLine(castedList.First());</code>

This approach performs the cast directly on each int element before boxing occurs, preventing the InvalidCastException. Using Select provides a cleaner and more efficient solution than relying on Cast<T> for this type of conversion.

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