ASP.NET Core Implicit Cast

While doing a code review I noticed the following code inside an ASP.NET(Core) controller:

Do you notice something special?

No? Let’s give you a hint. Take a look at the return type; ActionResult<IEnumerable<Product>> and then take a look at the type that is actually returned; List<Product>. Although both types don’t match this code compiles and runs perfectly.

What is the magic happening that is happening here? This all works thanks to the fact the ActionResult<T> type supports implicit casting.

Now what happens if we update the code to the following:

This time you get a compiler exception:

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<eShopExample.Product>' to 'Microsoft.AspNetCore.Mvc.ActionResult<System.Collections.Generic.IEnumerable<eShopExample.Product>>'

Whoops! Why this no longer works?

The reason is that where before we were returning a concrete type (List<T>), now we are returning an interface (IEnumerable<T>). C# doesn't support implicit cast operators on interfaces.

To fix this we can add a ToList() to convert the interface to a concrete type.