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类型，但可能需要创建一个类型的实例。 你如何从Type获得一个新的对象实例？
根System命名空间中的Activator类非常强大。
有很多重载参数传递给构造函数等等。 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");
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>();
编译表达式是最好的方式！ （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) {}
}
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