ASP.NET MVC中Application变量相关问题及Application_Start中Model.tt取值咨询

Hey there! Let's break down all your questions about Application variables in ASP.NET MVC clearly—since you're new to this, I'll keep things practical and avoid unnecessary jargon.

Application variables are global state containers in ASP.NET that are shared across all users and requests in your entire application. Their lifecycle starts when the application first launches and ends when the application shuts down (like when you restart the server or recycle the app pool).

They’re perfect for storing data that doesn’t change often and needs to be accessible everywhere—think site-wide settings (like a site name), public dropdown options, or cached lookup data. Just remember: since they’re shared, you need to use locking when modifying them to avoid concurrency issues.

You typically initialize Application variables in the Application_Start() method (found in Global.asax.cs), since this method runs exactly once when the app starts. You can also set them elsewhere, but always handle concurrency if you do.

Initializing in Application_Start() #

protected void Application_Start()
{
    // Basic setup for MVC
    AreaRegistration.RegisterAllAreas();
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);

    // Initialize a simple string Application variable
    Application["SiteName"] = "My Awesome MVC Site";

    // Initialize with a complex object (e.g., data from a database)
    using (var dbContext = new YourAppDbContext())
    {
        var publicCategories = dbContext.Categories.Where(c => c.IsPublic).ToList();
        Application["PublicCategories"] = publicCategories;
    }
}

Setting/Modifying in Other Places (e.g., Controllers) #

If you need to update an Application variable after startup, always use a lock to prevent race conditions:

public ActionResult UpdateGlobalCounter()
{
    lock (Application) // Lock the Application object to avoid concurrent edits
    {
        var currentCount = Application["VisitorCounter"] as int? ?? 0;
        Application["VisitorCounter"] = currentCount + 1;
    }
    return RedirectToAction("Index");
}

Where to Declare #

You shouldn’t "declare" Application variables directly in controllers—controllers are created per request, so any variable set there would only exist for that specific request. Instead, initialize global Application variables in Application_Start() as shown above.

How to Access #

Controllers have direct access to the Application property (of type HttpApplicationStateBase). You can retrieve values like this:

public ActionResult Index()
{
    // Retrieve and cast the value (since Application stores objects)
    var siteName = Application["SiteName"] as string;
    var publicCategories = Application["PublicCategories"] as List<Category>;

    // Pass data to the view
    ViewBag.SiteName = siteName;
    return View(publicCategories);
}

Always use as for casting to avoid runtime errors if the key doesn’t exist or the type is wrong—you can also add null checks for safety.

First, a quick clarification: Model.tt is a T4 template, which runs at design time (when you generate or update your entity classes) rather than at runtime. But if your goal is to fetch database data at runtime (during app startup) and store it in Application variables for your app’s models to use, here’s how:

In Application_Start(), you can instantiate your DbContext, query the database for the model data you need, and store it in an Application variable. Since Application_Start() runs once in a single thread, you don’t need to lock here.

Example:

protected void Application_Start()
{
    // Standard MVC setup
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);

    // Fetch model data from database
    using (var dbContext = new YourAppDbContext())
    {
        // Get a single model instance (e.g., site configuration)
        var siteSettings = dbContext.SiteSettings.FirstOrDefault();
        if (siteSettings != null)
        {
            Application["SiteSettings"] = siteSettings;
        }

        // Get a list of model objects (e.g., product types)
        var productTypes = dbContext.ProductTypes.ToList();
        Application["ProductTypes"] = productTypes;
    }
}

If you actually want to use database data in your Model.tt template (at design time), that’s a separate task. You’d write database queries directly in the T4 template itself, since it runs outside the application’s runtime context. For example:

<#
using (var db = new YourAppDbContext())
{
    var tables = db.Database.SqlQuery<string>("SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE = 'BASE TABLE'").ToList();
    // Use these table names to generate entity classes
}
#>

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内容的提问来源于stack exchange，提问作者Smit Kotadia