从Type获取新的对象实例
在编译时，人们可能并不总是知道对象的types，但可能需要创build一个types的实例。 你如何从一个types获得一个新的对象实例？
根System命名空间中的Activator类非常强大。
有很多重载的parameter passing给构造函数等。 查看文档：
http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx
或（新path）
https://docs.microsoft.com/en-us/dotnet/api/system.activator.createinstance
这里有一些简单的例子：
ObjectType instance = (ObjectType)Activator.CreateInstance(objectType); ObjectType instance = (ObjectType)Activator.CreateInstance("MyAssembly","MyNamespace.ObjectType");
ObjectType instance = (ObjectType)Activator.CreateInstance(objectType);
Activator类有一个通用的变体，这使得这一点更容易：
ObjectType instance = Activator.CreateInstance<ObjectType>();
编译expression式是最好的办法！ （用于在运行时反复创build实例的性能）。
static readonly Func<X> YCreator = Expression.Lambda<Func<X>>( Expression.New(typeof(Y).GetConstructor(Type.EmptyTypes)) ).Compile(); X x = YCreator();
统计（2012）：
Iterations: 5000000 00:00:00.8481762, Activator.CreateInstance(string, string) 00:00:00.8416930, Activator.CreateInstance(type) 00:00:06.6236752, ConstructorInfo.Invoke 00:00:00.1776255, Compiled expression 00:00:00.0462197, new
统计（2015，.net 4.5，x64）：
Iterations: 5000000 00:00:00.2659981, Activator.CreateInstance(string, string) 00:00:00.2603770, Activator.CreateInstance(type) 00:00:00.7478936, ConstructorInfo.Invoke 00:00:00.0700757, Compiled expression 00:00:00.0286710, new
统计（2015，.net 4.5，x86）：
Iterations: 5000000 00:00:00.3541501, Activator.CreateInstance(string, string) 00:00:00.3686861, Activator.CreateInstance(type) 00:00:00.9492354, ConstructorInfo.Invoke 00:00:00.0719072, Compiled expression 00:00:00.0229387, new
统计（2017，LINQPad 5.22.02 / x64 / .NET 4.6）：
Iterations: 5000000 No args 00:00:00.3897563, Activator.CreateInstance(string assemblyName, string typeName) 00:00:00.3500748, Activator.CreateInstance(Type type) 00:00:01.0100714, ConstructorInfo.Invoke 00:00:00.1375767, Compiled expression 00:00:00.1337920, Compiled expression (type) 00:00:00.0593664, new Single arg 00:00:03.9300630, Activator.CreateInstance(Type type) 00:00:01.3881770, ConstructorInfo.Invoke 00:00:00.1425534, Compiled expression 00:00:00.0717409, new
完整代码：
static X CreateY_New() { return new Y(); } static X CreateY_New_Arg(int z) { return new Y(z); } static X CreateY_CreateInstance() { return (X)Activator.CreateInstance(typeof(Y)); } static X CreateY_CreateInstance_String() { return (X)Activator.CreateInstance("Program", "Y").Unwrap(); } static X CreateY_CreateInstance_Arg(int z) { return (X)Activator.CreateInstance(typeof(Y), new object[] { z, }); } private static readonly System.Reflection.ConstructorInfo YConstructor = typeof(Y).GetConstructor(Type.EmptyTypes); private static readonly object[] Empty = new object[] { }; static X CreateY_Invoke() { return (X)YConstructor.Invoke(Empty); } private static readonly System.Reflection.ConstructorInfo YConstructor_Arg = typeof(Y).GetConstructor(new[] { typeof(int), }); static X CreateY_Invoke_Arg(int z) { return (X)YConstructor_Arg.Invoke(new object[] { z, }); } private static readonly Func<X> YCreator = Expression.Lambda<Func<X>>( Expression.New(typeof(Y).GetConstructor(Type.EmptyTypes)) ).Compile(); static X CreateY_CompiledExpression() { return YCreator(); } private static readonly Func<X> YCreator_Type = Expression.Lambda<Func<X>>( Expression.New(typeof(Y)) ).Compile(); static X CreateY_CompiledExpression_Type() { return YCreator_Type(); } private static readonly ParameterExpression YCreator_Arg_Param = Expression.Parameter(typeof(int), "z"); private static readonly Func<int, X> YCreator_Arg = Expression.Lambda<Func<int, X>>( Expression.New(typeof(Y).GetConstructor(new[] { typeof(int), }), new[] { YCreator_Arg_Param, }), YCreator_Arg_Param ).Compile(); static X CreateY_CompiledExpression_Arg(int z) { return YCreator_Arg(z); } static void Main(string[] args) { const int iterations = 5000000; Console.WriteLine("Iterations: {0}", iterations); Console.WriteLine("No args"); foreach (var creatorInfo in new[] { new {Name = "Activator.CreateInstance(string assemblyName, string typeName)", Creator = (Func<X>)CreateY_CreateInstance}, new {Name = "Activator.CreateInstance(Type type)", Creator = (Func<X>)CreateY_CreateInstance}, new {Name = "ConstructorInfo.Invoke", Creator = (Func<X>)CreateY_Invoke}, new {Name = "Compiled expression", Creator = (Func<X>)CreateY_CompiledExpression}, new {Name = "Compiled expression (type)", Creator = (Func<X>)CreateY_CompiledExpression_Type}, new {Name = "new", Creator = (Func<X>)CreateY_New}, }) { var creator = creatorInfo.Creator; var sum = 0; for (var i = 0; i < 1000; i++) sum += creator().Z; var stopwatch = new Stopwatch(); stopwatch.Start(); for (var i = 0; i < iterations; ++i) { var x = creator(); sum += xZ; } stopwatch.Stop(); Console.WriteLine("{0}, {1}", stopwatch.Elapsed, creatorInfo.Name); } Console.WriteLine("Single arg"); foreach (var creatorInfo in new[] { new {Name = "Activator.CreateInstance(Type type)", Creator = (Func<int, X>)CreateY_CreateInstance_Arg}, new {Name = "ConstructorInfo.Invoke", Creator = (Func<int, X>)CreateY_Invoke_Arg}, new {Name = "Compiled expression", Creator = (Func<int, X>)CreateY_CompiledExpression_Arg}, new {Name = "new", Creator = (Func<int, X>)CreateY_New_Arg}, }) { var creator = creatorInfo.Creator; var sum = 0; for (var i = 0; i < 1000; i++) sum += creator(i).Z; var stopwatch = new Stopwatch(); stopwatch.Start(); for (var i = 0; i < iterations; ++i) { var x = creator(i); sum += xZ; } stopwatch.Stop(); Console.WriteLine("{0}, {1}", stopwatch.Elapsed, creatorInfo.Name); } } public class X { public X() { } public X(int z) { this.Z = z; } public int Z; } public class Y : X { public Y() {} public Y(int z) : base(z) {} }
这个问题的一个实现就是尝试调用Type的无参数构造函数：
public static object GetNewObject(Type t) { try { return t.GetConstructor(new Type[] { }).Invoke(new object[] { }); } catch { return null; } }
这是一个通用方法中包含的相同方法：
public static T GetNewObject<T>() { try { return (T)typeof(T).GetConstructor(new Type[] { }).Invoke(new object[] { }); } catch { return default(T); } }
如果这是在应用程序实例中被调用的东西，那么编译和cachingdynamic代码要快很多，而不是使用激活器或ConstructorInfo.Invoke() 。 dynamic编译的两个简单选项是编译Linqexpression式或一些简单的IL操作码和DynamicMethod 。 无论哪种方式，当你开始进入紧密的循环或多次调用时，差别是巨大的。
它非常简单。 假设你的类名是Car ，命名空间是Vehicles ，然后将parameter passing为Vehicles.Car ，它返回Cartypes的对象。 像这样，你可以dynamic地创build任何类的任何实例。
public object GetInstance(string strNamesapace) { Type t = Type.GetType(strNamesapace); return Activator.CreateInstance(t); }
如果您的完全限定名称 （即在这种情况下为Vehicles.Car ）在另一个程序集中，则Type.GetType将为空。 在这种情况下，您可以遍历所有程序集并findType 。 为此，您可以使用下面的代码
public object GetInstance(string strFullyQualifiedName) { Type type = Type.GetType(strFullyQualifiedName); if (type != null) return Activator.CreateInstance(type); foreach (var asm in AppDomain.CurrentDomain.GetAssemblies()) { type = asm.GetType(strFullyQualifiedName); if (type != null) return Activator.CreateInstance(type); } return null; }
您可以通过调用上述方法来获取实例。
object objClassInstance = GetInstance("Vehicles.Car");
不使用reflection：
private T Create<T>() where T : class, new() { return new T(); }
如果你想使用默认的构造函数，那么前面介绍的使用System.Activator的解决scheme可能是最方便的。 但是，如果types缺less默认构造函数，或者您必须使用非默认构造函数，则选项是使用reflection或System.ComponentModel.TypeDescriptor 。 在反思的情况下，只知道types名称（使用其名称空间）就足够了。
使用reflection的示例：
ObjectType instance = (ObjectType)System.Reflection.Assembly.GetExecutingAssembly().CreateInstance( typeName: objectType.FulName, // string including namespace of the type ignoreCase: false, bindingAttr: BindingFlags.Default, binder: null, // use default binder args: new object[] { args, to, constructor }, culture: null, // use CultureInfo from current thread activationAttributes: null );
使用TypeDescriptor示例：
ObjectType instance = (ObjectType)System.ComponentModel.TypeDescriptor.CreateInstance( provider: null, // use standard type description provider, which uses reflection objectType: objectType, argTypes: new Type[] { types, of, args }, args: new object[] { args, to, constructor } );
不是genericsT t = new T(); 工作？
鉴于这个问题激活器将工作时，有一个无参数ctor。 如果这是一个约束考虑使用
System.Runtime.Serialization.FormatterServices.GetSafeUninitializedObject()
我可以跨越这个问题，因为我正在为任意类实现一个简单的CloneObject方法（带有默认构造函数）
使用generics方法可以要求types实现New（）。
Public Function CloneObject(Of T As New)(ByVal src As T) As T Dim result As T = Nothing Dim cloneable = TryCast(src, ICloneable) If cloneable IsNot Nothing Then result = cloneable.Clone() Else result = New T CopySimpleProperties(src, result, Nothing, "clone") End If Return result End Function
非generics假定types有一个默认的构造函数，如果没有的话，会捕获一个exception。
Public Function CloneObject(ByVal src As Object) As Object Dim result As Object = Nothing Dim cloneable As ICloneable Try cloneable = TryCast(src, ICloneable) If cloneable IsNot Nothing Then result = cloneable.Clone() Else result = Activator.CreateInstance(src.GetType()) CopySimpleProperties(src, result, Nothing, "clone") End If Catch ex As Exception Trace.WriteLine("!!! CloneObject(): " & ex.Message) End Try Return result End Function
public AbstractType New { get { return (AbstractType) Activator.CreateInstance(GetType()); } }
