858

人们可能并不总是在编译时知道Type对象的 ，但可能需要创建Type.

如何从 a 中获取新的对象实例Type？

4

11 回答 11

995

Activator根命名空间中的类System非常强大。

将参数传递给构造函数等有很多重载。在以下位置查看文档：

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

下面是一些简单的例子：

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

ObjectType instance = (ObjectType)Activator.CreateInstance("MyAssembly","MyNamespace.ObjectType");
于 2008-08-03T16:35:34.233 回答
173 
ObjectType instance = (ObjectType)Activator.CreateInstance(objectType);

该类Activator有一个通用变体，使这更容易一些：

ObjectType instance = Activator.CreateInstance<ObjectType>();
于 2008-08-25T13:33:25.257 回答
167

编译表达式是最好的方法！（为了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）：

    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

统计数据（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

统计数据（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

统计数据（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

统计数据（2019，x64/.NET 4.8）：

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

统计数据（2019，x64/.NET Core 3.0）：

Iterations: 5000000
No args
00:00:00.3226895, Activator.CreateInstance(string assemblyName, string typeName)
00:00:00.2786803, Activator.CreateInstance(Type type)
00:00:00.6183554, ConstructorInfo.Invoke
00:00:00.0483217, Compiled expression
00:00:00.0485119, Compiled expression (type)
00:00:00.0434534, new
Single arg
00:00:03.4389401, Activator.CreateInstance(Type type)
00:00:01.0803609, ConstructorInfo.Invoke
00:00:00.0554756, Compiled expression
00:00:00.0462232, 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 += x.Z;
        }
        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 += x.Z;
        }
        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) {}
}
于 2015-04-30T16:13:00.737 回答
25

它很简单。假设您的类名是Car并且命名空间是Vehicles，然后将参数作为Vehicles.Car返回类型的对象传递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，则Type.GetType将为空。在这种情况下，您需要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;
 }

您可以通过调用上述方法来获取实例。

object objClassInstance = GetInstance("Vehicles.Car");
于 2014-11-03T06:11:55.120 回答
19

不使用反射：

private T Create<T>() where T : class, new()
{
    return new T();
}
于 2015-06-30T12:51:58.807 回答
16

如果这是为了在应用程序实例中被大量调用的东西，编译和缓存动态代码会快得多，而不是使用激活器或ConstructorInfo.Invoke(). 动态编译的两个简单选项是编译的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次调用时，差异都会很大。

于 2008-08-25T13:31:35.433 回答
12

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，如果该类型缺少默认构造函数或者您必须使用非默认构造函数，则可以选择使用反射或System.ComponentModel.TypeDescriptor. 在反射的情况下，只知道类型名称（及其命名空间）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
    );

使用示例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 }
    );
于 2013-07-22T21:03:41.997 回答
10

通用T t = new T();不行吗？

于 2010-08-17T14:30:14.463 回答
10

鉴于此问题，当存在无参数 ctor 时，Activator 将起作用。如果这是一个约束考虑使用

System.Runtime.Serialization.FormatterServices.GetSafeUninitializedObject()
于 2015-06-30T16:35:24.747 回答
6 
public AbstractType New
{
    get
    {
        return (AbstractType) Activator.CreateInstance(GetType());
    }
}
于 2012-09-09T23:08:58.957 回答
3

我可以解决这个问题，因为我希望为任意类（使用默认构造函数）实现一个简单的 CloneObject 方法

使用泛型方法，您可以要求类型实现 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

使用非泛型假设该类型具有默认构造函数，如果没有，则捕获异常。

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
于 2015-03-24T18:10:05.570 回答