如何从Type创建新的对象实例

Question

人们可能并不总是Type在编译时知道对象,但可能需要创建一个实例Type.你如何从一个新的对象实例Type？

Answer 1

Activator根System命名空间中的类非常强大.

将参数传递给构造函数等有很多重载.查看以下文档:

http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx

或(新路径)

https://docs.microsoft.com/en-us/dotnet/api/system.activator.createinstance

以下是一些简单的例子:

ObjectType instance = (ObjectType)Activator.CreateInstance(objectType);

ObjectType instance = (ObjectType)Activator.CreateInstance("MyAssembly","MyNamespace.ObjectType");

Answer 2

ObjectType instance = (ObjectType)Activator.CreateInstance(objectType);

该Activator班有一个通用的变种,使这是一个更容易一些:

ObjectType instance = Activator.CreateInstance<ObjectType>();

Answer 3

编译表达式是最好的方法!(用于在运行时重复创建实例的性能).

static readonly Func<X> YCreator = Expression.Lambda<Func<X>>(
   Expression.New(typeof(Y).GetConstructor(Type.EmptyTypes))
 ).Compile();

X x = YCreator();

统计(2012年):

    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年,.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年,.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年,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:

Iterations: 5000000
No args
00:00:00.3287835, Activator.CreateInstance(string assemblyName, string typeName)
00:00:00.3122015, Activator.CreateInstance(Type type)
00:00:00.8035712, ConstructorInfo.Invoke
00:00:00.0692854, Compiled expression
00:00:00.0662223, Compiled expression (type)
00:00:00.0337862, new
Single arg
00:00:03.8081959, Activator.CreateInstance(Type type)
00:00:01.2507642, ConstructorInfo.Invoke
00:00:00.0671756, Compiled expression
00:00:00.0301489, new

Answer 4

此问题的一个实现是尝试调用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;
    }
}

这是一种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);
    }
}

Answer 5

它非常简单.假设您的类名是Car和命名空间Vehicles,然后传递Vehicles.Car返回类型对象的参数Car.像这样,您可以动态创建任何类的任何实例.

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,Vehicles.Car在这种情况下)在另一个程序集中,Type.GetType则将为null.在这种情况下,你循环遍历所有程序集并找到Type.为此,您可以使用以下代码

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;
 }

您可以通过调用上面的方法来获取实例.

object objClassInstance = GetInstance("Vehicles.Car");

Answer 6

如果这是在应用程序实例中被大量调用的东西,那么编译和缓存动态代码而不是使用激活器或更快的速度要快得多ConstructorInfo.Invoke().动态编译的两个简单选项是编译Linq表达式或一些简单的IL操作码和DynamicMethod.无论哪种方式,当您开始进入紧密循环或多个呼叫时,差异是巨大的.

Answer 7

通用T t = new T();工作不会吗？

Answer 8

如果您想使用默认构造函数,那么System.Activator前面介绍的解决方案可能是最方便的.但是,如果类型缺少默认构造函数或者您必须使用非默认构造函数,则选项是使用反射或System.ComponentModel.TypeDescriptor.在反射的情况下,只知道类型名称(及其名称空间)就足够了.

使用反射的示例:

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
    );

示例使用TypeDescriptor:

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 }
    );

Answer 9

不使用反射:

private T Create<T>() where T : class, new()
{
    return new T();
}

Answer 10

鉴于此问题,当存在无参数的ctor时,Activator将工作.如果这是一个约束考虑使用

System.Runtime.Serialization.FormatterServices.GetSafeUninitializedObject()

Answer 11

public AbstractType New
{
    get
    {
        return (AbstractType) Activator.CreateInstance(GetType());
    }
}