使用委托调用构造函数

Question

我发现这个但尝试使用它但失败了。

如何使用反射创建对象并通过将其放入委托中来使其快速？

        DynamicMethod dm = new DynamicMethod("MyCtor", t, new Type[] { });            
        var ctor = t.GetConstructor(new Type[] { });
        ILGenerator ilgen = dm.GetILGenerator();
        ilgen.Emit(OpCodes.Ldarg_0);
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);
        var d = (Func<T>)dm.CreateDelegate(t);
        dm.Invoke(null, new object[] { });

在将其放入 deleage 之前，我至少尝试调用它，当我执行上述操作时，我收到错误

An unhandled exception of type 'System.Reflection.TargetInvocationException' occurred in mscorlib.dll

附加信息：调用目标已引发异常。

如果我调用 d() 相反，我会得到异常

An unhandled exception of type 'System.ArgumentException' occurred in mscorlib.dll

Additional information: Type must derive from Delegate.

如何将无参数构造函数放入委托中并调用它？

Answer 1

如果您可以访问 .NET 3.5（您使用 Func 建议这样做），您可能会发现 Expression 比 ILGenerator 更容易

class Foo { }
static void Main() {
    Func<Foo> func = GetCtor<Foo>(); // cache this somewhere!
    Foo foo = func();
}
static Func<T> GetCtor<T>() {
    Type type = typeof(T);
    Expression body = Expression.New(type);
    return Expression.Lambda<Func<T>>(body).Compile();        
}

：易于扩展以使用特定的构造函数、传递参数或添加构造函数后属性绑定；转换、转换等（请参阅此相关答案）。如果您有特定的场景，我很乐意添加一个示例。

另请注意，您应该缓存并重用任何此类构造函数 - 否则您将失去好处（即不要在每次调用时重新创建委托）。

Answer 2

试试这个 -

Action myCtor = CreateCtor(t, Type.EmptyTypes, typeof(Action));

public static Delegate CreateCtor(Type type, Type[] parameterTypes, Type delegateType, string typeParameterName)
{
    var ctorInfo = type.GetConstructor(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance, null, parameterTypes, null);
    if (ctorInfo == null)
    {
        string parameterString = string.Empty;
        if(parameterTypes.Length > 0)
        {
            string[] parameterStrings = new string[parameterTypes.Length];
            for(int i = 0; i < parameterTypes.Length; ++i)
            {
                parameterStrings[i] = parameterTypes[i].ToString();
            }
            parameterString = string.Join(",", parameterStrings);
        }
        throw new ArgumentException(string.Format("Type '{0}' does not define .ctor({1}).", type, parameterString), typeParameterName);
    }

    bool isVisible = type.IsVisible && (ctorInfo.IsPublic && !ctorInfo.IsFamilyOrAssembly);

    DynamicMethod dynamicCtor = new DynamicMethod(Guid.NewGuid().ToString("N"), type, parameterTypes, ctorInfo.Module, !isVisible);
    var il = dynamicCtor.GetILGenerator();
    for (int i = 0; i < parameterTypes.Length; ++i)
    {
        switch (i)
        {
            case 0: il.Emit(OpCodes.Ldarg_0); break;
            case 1: il.Emit(OpCodes.Ldarg_1); break;
            case 2: il.Emit(OpCodes.Ldarg_2); break;
            case 3: il.Emit(OpCodes.Ldarg_3); break;
            default: il.Emit(OpCodes.Ldarg, i); break;
        }
    }
    il.Emit(OpCodes.Newobj, ctorInfo);
    il.Emit(OpCodes.Ret);
    return dynamicCtor.CreateDelegate(delegateType);
}

Answer 3

构造函数没有参数，因此您不应在堆栈上加载参数 ilgen.Emit(OpCodes.Ldarg_0)：

class Program
{
    static void Main()
    {
        var t = typeof(Program);
        var dm = new DynamicMethod("MyCtor", t, new Type[0], t.Module);
        var ctor = t.GetConstructor(new Type[0]);
        ILGenerator ilgen = dm.GetILGenerator();
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);
        var del = (Func<Program>)dm.CreateDelegate(typeof(Func<Program>));
        var instance = del();
        Console.WriteLine(instance);
    }
}

Answer 4

构造委托的通用方法，直接调用构造函数。
自动搜索具有给定委托类型签名的给定类型的构造函数并构造该类型的委托。此处的代码：

/// <summary>
/// Reflective object construction helper.
/// All methods are thread safe.
/// </summary>
public static class Constructor
{
    /// <summary>
    /// Searches an instanceType constructor with delegateType-matching signature and constructs delegate of delegateType creating new instance of instanceType.
    /// Instance is casted to delegateTypes's return type. 
    /// Delegate's return type must be assignable from instanceType.
    /// </summary>
    /// <param name="delegateType">Type of delegate, with constructor-corresponding signature to be constructed.</param>
    /// <param name="instanceType">Type of instance to be constructed.</param>
    /// <returns>Delegate of delegateType wich constructs instance of instanceType by calling corresponding instanceType constructor.</returns>
    public static Delegate Compile(Type delegateType,Type instanceType)
    {
        if (!typeof(Delegate).IsAssignableFrom(delegateType))
        {
            throw new ArgumentException(String.Format("{0} is not a Delegate type.",delegateType.FullName),"delegateType");
        }
        var invoke = delegateType.GetMethod("Invoke");
        var parameterTypes = invoke.GetParameters().Select(pi => pi.ParameterType).ToArray();
        var resultType = invoke.ReturnType;
        if(!resultType.IsAssignableFrom(instanceType))
        {
            throw new ArgumentException(String.Format("Delegate's return type ({0}) is not assignable from {1}.",resultType.FullName,instanceType.FullName));
        }
        var ctor = instanceType.GetConstructor(
            BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null, parameterTypes, null);
        if(ctor == null)
        {
            throw new ArgumentException("Can't find constructor with delegate's signature","instanceType");
        }
        var parapeters = parameterTypes.Select(Expression.Parameter).ToArray();

        var newExpression = Expression.Lambda(delegateType,
            Expression.Convert(Expression.New(ctor, parapeters), resultType),
            parapeters);
        var @delegate = newExpression.Compile();
        return @delegate;
    }
    public static TDelegate Compile<TDelegate>(Type instanceType)
    {
        return (TDelegate) (object) Compile(typeof (TDelegate), instanceType);
    }
}

是 Yappi 项目源代码的一部分。使用它，您可以构造委托，调用给定类型的任何构造函数，包括带参数的构造函数（除了 ref 和 out 参数）。

示例用法：

var newList = Constructor.Compile<Func<int, IList<String>>>(typeof (List<String>));
var list = newList(100);

构造委托后，将其存储在静态字典中或具有泛型参数的类的静态字段中的某个位置。不要每次都构建新的委托。使用一个委托来构造给定类型的多个实例。