如何从Type创建新的对象实例

时间：2008-08-03 16:29:04

标签： c# .net performance reflection types

在编译时可能并不总是知道对象的Type，但可能需要创建Type的实例。如何从Type获得新的对象实例？

13 个答案:

答案 0 :(得分：807)

根Activator命名空间中的System类非常强大。

将参数传递给构造函数等有很多重载。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

以下是一些简单的例子：

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

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

答案 1 :(得分：128)

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

Activator类有一个通用的变体，使这更容易：

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

答案 2 :(得分：104)

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

统计（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

统计信息（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) {}
}

答案 3 :(得分：41)

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;
    }
}

以下是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);
    }
}

答案 4 :(得分：13)

非常简单。假设您的类名为Car且命名空间为Vehicles，然后将参数传递为Vehicles.Car，返回类型为Car的对象。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Fully Qualified Name（即本例中为Vehicles.Car）位于另一个程序集中，则Type.GetType将为空。在这种情况下，您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;
 }

您可以通过调用上述方法获取实例。

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

答案 5 :(得分：12)

如果这是在应用程序实例中大量调用的东西，那么编译和缓存动态代码而不是使用激活器或ConstructorInfo.Invoke()要快得多。动态编译的两个简单选项是Linq Expressions或一些简单的IL opcodes and DynamicMethod。无论哪种方式，当您开始进入紧密循环或多次呼叫时，差异是巨大的。

答案 6 :(得分：9)

通用T t = new T();不会起作用吗？

答案 7 :(得分：9)

不使用反射：

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

答案 8 :(得分：8)

如果您想使用默认构造函数，那么前面介绍的使用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。在反射的情况下，仅知道类型名称（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
    );

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 }
    );

答案 9 :(得分：5)

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，请考虑使用

    <select name="syear" class="inputbox" id="Select1">
     <option value="104" selected> 104</option>
     <option value="105">105</option>
    </select> 
     YEAR <select name="smonth" class="inputbox" id="Select2"> 
     <option value="01"> 1</option> 
     <option value="02"> 2</option> 
     <option value="03"> 3</option> 
     <option value="04"> 4</option> 
     <option value="05"> 5</option> 
     <option value="06"> 6</option> 
     <option value="07" selected> 7</option> 
     <option value="08"> 8</option> 
     <option value="09"> 9</option> 
     <option value="10"> 10</option> 
     <option value="11"> 11</option> 
     <option value="12"> 12</option> 
    </select>

答案 10 :(得分：3)

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

答案 11 :(得分：3)

我可以跨越这个问题，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

使用非泛型假设类型具有默认构造函数和catch 如果没有，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

答案 12 :(得分：0)

对于使用以上实现的答案的任何人来说，都是额外的东西：

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

请注意-如果您的构造函数不是“公共”的，则会出现以下错误：

“ System.MissingMethodException：'未定义无参数构造函数 为此对象。”

您的类可以是Internal / Friend或您需要的任何类，但构造函数必须是公共的。

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