[assembly: WebActivator.PreApplicationStartMethod(typeof(A.InitClass1), "PreStart")]
[assembly: WebActivator.PostApplicationStartMethod(typeof(A.InitClass1), "PostStart")]
[assembly: WebActivator.ApplicationShutdownMethod(typeof(A.InitClass1), "ShutDown")]

[assembly: WebActivator.PreApplicationStartMethod(typeof(A.InitClass1), "PreStart")]
[assembly: WebActivator.PreApplicationStartMethod(typeof(A.InitClass2), "PreStart")]
[assembly: WebActivator.PreApplicationStartMethod(typeof(A.InitClass3), "PreStart")]

[AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]

using System;
using System.Reflection;

namespace WebActivator
{
    // Base class of all the activation attributes
    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
    public abstract class BaseActivationMethodAttribute : Attribute
    {
        private Type _type;
        private string _methodName;

        public BaseActivationMethodAttribute(Type type, string methodName)
        {
            _type = type;
            _methodName = methodName;
        }

        public Type Type { get { return _type; } }

        public string MethodName { get { return _methodName; } }

        public int Order { get; set; }


        public void InvokeMethod()
        {
            // Get the method
            MethodInfo method = Type.GetMethod(
                MethodName,
                BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public);

            if (method == null)
            {
                throw new ArgumentException(
                    String.Format("The type {0} doesn‘t have a static method named {1}",
                        Type, MethodName));
            }

            // Invoke it
            method.Invoke(null, null);
        }
    }
}

using System.Collections.Generic;
using System.Linq;
using System.Reflection;

namespace WebActivator
{
    static class AssemblyExtensions
    {
        // Return all the attributes of a given type from an assembly
        public static IEnumerable<T> GetActivationAttributes<T>(this Assembly assembly) where T : BaseActivationMethodAttribute
        {
            return assembly.GetCustomAttributes(
                typeof(T),
                inherit: false).OfType<T>();
        }
    }
}

// 加载所有获取的程序集
private static IEnumerable<Assembly> Assemblies
{
    get
    {
        if (_assemblies == null)
        {
            // Cache the list of relevant assemblies, since we need it for both Pre and Post
            _assemblies = new List<Assembly>();
            foreach (var assemblyFile in GetAssemblyFiles())
            {
                try
                {
                    // Ignore assemblies we can‘t load. They could be native, etc...
                    _assemblies.Add(Assembly.LoadFrom(assemblyFile));
                }
                catch
                {
                }
            }
        }

        return _assemblies;
    }
}

// 获取程序集文件路径集合
private static IEnumerable<string> GetAssemblyFiles()
{
    // When running under ASP.NET, find assemblies in the bin folder.
    // Outside of ASP.NET, use whatever folder WebActivator itself is in
    string directory = HostingEnvironment.IsHosted
        ? HttpRuntime.BinDirectory
        : Path.GetDirectoryName(typeof(ActivationManager).Assembly.Location);
    return Directory.GetFiles(directory, "*.dll");
}

// Return all the App_Code assemblies
private static IEnumerable<Assembly> AppCodeAssemblies
{
    get
    {
        // Return an empty list if we;re not hosted or there aren‘t any
        if (!HostingEnvironment.IsHosted || !_hasInited || BuildManager.CodeAssemblies == null)
        {
            return Enumerable.Empty<Assembly>();
        }

        return BuildManager.CodeAssemblies.OfType<Assembly>();
    }
}

public static void RunPreStartMethods()
{
    RunActivationMethods<PreApplicationStartMethodAttribute>();
}

public static void RunPostStartMethods()
{
    RunActivationMethods<PostApplicationStartMethodAttribute>();
}

public static void RunShutdownMethods()
{
    RunActivationMethods<ApplicationShutdownMethodAttribute>();
}

// Call the relevant activation method from all assemblies
private static void RunActivationMethods<T>() where T : BaseActivationMethodAttribute
{
    foreach (var assembly in Assemblies.Concat(AppCodeAssemblies))
    {
        foreach (BaseActivationMethodAttribute activationAttrib in assembly.GetActivationAttributes<T>().OrderBy(att => att.Order))
        {
            activationAttrib.InvokeMethod();
        }
    }
}

class StartMethodCallingModule : IHttpModule
{
    private static object _lock = new object();
    private static int _initializedModuleCount;

    public void Init(HttpApplication context)
    {
        lock (_lock)
        {
            // Keep track of the number of modules initialized and
            // make sure we only call the post start methods once per app domain
            if (_initializedModuleCount++ == 0)
            {
                RunPostStartMethods();
            }
        }
    }

    public void Dispose()
    {
        lock (_lock)
        {
            // Call the shutdown methods when the last module is disposed
            if (--_initializedModuleCount == 0)
            {
                RunShutdownMethods();
            }
        }
    }
}

public static void Run()
{
    if (!_hasInited)
    {
        RunPreStartMethods();

        // Register our module to handle any Post Start methods. But outside of ASP.NET, just run them now
        if (HostingEnvironment.IsHosted)
        {
            Microsoft.Web.Infrastructure.DynamicModuleHelper.DynamicModuleUtility.RegisterModule(typeof(StartMethodCallingModule));
        }
        else
        {
            RunPostStartMethods();
        }

        _hasInited = true;
    }
}

[assembly: PreApplicationStartMethod(typeof(WebActivator.ActivationManager), "Run")]