Why Does `Enumerable.Cast` Throw an `InvalidCastException` in .NET 3.5 SP1?

In .NET 3.5 SP1 Enumerable.Cast throws InvalidCastException

In .NET 3.5 SP1, a special behavior was observed when using the Enumerable.Cast method to convert a collection of one type to another type. The following code:

IEnumerable<int> list = new List<int>() { 1 };
IEnumerable<long> castedList = list.Cast<long>();
Console.WriteLine(castedList.First());

throws InvalidCastException unexpectedly. This puzzling error also occurs when using LINQ syntax, as shown in the following code:

var list = new[] { 1 };
var castedList = from long l in list select l;
Console.WriteLine(castedList.First());

Reveal hidden behavior

At first it seems confusing why this conversion fails. However, a blog post sheds light on the potential issues. In .NET 3.5 SP1, the Cast<t>()</t> method was modified to act on IEnumerable instead of IEnumerable<t></t>. Therefore, each element in the collection is boxed into System.Object before reaching the conversion stage.

Exposed root cause

This fundamental shift leads to a subtle problem. InvalidCastException is essentially caused by trying the following conversion:

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

As this code shows, converting int directly to long produces the expected results; however, converting a boxed int to long fails. This explains why the initial code snippet and its LINQ counterpart don't work as expected.

Looking for solutions

To avoid this exception, the conversion must be performed explicitly:

var castedList = list.Select(i => (long)i);

This alternative method successfully converts each element without encountering an invalid conversion exception.

This unusual behavior reminds us that some subtle behind-the-scenes mechanisms may affect code execution. To avoid similar pitfalls in the future, it's worth exploring alternatives or handling the conversion explicitly.

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