Expression.Call groupBy 然后选择？

Question

我正在尝试使用表达式树来构建一对嵌套的组，并且完全被 Select 及其对参数的期望所困扰。我最终想要做的是通过表达式树来构建它；

var queryNestedGroups = products.GroupBy(x => x.Category)
                .Select(p => new
                {
                    key = p.Key,
                    objects = p.ToList().GroupBy(y => y.Subcategory)
                        .Select(y => new { key = y.Key, objects = y.ToList() })
                })
                .AsQueryable();

这是我目前的尝试（产品是一个列表）；

var data = Expression.Constant(products);
var arg = Expression.Parameter(typeof(Product), "arg");
var nameProperty = Expression.PropertyOrField(arg, "Category");

var groupByLambda = Expression.Lambda<Func<Product, string>>(nameProperty, arg);
var groupByExpression = Expression.Call(
    typeof(Queryable),
    "GroupBy",
    new Type[] { typeof(Product), typeof(string) },
    data,
    groupByLambda);

var parameterExp = Expression.Parameter(typeof(IGrouping<string, Product>), "p");
var keyProp = Expression.PropertyOrField(parameterExp, "Key");
ConstructorInfo constructorInfo = typeof(object)
    .GetConstructor(new[] { typeof(string), typeof(Product) });

Type anonymousResultType = new { Key = "abc", Values = new List<Product>() }.GetType();
var exp = Expression.New(
            anonymousResultType.GetConstructor(new[] { typeof(string), typeof(List<Product>) }),
            Expression.Constant("def"),
            Expression.Constant(new List<Product>()));
var selectLambda = Expression.Lambda(exp);

var selectExpression = Expression.Call(
    typeof(Queryable),
    "Select",
    new Type[] { typeof(List<Product>), selectLambda.Body.Type },
    data,
    selectLambda); 

var finalExpression = Expression.Lambda(groupByExpression);

一切进展顺利，除了我在 var selectExpression = ... 上遇到异常，告诉我我的类型参数和参数错误。不幸的是，它没有告诉我哪些参数以及它们错误的原因。我已经尝试了我能想到的所有排列。所以有两个问题；

我怎么知道是什么

1. 正是它想要的类型？
2. 在这种情况下，正确的类型/参数是什么？

Answer 1

以下是执行您想要的操作的代码。每个选择都需要在表达式树中有自己的 lamda 投影。您还有两种不同的匿名类型，一种是内部匿名类型的 IEnumerable，一种是产品列表。

此外，由于它是对不需要 Queryable 的对象的 linq，因此您可以只使用 Enumerable 和 p.ToList().GroupBy(y => y.Subcategory) 不需要 ToList，所以我没有转换它.

如果你不使用匿名类型并且有具体的类，它会更简单。尤其是最后。由于它不能是强类型的，你只需要编译它，然后 DynamicInvoke 它。

// This could be a parameter
var data = Expression.Constant(products);

var outterGroupByarg = Expression.Parameter(typeof(Product), "x");
var outterGroupNameProperty = Expression.PropertyOrField(outterGroupByarg, "Category");
var outterGroupByLambda = Expression.Lambda<Func<Product, string>>(outterGroupNameProperty, outterGroupByarg);
var outterGroupByExpression = Expression.Call(typeof(Enumerable), "GroupBy", new [] { typeof(Product), typeof(string) },
                                         data, outterGroupByLambda);

var outterSelectParam = Expression.Parameter(typeof (IGrouping<string, Product>), "p");

var innerGroupByarg = Expression.Parameter(typeof(Product), "y");
var innerGroupNameProperty = Expression.PropertyOrField(innerGroupByarg, "Subcategory");
var innerGroupByLambda = Expression.Lambda<Func<Product, string>>(innerGroupNameProperty, innerGroupByarg);

var innerGroupByExpression = Expression.Call(typeof(Enumerable), "GroupBy", new[] { typeof(Product), typeof(string) },
                                         outterSelectParam, innerGroupByLambda);

var innerAnonymousType = new {Key = "abc", objects = new List<Product>()};

var innerSelectProjectionarg = Expression.Parameter(typeof(IGrouping<string, Product>), "y");
var innerKeyProp = Expression.Property(innerSelectProjectionarg, "Key");

var innerToList = Expression.Call(typeof (Enumerable), "ToList", new[] {typeof (Product)},
                                  innerSelectProjectionarg);

var innerAnonymousResultType = innerAnonymousType.GetType();
var innerAnonymousConstructor =
    innerAnonymousResultType.GetConstructor(new[] {typeof (string), typeof (List<Product>)});
var innerAnonymous = Expression.New(innerAnonymousConstructor, innerKeyProp, innerToList);

var innerSelectProjection =
    Expression.Lambda(typeof(Func<,>).MakeGenericType(typeof(IGrouping<string, Product>), innerAnonymousResultType), innerAnonymous,
                      innerSelectProjectionarg);

var innerSelectExpression = Expression.Call(typeof(Enumerable), "Select", new [] { typeof(IGrouping<string, Product>), innerAnonymousResultType },
                            innerGroupByExpression, innerSelectProjection);


var outterAnonymousType = new {Key = "abc", Values = new[] {innerAnonymousType}.AsEnumerable()};
var outterAnonymousResultType = outterAnonymousType.GetType();
var outterAnonymousConstructor =
    outterAnonymousResultType.GetConstructor(new[] { typeof(string), typeof(IEnumerable<>).MakeGenericType(innerAnonymousResultType) });

var outterKeyProp = Expression.PropertyOrField(outterSelectParam, "Key");
var outterAnonymous = Expression.New(outterAnonymousConstructor, outterKeyProp, innerSelectExpression);
var outterSelectProjection =
    Expression.Lambda(
        typeof (Func<,>).MakeGenericType(typeof (IGrouping<string, Product>), outterAnonymousResultType),
        outterAnonymous,
        outterSelectParam);

var outterSelect = Expression.Call(typeof(Enumerable), "Select", new[] { typeof(IGrouping<string, Product>), outterAnonymousResultType },
                                  outterGroupByExpression, outterSelectProjection);

// Lamda is a func with no input because list of products was set as a constant and not a parameter
var finial =
    Expression.Lambda(
        typeof (Func<>).MakeGenericType(typeof (IEnumerable<>).MakeGenericType(outterAnonymousResultType)),
        outterSelect);