CodeGo.net>如何创建Lambda并将其添加到使用反射的动作

假设在C#中,我的类具有任意数量的Action,可以具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...
}

而且我正在此类的实例上注册一些调试lambda,这些lambda仅打印出动作字段的名称：

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lambda,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 
        }   
    }
}

我可以在不带参数的情况下获得Actions的预期结果-上面的代码确实打印出“被调用”-但我不知道如何对泛型执行此操作.

我相信我知道我需要做什么,但不知道如何做：

>使用反射来创建动作< T1,T2,...>.使用actionArgTypes中的类型的对象,该对象包装对回调操作的调用.
>将此新创建的对象添加到由字段指定的通用操作.

我将如何去做,或者达到添加这种调试回调的预期效果的类似方法？

解决方法:

这是一个使用Expressions的相当简单的实现,可以直接使用ILGenerator,但是在这种情况下这不值得花钱.

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,应与.net framework 2.0以及.net core一起使用.在现实生活中的应用程序中,应该进行检查,缓存以及可能是整个assemblybuilder：

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);
    }
}

标签：system-reflection,reflection,delegates,c,lambda
来源： https://codeday.me/bug/20191108/2008854.html