采用TypeBuilder创建基类直通构造 [英] Using TypeBuilder to create a pass-through constructor for the base class

问题描述

说我有一个飞船类，像这样：

Say I have a SpaceShip class, like so:

public class SpaceShip {
    public SpaceShip() {  }
    public SpaceShip(IRocketFuelSource fuelSource) {  }
}

我想用 TypeBuilder 来创建一个类型在运行时从飞船继承，并定义为每个飞船子里的一个构造函数。我不需要构造实际的做的除了通过它们的参数到父任何东西（传递构造函数）。例如，生成的类型会是这个样子，如果在C＃中表示：

I want to use TypeBuilder to create a type at run-time which inherits from SpaceShip, and defines one constructor for each of the ones in SpaceShip. I don't need the constructors to actually do anything except pass their arguments up to the parent ("pass-through" constructors). For example, the generated type would look something like this if expressed in C#:

public class SpaceShipSubClass : SpaceShip {
    public SpaceShipSubClass() : base() {  }
    public SpaceShipSubClass(IRocketFuelSource fuelSource) : base(fuelSource) {  }
}

要复杂的事情了一下，我是不知道生成的类型将是哪一类的，直到运行时继承（所以我必须考虑到任何数量的构造函数，可能带有默认参数）。

To complicate things a bit, I don't actually know which class the generated type will be inheriting from until run-time (so I'll have to take into account any number of constructors, possibly with default parameters).

这可能吗？我想我可以计算出来，如果我有一个大方向入手，这只是我完全新的 TypeBuilder 。

Is this possible? I think I could figure it out if I had a general direction to start with, it's just that I'm completely new to TypeBuilder.

谢谢！

推荐答案

好吧，我就在网上找到任何东西，所以我最终实现我自己。这应该有助于开始有人写一些代理了。

Alright, I couldn't find anything online, so I ended up implementing my own. This should help start off anyone writing some sort of proxy, too.

public static class TypeBuilderHelper
{
    /// <summary>Creates one constructor for each public constructor in the base class. Each constructor simply
    /// forwards its arguments to the base constructor, and matches the base constructor's signature.
    /// Supports optional values, and custom attributes on constructors and parameters.
    /// Does not support n-ary (variadic) constructors</summary>
    public static void CreatePassThroughConstructors(this TypeBuilder builder, Type baseType)
    {
        foreach (var constructor in baseType.GetConstructors()) {
            var parameters = constructor.GetParameters();
            if (parameters.Length > 0 && parameters.Last().IsDefined(typeof(ParamArrayAttribute), false)) {
                //throw new InvalidOperationException("Variadic constructors are not supported");
                continue;
            }

            var parameterTypes = parameters.Select(p => p.ParameterType).ToArray();
            var requiredCustomModifiers = parameters.Select(p => p.GetRequiredCustomModifiers()).ToArray();
            var optionalCustomModifiers = parameters.Select(p => p.GetOptionalCustomModifiers()).ToArray();

            var ctor = builder.DefineConstructor(MethodAttributes.Public, constructor.CallingConvention, parameterTypes, requiredCustomModifiers, optionalCustomModifiers);
            for (var i = 0; i < parameters.Length; ++i) {
                var parameter = parameters[i];
                var parameterBuilder = ctor.DefineParameter(i + 1, parameter.Attributes, parameter.Name);
                if (((int)parameter.Attributes & (int)ParameterAttributes.HasDefault) != 0) {
                    parameterBuilder.SetConstant(parameter.RawDefaultValue);
                }

                foreach (var attribute in BuildCustomAttributes(parameter.GetCustomAttributesData())) {
                    parameterBuilder.SetCustomAttribute(attribute);
                }
            }

            foreach (var attribute in BuildCustomAttributes(constructor.GetCustomAttributesData())) {
                ctor.SetCustomAttribute(attribute);
            }

            var emitter = ctor.GetILGenerator();
            emitter.Emit(OpCodes.Nop);

            // Load `this` and call base constructor with arguments
            emitter.Emit(OpCodes.Ldarg_0);
            for (var i = 1; i <= parameters.Length; ++i) {
                emitter.Emit(OpCodes.Ldarg, i);
            }
            emitter.Emit(OpCodes.Call, constructor);

            emitter.Emit(OpCodes.Ret);
        }
    }


    private static CustomAttributeBuilder[] BuildCustomAttributes(IEnumerable<CustomAttributeData> customAttributes)
    {
        return customAttributes.Select(attribute => {
            var attributeArgs = attribute.ConstructorArguments.Select(a => a.Value).ToArray();
            var namedPropertyInfos = attribute.NamedArguments.Select(a => a.MemberInfo).OfType<PropertyInfo>().ToArray();
            var namedPropertyValues = attribute.NamedArguments.Where(a => a.MemberInfo is PropertyInfo).Select(a => a.TypedValue.Value).ToArray();
            var namedFieldInfos = attribute.NamedArguments.Select(a => a.MemberInfo).OfType<FieldInfo>().ToArray();
            var namedFieldValues = attribute.NamedArguments.Where(a => a.MemberInfo is FieldInfo).Select(a => a.TypedValue.Value).ToArray();
            return new CustomAttributeBuilder(attribute.Constructor, attributeArgs, namedPropertyInfos, namedPropertyValues, namedFieldInfos, namedFieldValues);
        }).ToArray();
    }
}

使用（假设你有一个 TypeBuilder 对象 - 看的这里为例）：

Usage (assuming you have a TypeBuilder object -- see here for an example):

var typeBuilder = ...;  // TypeBuilder for a SpaceShipSubClass
typeBuilder.CreatePassThroughConstructors(typeof(SpaceShip));
var subType = typeBuilder.CreateType();  // Woo-hoo, proxy constructors!

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