从Type获取新的对象实例

seaque 发布于 2018-02-05 .net 最后更新 2018-02-05 01:01 496 浏览

在编译时,人们可能并不总是知道对象的类型,但可能需要创建一个类型的实例。你如何从一个类型获得一个新的对象实例?

已邀请:

seaque

赞同来自:

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);
    }
}

iqui

赞同来自:

System命名空间中的Activator类非常强大。 有很多重载的参数传递给构造函数等。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
这里有一些简单的例子:
ObjectType instance = (ObjectType)Activator.CreateInstance(objectType);
ObjectType instance = (ObjectType)Activator.CreateInstance("MyAssembly","MyNamespace.ObjectType");

uharum

赞同来自:

如果这是在应用程序实例中被调用的东西,那么编译和缓存动态代码要快得多,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Linq Expressions或者一些简单的IL opcodes and 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。

rqui

赞同来自:

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>();

nodio

赞同来自:

普通的T t = new T();不工作吗?

yvitae

赞同来自:

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

oqui

赞同来自:

如果你想使用默认的构造函数,那么使用System.Activator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。在反思的情况下,只知道类型名称(使用其名称空间)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
    );
使用TypeDescriptor的示例:
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 }
    );

grem

赞同来自:

它非常简单。假设你的类名是Car,名字空间是Vehicles,那么传递参数为Vehicles.Car,它返回Car类型的对象。像这样,你可以动态地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Fully Qualified Name(即本例中的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找到Type。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");

rqui

赞同来自:

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) 使用泛型方法可以要求类型实现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
用非泛型假定类型有一个默认的构造函数和catch 如果不是,则为例外。
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

grem

赞同来自:

编译表达式是最好的办法! (用于在运行时反复创建实例的性能)。

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(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 += 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) {}
}

et_qui

赞同来自:

不使用反射:

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

kex

赞同来自:

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()