如何创建lambdas并使用反射将它们添加到操作中

Question

假设在C＃中我有一个具有任意数量的Actions的类，它可以有任意数量的泛型参数：

public class Container
{
    public Action a;
    public Action<float> b;
    public Action<int, float> c;
    // etc...
}

我正在这个类的实例上注册一些调试lambda，它只打印出action的字段的名称：

public static void Main()
{
    Container container = new Container();

    container.a += () => Console.WriteLine("a was called");
    container.b += (temp1) => Console.WriteLine("b was called");
    container.c += (temp1, temp2) => Console.WriteLine("c was called");

    container.a();
    container.b(1.5f);
    container.c(1, 1.5f);
}

---

我想使用反射自动创建这些调试lambdas，如下所示：

public static void Main()
{
    Container container = new Container();

    GenerateDebug(container);

    if(container.a != null) container.a();
    if(container.b != null) container.b(1.5f);
    if(container.c != null) container.c(1, 1.5f);
}

public static void GenerateDebug(Container c)
{
    Type t = c.GetType();
    FieldInfo[] fields = t.GetFields(BindingFlags.Instance | BindingFlags.Public);
    foreach(FieldInfo field in fields)
    {
        Action callback = () => Console.WriteLine(field.Name + " was called");

        Type[] actionArgTypes = field.FieldType.GetGenericArguments();
        if(actionArgTypes.Length == 0)
        {
            Action action = field.GetValue(c) as System.Action;
            action += callback;
            field.SetValue(c, action);
        }
        else
        {
            // 1. Create an Action<T1, T2, ...> object that takes the types in 'actionArgTypes' which wraps the 'callback' action
            // 2. Add this new lambda to the current Action<T1, T2, ...> field 
        }   
    }
}

我能够在没有参数的情况下获得所需的动作结果 - 上面的代码确实打印出"a was called" - 但我不知道如何为泛型做这件事。

我相信我知道我需要做什么，而不是如何：

1. 使用反射来创建Action<T1, T2, ...> object，使用actionArgTypes中的类型，它包含对callback动作的调用。
2. 将此新创建的对象添加到字段指定的通用操作中。

我将如何做到这一点，或类似的，以实现添加这样的调试回调所需的效果？

Answer 1

这是一个使用Expressions的相当简单的实现，可以直接使用ILGenerator，但在这种情况下这不值得麻烦。

public static void GenerateDebug(Container c)
{
    Type t = c.GetType();
    FieldInfo[] fields = t.GetFields(BindingFlags.Instance | BindingFlags.Public);
    foreach(FieldInfo field in fields)
    {
        var fieldName = field.Name;
        Type[] actionArgTypes = field.FieldType.GetGenericArguments();
        // Create paramter expression for each argument
        var parameters = actionArgTypes.Select(Expression.Parameter).ToArray();
        // Create method call expression with a constant argument
        var writeLineCall = Expression.Call(typeof(Console).GetMethod("WriteLine", new [] {typeof(string)}), Expression.Constant(fieldName + " was called"));
        // Create and compile lambda using the fields type
        var lambda = Expression.Lambda(field.FieldType, writeLineCall, parameters);
        var @delegate = lambda.Compile();
        var action = field.GetValue(c) as Delegate;
        // Combine and set delegates
        action = Delegate.Combine(action, @delegate);
        field.SetValue(c, action);
    }
}

这是使用ILGenerator的相同功能，它应该与.net framework 2.0+以及.net core一起使用。在现实生活中，应该有检查，缓存，可能还有整个装配架：

public static void GenerateDebug(Container c)
{
    Type t = c.GetType();
    FieldInfo[] fields = t.GetFields(BindingFlags.Instance | BindingFlags.Public);
    foreach(FieldInfo field in fields)
    {
        var fieldName = field.Name;
        Type[] actionArgTypes = field.FieldType.GetGenericArguments();

        var dm = new DynamicMethod(fieldName, typeof(void), actionArgTypes);
        var il = dm.GetILGenerator();
        il.Emit(OpCodes.Ldstr, fieldName + " was called using ilgen");
        il.Emit(OpCodes.Call, typeof(Console).GetMethod("WriteLine", new [] {typeof(string)}));
        il.Emit(OpCodes.Ret);

        var @delegate = dm.CreateDelegate(field.FieldType);
        var action = field.GetValue(c) as Delegate;
        // Combine and set delegates
        action = Delegate.Combine(action, @delegate);
        field.SetValue(c, action);
    }
}