构建后如何自动实例化单例？

Question

I have an ASP.NET 2 C# application that uses a Singleton to load a large list of Data from a Database.

The first request for this singleton very slow (as expected). The subsequent ones are very fast. Is there anyway to instantiate the singleton as soon as the project is built?

Answer 1

There is no way of populating a singleton from the database directly after it has been built without running the actual application. You could potentially speed it up by having the singleton be populated on a seperate thread from the rest of the application (in Application_Start ) this however will not solve the slowdown issue if the singleton is used before it is fully loaded (and also requires the addition of locking logic).

Is there a specific reason why you're loading a large amount of data from the database into the singleton?

Answer 2

You could put the code to populate the singleton in the Application_Start method in global.asax.cs.

From MSDN :

Called when the first resource (such as a page) in an ASP.NET application is requested. The Application_Start method is called only one time during the life cycle of an application. You can use this method to perform startup tasks such as loading data into the cache and initializing static values.

Answer 3

Create it as an App_Start configuration class with WebActivator .

Assuming your application is setup as a virtual directory the code in App_Start will be called immediately after build.

Note, this will give you the appearance that your builds are "slow". Also note, if you get an exception during this code you will actually get a "build failure" with an arcane error message similar to:

Exception has been thrown by the target of an invocation.. ASPRUNTIME ProjectName

This is means you need to browse to your app in a browser to see the real error message.

We encountered all of this when we tied in EF Migrations into a App_Start configuration.