Why Does `IEnumerable.Cast()` Throw an `InvalidCastException` in .NET 3.5 SP1?

Unravel the mystery of Enumerable.Cast()thrownInvalidCastException

In .NET 3.5 SP1, a Cast<T>() exception was puzzlingly thrown when converting IEnumerable using InvalidCastException. Consider the following code:

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

Why does this code throw an exception?

This unusual behavior stems from changes in the behavior of the Cast<T>() method between .NET 3.5 and .NET 3.5 SP1. Prior to SP1, Cast<T>() acted on IEnumerable<T>, allowing direct conversion. However, in SP1, it was modified to work on IEnumerable, which means the element is unboxed to System.Object before being converted.

This unboxing operation results in InvalidCastException because unlike converting int directly to long, converting a boxed int to long is not allowed. Effectively, this code attempts to do the following:

<code class="language-csharp">int i = 1;
object o = i;
long l = (long)o;</code>

This will also result in the same InvalidCastException.

Solutions and explanations

To solve this problem, you can use lambda expressions to explicitly convert the elements:

<code class="language-csharp">var castedList = list.Select(i => (long)i);</code>

This will successfully convert the element to long.

The reason for this behavior of

is that Cast<T>() is an extension method of IEnumerable, not of IEnumerable<T>. When elements reach the transformation stage, they have already been boxed as object. This results in object when converting from InvalidCastException to a specific type.

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