反射發射創建類實例

Question

我想使用反射發射來創建具有任意構造函數參數的類的實例。這是我的代碼看起來像：

public delegate object ObjectActivator(params object[] args); 
static void Main(string[] args) 
{ 
    var ao = new { ID = 10000, FName = "Sample", SName = "Name" }; 
    var t = ao.GetType(); 
    var info = t.GetConstructor(new Type[] { typeof(int), typeof(string), typeof(string) }); 
    var objActivatorEmit = GetActivatorEmit(info); 
    var obj = createdActivatorEmit(4, "Foo", "Bar"); 
} 
public static ObjectActivator GetActivatorEmit(ConstructorInfo ctor) 
{ 
    ParameterInfo[] paramsInfo = ctor.GetParameters(); 
    DynamicMethod method = new DynamicMethod("CreateInstance", typeof(object), new Type[] { typeof(object[]) }); 
    ILGenerator gen = method.GetILGenerator(); 
    for (int i = 0; i < paramsInfo.Length; i++) 
    { 
     Type t = paramsInfo[i].ParameterType; 
     gen.Emit(OpCodes.Ldarg_0); // Push array (method argument) 
     gen.Emit(OpCodes.Ldc_I4, i); // Push i 
     gen.Emit(OpCodes.Ldelem_Ref); // Pop array and i and push array[i] 
     if(t.IsValueType) 
     { 
      gen.Emit(OpCodes.Unbox_Any, t); // Cast to Type t 
     } 
     else 
     { 
      gen.Emit(OpCodes.Castclass, t); //Cast to Type t 
     } 
    } 
    gen.Emit(OpCodes.Newobj, ctor); 
    gen.Emit(OpCodes.Ret); 
    return (ObjectActivator)method.CreateDelegate(typeof(ObjectActivator)); 
} 

代碼失敗，並MethodAccessException並顯示錯誤消息Attempt by method 'DynamicClass.CreateInstance(System.Object[])' to access method '<>f__AnonymousType1'3<System.Int32,System.__Canon,System.__Canon>..ctor(Int32, System.__Canon, System.__Canon)' failed.。

什麼問題？

Answer 1

的錯誤信息表明匿名類型的構造是不公開的。我覺得匿名類型構造函數總是internal，所以你需要使用一個不同的DynamicMethod構造跳過可視性檢查：

DynamicMethod method = new DynamicMethod("CreateInstance", typeof(object), new Type[] { typeof(object[]) }, true); 

注意，這不是在部分信任情況下工作。

Answer 2

你不需要使用Reflection.Emit，我建議你不要。除非你知道自己在做什麼或者有某些其他API無法滿足的特殊需求，否則最好不要離開。

有3個替代方案更容易得到正確的。檢查出來：

using System; 
using System.Linq; 
using System.Linq.Expressions; 
using System.Reflection; 

public static class App 
{ 
    public delegate object ObjectActivator(params object[] args); 

    public static void Main(string[] args) 
    { 
     var ao = new { ID = 10000, FName = "Sample", SName = "Name" }; 
     var t = ao.GetType(); 
     var info = t.GetConstructor(new[] { typeof(int), typeof(string), typeof(string) }); 
     if (info == null) 
     { 
      throw new Exception("Info is null"); 
     } 

     // This uses System.Linq.Expressions to create the delegate 
     var activator = GetActivator(info); 
     var newObj1 = activator(4, "Foo", "Bar"); 

     // This invokes the ConstructorInfo directly 
     var newObj2 = info.Invoke(new object[] { 4, "Foo", "Bar" }); 

     // This uses System.Activator to dynamically create the instance 
     var newObj3 = Activator.CreateInstance(t, 4, "Foo", "Bar"); 
    } 

    // This uses System.Linq.Expressions to generate a delegate 
    public static ObjectActivator GetActivator(ConstructorInfo ctor) 
    { 
     var args = Expression.Parameter(typeof(object[]), "args"); 
     var parameters = ctor.GetParameters().Select((parameter, index) => Expression.Convert(Expression.ArrayIndex(args, Expression.Constant(index)), parameter.ParameterType)); 
     return Expression.Lambda<ObjectActivator>(Expression.New(ctor, parameters), args).Compile(); 
    } 
} 

注：靈感GetActivator方法從this post