制作 lambda 表达式以从泛型 class 调用方法

Question

我有这些简单的界面：

public interface IQuery<TResult> { }

public interface IQueryHandler<in TQuery, out TResult> 
    where TQuery : IQuery<TResult> {
    TResult Handle(TQuery query);
}

并且有一些实现。我正在尝试创建一个表达式树以在指定的处理程序上调用 Handle 方法。我的意思是：

var query = new MyQuery(); // which MyQuery implements IQuery<int>
object handler = _someServiceProvider
    .Get<IQueryHandler<MyQuery, int>>();

还有一个MyQueryHandler:

public class MyQueryHandler : IQueryHandler<MyQuery, int> {
    public int Handle(MyQuery query) { return 20; }
}

现在，我正在尝试创建一个 Func<object, MyQuery, int> 来像这样调用：

var func = GetMethod<MyQuery, int>(handler);
var result = func(handler, query);

这是我的 GetMethod 实现：

    private Func<object, TQuery, TResult> GetMethod<TQuery, TResult>(object obj)
        where TQuery : IQuery<TResult> {

        var methodInfo = obj.GetType().GetMethod(nameof(IQueryHandler<TQuery, TResult>.Handle));

        var insExp = Expression.Parameter(typeof(object), "ins");

        var inputExp = Expression.Parameter(typeof(TQuery), "query");

        var instanceExp = Expression.Variable(obj.GetType(), "instance");

        var assExp = Expression.Assign(instanceExp, Expression.Convert(insExp, obj.GetType()));

        var castExp = Expression.Convert(inputExp, methodInfo.GetParameters()[0].ParameterType);

        var callExp = Expression.Call(instanceExp, methodInfo, castExp);

        var blockExp = Expression.Block(new Expression[] {
            insExp,
            inputExp,
            instanceExp,
            assExp,
            castExp,
            callExp
        });

        var func =
            Expression.Lambda<Func<object, TQuery, TResult>>(
                blockExp,
                insExp,
                inputExp).Compile();
        return func;
    }

但是，当我尝试编译 Lambda 时，出现此错误：

An exception of type 'System.InvalidOperationException' occurred in System.Core.dll but was not handled in user code

Additional information: variable 'instance' of type 'Namespace.MyQueryHandler' referenced from scope '', but it is not defined

我哪里错了？我错过了什么？你有什么主意吗？提前致谢。

Answer 1

不太清楚你在GetMethod中试图用你的表达式做什么，所以我不会使用它并完全从头开始写。

如果您想将处理程序和查询都传递给您的方法 - 您不需要将任何实例传递给 GetMethod:

private static Func<object, TQuery, TResult> GetMethod<TQuery, TResult>()
    where TQuery : IQuery<TResult> {
    // "query" paramter
    var query = Expression.Parameter(typeof(TQuery), "query");
    // "handler" parameter
    var handler = Expression.Parameter(typeof(object), "handler");
    // convert your "object" parameter to handler type (not type safe of course)
    // ((IQueryHandler<TQuery, TResult>) handler).Handle(query)
    var body = Expression.Call(Expression.Convert(handler, typeof(IQueryHandler<TQuery, TResult>)), "Handle", new Type[0], query);
    //(handler, query) => ((IQueryHandler<TQuery, TResult>) handler).Handle(query);
    return Expression.Lambda<Func<object, TQuery, TResult>>(body, handler, query).Compile();
}

object handler = new MyQueryHandler();
var func = GetMethod<MyQuery, int>();
var result = func(handler, query);

如果您做将 handler 实例传递给 GetMethod - 您以后不需要再次将同一实例传递给您创建的 func - 你可以像这样重用相同的实例（当然假设这适合你的场景）：

private static Func<TQuery, TResult> GetMethod<TQuery, TResult>(object obj)
    where TQuery : IQuery<TResult> {
    // parameter
    var query = Expression.Parameter(typeof(TQuery), "query");
    // note Expression.Constant here - you use the same instance for every call
    var body = Expression.Call(Expression.Constant(obj), "Handle", new Type[0], query);
    return Expression.Lambda<Func<TQuery, TResult>>(body, query).Compile();
}

并使用它：

var query = new MyQuery(); // which MyQuery implements IQuery<int>
object handler = new MyQueryHandler();
var func = GetMethod<MyQuery, int>(handler);
var result = func(query);

Answer 2

据我所知，您正在尝试编写此函数：

TResult f(object ins, TQuery query)
{
    var instance = (MyQueryHandler)ins;
    return instance.Handle(query);
}

要使用表达式树执行此操作，您必须在 Expression.Block 中声明变量，但仅指定上面的两个语句，而不是所有子表达式：

var blockExp = Expression.Block(new[] { instanceExp }, new Expression[] {
    assExp,
    callExp
});

但更简单的选择是改为编写以下函数：

TResult f(object ins, TQuery query)
{
    return ((MyQueryHandler)ins).Handle(query);
}

看起来像这样：

var callExp = Expression.Call(
    Expression.Convert(insExp, obj.GetType()), methodInfo, castExp);

var func =
    Expression.Lambda<Func<object, TQuery, TResult>>(
        callExp,
        insExp,
        inputExp).Compile();

制作 lambda 表达式以从泛型 class 调用方法

Making a lambda expression to call a method from a generic class

c#

lambda

func

expression-trees

c#-4.0