Question

I’ve got a simple ASP.NET MVC controller. Inside a few action methods, I access a resource which I’ll say is expensive.

So I thought, why not make it static. So instead of doing double checked locking I thought I can leverage the use of Lazy<T> in .NET 4.0. Call the expensive service once instead of multiple times.

So, if this is my pseduo code, how can I change it do use Lazy<T>.
For this contrite example, I’ll use the File System as the expensive resource
So with this example, instead of getting all the files from the destination path, every time a request calls that ActionMethod, I was hoping to use Lazy to hold that list of files .. which of course, makes the call the first time only.

Next assumption: don’t worry if the content is changed. That’s out of scope, here.

public class FooController : Controller
{
    private readonly IFoo _foo;
    public FooController(IFoo foo)
    {
        _foo = foo;
    }

    public ActionResult PewPew()
    {
        // Grab all the files in a folder.
        // nb. _foo.PathToFiles = "/Content/Images/Harro"
        var files = Directory.GetFiles(Server.MapPath(_foo.PathToFiles));

        // Note: No, I wouldn't return all the files but a concerete view model
        //       with only the data from a File object, I require.
        return View(files);
    }
}

Answer 1

In your example, the result of Directory.GetFiles depends on the value of _foo, which is not static. Therefore you cannot use a static instance of Lazy<string[]> as a shared cache between all instances of your controller.

The ConcurrentDictionary<TKey, TValue> sounds like something that is closer to what you want.

// Code not tested, blah blah blah...
public class FooController : Controller
{
    private static readonly ConcurrentDictionary<string, string[]> _cache
        = new ConcurrentDictionary<string, string[]>();

    private readonly IFoo _foo;
    public FooController(IFoo foo)
    {
        _foo = foo;
    }

    public ActionResult PewPew()
    {
        var files = _cache.GetOrAdd(Server.MapPath(_foo.PathToFiles), path => {
            return Directory.GetFiles(path);
        });

        return View(files);
    }
}