构建表达式树以调用具有 1 或 2 个参数的函数

Question

所以，尝试一下表达式树。 这是一个想法：我想返回一个Func<long, byte?, object>对象。 根据我想要使用该函数的类型，我将使用以下两种方法之一： LoadById(long, byte?)或LoadByID(long) 。 两者都返回一个对象。 所以我正在尝试执行以下操作：根据类型是否实现某个接口，我使用classToUseFunctionOn.LoadById(long, byte?)或classToUseFunctionOn.LoadByID(long) 。 所以，基本上我希望如果它实现了接口，则返回以下代码： (long id, byte? options) => new TestFacade().LoadById(id, options) and (long id, byte? options) => new TestFacade().LoadByID(id)。 我只是不知道该怎么做。 最后几行出错了。 Lambda 调用声明参数数量不正确。 以下是我到目前为止的代码：

    private static Func GetDataExtractorForTypeWithId(Type type)
    {
        var paramId = Expression.Parameter(typeof(long), "id");
        ParameterExpression paramOptions = null;
        //gets the ConstructorInfo for the constructor of type T with a single parameter of type IDataReader
        var facadetype = GetFacadeType(type.Name);
        MethodInfo loadMethod;
        var linkedEntitiesInterface = facadetype.GetInterface(typeof(IFacadeLoadLinkedEntities).Name);
        var lamdaParameterExpressions = new List() { paramId };
        if (linkedEntitiesInterface != null)
        {
            loadMethod = facadetype.GetMethod("LoadById");
            paramOptions = Expression.Parameter(linkedEntitiesInterface.GetGenericTypeDefinition().GenericTypeArguments[1], "options");
            lamdaParameterExpressions.Add(paramOptions);
        }
        else
        {
            paramOptions = Expression.Parameter(typeof(byte?));
            loadMethod = facadetype.GetMethod("LoadByID", new Type[1]{typeof(long)});
        }
        var facadeConstructor = facadetype.GetConstructor(new Type[0]);
        var newFacade = Expression.New(facadeConstructor);
        var callLoad = Expression.Call(newFacade, loadMethod, lamdaParameterExpressions);
        lamdaParameterExpressions.Add(paramOptions);
        var returnValue = Expression.Parameter(typeof(object));
        lamdaParameterExpressions.Add(returnValue);
        var entityVariable = Expression.Variable(typeof(object), "entity");
        Expression.Assign(entityVariable, callLoad);
        var lambda = Expression.Lambda>(
            entityVariable, lamdaParameterExpressions.ToArray());
        //compiles the Expression to a usable delegete.
        return lambda.Compile();
    }

Answer 1

与此同时，我找到了让它工作的方法：

    private static Func GetDataExtractorForTypeWithId(Type type)
            {
                var paramId = Expression.Parameter(typeof(long), "id");
                ParameterExpression paramOptions = null;
                var facadetype = GetFacadeType(type.Name) ?? typeof(AzzFacade).MakeGenericType(type);
                MethodInfo loadMethod;
                var linkedEntitiesInterface = facadetype.GetInterface(typeof(IFacadeLoadLinkedEntities).Name);
                if (linkedEntitiesInterface != null)
                {
                    loadMethod = facadetype.GetMethod("LoadById");
                    paramOptions = Expression.Parameter(typeof(byte), "options");
                }
                else
                {
                    paramOptions = Expression.Parameter(typeof(byte));
                    loadMethod = facadetype.GetMethod("LoadByID", new Type[1]{typeof(long)});
                }
                var facadeConstructor = facadetype.GetConstructor(new Type[0]);
                if(facadeConstructor==null)
                    throw new NullReferenceException($"No parameterless constructor found for facade for type {type.Name}");
                MethodCallExpression callLoad;
                var newFacade = Expression.New(facadeConstructor);
                if (linkedEntitiesInterface != null)
                {
                    var conversionExpression = Expression.Convert(paramOptions, linkedEntitiesInterface.GetGenericArguments()[1]);
                    // ReSharper disable once AssignNullToNotNullAttribute
                    callLoad = Expression.Call(newFacade, loadMethod, paramId, conversionExpression);
                }
                else
                {
                    // ReSharper disable once AssignNullToNotNullAttribute
                    callLoad = Expression.Call(newFacade, loadMethod, paramId);
                }

                var lambda = Expression.Lambda>(
                    // ReSharper disable once AssignNullToNotNullAttribute
                    callLoad, paramId, paramOptions);
                //compiles the Expression to a usable delegate.
                return lambda.Compile();
            }