FactoryGenerator.cs source code in C# .NET

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                         / DotNET / DotNET / 8.0 / untmp / whidbey / REDBITS / ndp / fx / src / xsp / System / Web / Util / FactoryGenerator.cs / 1 / FactoryGenerator.cs
                        

                        
                            //------------------------------------------------------------------------------ 
// 
//     Copyright (c) Microsoft Corporation.  All rights reserved.
// 
//----------------------------------------------------------------------------- 

#if !DONTUSEFACTORYGENERATOR 
 
using System;
using System.Reflection; 
using System.Reflection.Emit;
using System.Threading;
using System.Security;
using System.Security.Permissions; 

namespace System.Web.Util { 
    /* 
     * Factory Generator class
     * A factory generator is useful for cases where a large number of late-bound 
     * classes need to be instantiated.
     *
     * Normally, to create an instance of type t, you call the following code:
     * 
     *      ISomeInterface o = (ISomeInterface) Activator.CreateInstance(t);
     * 
     * This assumes that the default constructor is used, and that the type t 
     * implements the interface ISomeInterface.
     * 
     * The factory generator, on the other hand, can use reflection emit APIs
     * to dynamically generate a class factory for t. The generated class has
     * the equivalent of the following code:
     * 
     *      class X : ISomeInterfaceFactory {
     *          public ISomeInterface CreateInstance() { 
     *              return new t(); 
     *          }
     *      } 
     *
     * It then instantiates and returns an object of this type. You can then
     * call CreateInstance to create an instance of the type, which is
     * significantly faster. 
     *
     * A single instance of a FactoryGenerator can generate factories for 
     * multiple types. However, it builds all these types into a single 
     * dynamically generated assembly. CLR implementation prevents this
     * assembly from being unloaded until the process exits. 
     *
     * The FactoryGenerator is (almost) a templated type. It takes two
     * types in its constructor:
     * 
     *   returnedType is the type common to all classes for which factories
     *      are to be generated. In the example above, this would be 
     *      ISomeInterface. 
     *   factoryInterface is the interface implemented by the dynamically
     *      generated class factory, and should include a method named 
     *      CreateInstance, that takes no parameters and returns an object
     *      of the type specified by returnedType. In the example above,
     *      this would be ISomeInterfaceFactory.
     * 
     * Copyright (c) 2003 Microsoft Corporation
     */ 
 
    internal class FactoryGenerator {
        private Type _factoryInterface; 
        private Type _returnedType;
        private MethodInfo _methodToOverride;
        private ModuleBuilder _dynamicModule = null;
        private Type[] _emptyParameterList = new Type[] { }; 
        private Type[] _interfacesToImplement;
 
        internal FactoryGenerator() : this(typeof(object), typeof(IWebObjectFactory)) {} 

        private FactoryGenerator(Type returnedType, Type factoryInterface) { 
            _returnedType = returnedType;
            _factoryInterface = factoryInterface;

            // Get the CreateInstance method, and make sure it has 
            // the correct signature.
 
            _methodToOverride = factoryInterface.GetMethod("CreateInstance", new Type[0]); 
            if (_methodToOverride.ReturnType != _returnedType) {
                throw new ArgumentException(SR.GetString(SR.FactoryInterface)); 
            }

            // This will be needed later, when building the dynamic class.
            _interfacesToImplement = new Type[1]; 
            _interfacesToImplement[0] = factoryInterface;
        } 
 
        // A type must be public and have a parameterless constructor, in order for us to be able to
        // create a factory for the type.  Throws if either of these conditions are not true. 
        internal static void CheckPublicParameterlessConstructor(Type type) {
            if (type == null) {
                throw new ArgumentNullException("type");
            } 

            if (!(type.IsPublic || type.IsNestedPublic)) { 
                throw new InvalidOperationException(SR.GetString(SR.FactoryGenerator_TypeNotPublic, type.Name)); 
            }
 
            ConstructorInfo constructor = type.GetConstructor(Type.EmptyTypes);
            if (constructor == null) {
                throw new InvalidOperationException(SR.GetString(SR.FactoryGenerator_TypeHasNoParameterlessConstructor, type.Name));
            } 
        }
 
        private static String GetUniqueCompilationName() { 
            return Guid.NewGuid().ToString().Replace('-', '_');
        } 

        [ReflectionPermission(SecurityAction.Assert, Flags=ReflectionPermissionFlag.ReflectionEmit)]
        Type GetFactoryTypeWithAssert(Type type) {
            CheckPublicParameterlessConstructor(type); 

            // Create the dynamic assembly if needed. 
            Type factoryType; 

            if (_dynamicModule == null) { 
                lock (this) {
                    if (_dynamicModule == null) {

                        // Use a unique name for each assembly. 
                        String name = GetUniqueCompilationName();
 
                        AssemblyName assemblyName = new AssemblyName(); 
                        assemblyName.Name = "A_" + name;
 
                        // Create a new assembly.
                        AssemblyBuilder newAssembly =
                           Thread.GetDomain().DefineDynamicAssembly(assemblyName,
                                                                    AssemblyBuilderAccess.Run, 
                                                                    null, //directory to persist assembly
                                                                    null, //evidence copied from caller 
                                                                    null, //requiredPermissions 
                                                                    null, //optionalPermissions
                                                                    null, //refusedPermissions 
                                                                    true //isSynchronized
                                                                    );

                        // Create a single module in the assembly. 
                        _dynamicModule = newAssembly.DefineDynamicModule("M_" + name);
                    } 
                } 
            }
 
            // Give the factory a unique name.

            String typeName = GetUniqueCompilationName();
            TypeBuilder factoryTypeBuilder = _dynamicModule.DefineType("T_" + typeName, 
                                                                       TypeAttributes.Public,
                                                                       typeof(Object), 
                                                                       _interfacesToImplement); 

            // Define the CreateInstance method. It must be virtual to be an interface implementation. 

            MethodBuilder method = factoryTypeBuilder.DefineMethod("CreateInstance",
                                                                   MethodAttributes.Public |
                                                                        MethodAttributes.Virtual, 
                                                                   _returnedType,
                                                                   null); 
 
            // Generate IL. The generated IL corresponds to "return new type()"
            //      newobj  
            //      ret

            ILGenerator il = method.GetILGenerator();
            ConstructorInfo cons = type.GetConstructor(Type.EmptyTypes); 
            il.Emit(OpCodes.Newobj, cons);
            il.Emit(OpCodes.Ret); 
 
            // Specify that this method implements CreateInstance from the inherited interface.
            factoryTypeBuilder.DefineMethodOverride(method, _methodToOverride); 

            // Bake in the type.
            factoryType = factoryTypeBuilder.CreateType();
 
            return factoryType;
        } 
 
        internal IWebObjectFactory CreateFactory(Type type) {
            Debug.Trace("FactoryGenerator", "Creating generator for type " + type.FullName); 

            Type factoryType = GetFactoryTypeWithAssert(type);

            // Create the type. This is the only place where Activator.CreateInstance is used, 
            // reducing the calls to it from 1 per instance to 1 per type.
            return (IWebObjectFactory) Activator.CreateInstance(factoryType); 
        } 
    }
 
}

#endif

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