Question

I have written a test ViewEngine which responds to requests with a view name of “About”, and added the following to the Application_Start method in Global.asax.

ViewEngines.Engines.Clear();
ViewEngines.Engines.Add(new RazorViewEngine());
ViewEngines.Engines.Add(new MyViewEngine());

The FindView method of MyViewEngine is as follows:

if (viewName == "About")
    return new ViewEngineResult(new MyView(), this);
return new ViewEngineResult(new List<string>{"Some arbitrary search location"});

As the Home controller responds to the action About, I would have expected the RazorViewEngine to handle a request for ~/Home/About (as it was added to the Engines collection first) but it’s not… MyViewEngine is handling calls to that URL instead.

Calls which result in views not named “About” are handled correctly by Razor.

Even if I reverse the order of the two Add statements above, I get the exact same behavior. If I comment out the code which registers my view engine, Razor does pick up ~/Home/About as I would expect.

Any ideas why MyViewEngine is creating views ahead of Razor when it shouldn’t be?

Answer 1

ASP.NET MVC asks the registered ViewEngines in their registration order as expected. However there is one catch. It’s in the ViewEngineCollection FindView method:

public virtual ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName)

This method iterates through the available ViewEngines asking for a view but it does it twice. Once with calling the FindView method with useCache=true and if none of the ViewEngines provided a result it iterates over them another time with useCache=false.
So in your case when it asks the RazorViewEngine with useCache=true it does not return a view because there is no chached one. But your view engine will always provide View. So you need to handle the useCache=true case in your implementation:

public ViewEngineResult FindView(ControllerContext controllerContext, string viewName, string masterName, bool useCache)
{
    if (useCache)
        return new ViewEngineResult(new List<string> { "Not in cache" });

    if (viewName == "About")
        return new ViewEngineResult(new MyView(), this);
    return new ViewEngineResult(new List<string> { "Some arbitrary search location" });
}