Question

我需要使用表达式语法来实现查询（因为我不知道编译时的类型）。例如查询如下：

from customer in Customers
join purchase in Purchases
    on customer.ID equals purchase.CustomerID
into outerJoin
from range in outerJoin.DefaultIfEmpty()
where 
    customer.Name == "SomeName" && 
    range.Description.Contains("SomeString") &&
    customer.ID == range.CustomerID
select 
    new { Customer = customer, Purchase = range }

我找到了实现这样的群组加入部分的方法：

ITable p = _dataContext.GetTable(typeof(Purchases));
ITable c = _dataContext.GetTable(typeof(Customers));

LambdaExpression outerSelectorLambda    = DynamicExpression.ParseLambda(typeof(Customers), null, "ID");
LambdaExpression innerSelectorLambda    = DynamicExpression.ParseLambda(typeof(Purchases), null, "CustomerID");

ParameterExpression param1 = Expression.Parameter(typeof(Customers), "customer");
ParameterExpression param2 = Expression.Parameter(typeof(IEnumerable<Purchases>), "purchases");

ParameterExpression[] parameters = new ParameterExpression[] { param1, param2 };

LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, "New(customers as customers, purchases as purchases)");

MethodCallExpression joinCall = 
 Expression.Call(
   typeof(Queryable),
   "GroupJoin",
   new Type[] { 
      typeof(Customers), 
      typeof(Purchases), 
      outerSelectorLambda.Body.Type, 
      resultsSelectorLambda.Body.Type 
   },
   c.Expression,
   p.Expression,
   Expression.Quote(outerSelectorLambda),
   Expression.Quote(innerSelectorLambda),
   Expression.Quote(resultsSelectorLambda)
);

但我无法弄清楚如何使用这种语法编写其余的查询。有人可以帮助我吗？

Answer 1

我刚刚在dynamic.cs中复制+粘贴了“join”实现，并进行了一些更改以使“GroupJoin”工作。

public static IQueryable GroupJoin(this IQueryable outer, IEnumerable inner, string outerSelector, string innerSelector, string resultsSelector, params object[] values)
{
    if (inner == null) throw new ArgumentNullException("inner");
    if (outerSelector == null) throw new ArgumentNullException("outerSelector");
    if (innerSelector == null) throw new ArgumentNullException("innerSelector");
    if (resultsSelector == null) throw new ArgumentNullException("resultsSelctor");

    LambdaExpression outerSelectorLambda = DynamicExpression.ParseLambda(outer.ElementType, null, outerSelector, values);
    LambdaExpression innerSelectorLambda = DynamicExpression.ParseLambda(inner.AsQueryable().ElementType, null, innerSelector, values);

    Type resultType = typeof(IEnumerable<>).MakeGenericType(inner.AsQueryable().ElementType);

    ParameterExpression[] parameters = new ParameterExpression[]
    {
        Expression.Parameter(outer.ElementType, "outer"), Expression.Parameter(resultType, "inner")
    };
    LambdaExpression resultsSelectorLambda = DynamicExpression.ParseLambda(parameters, null, resultsSelector, values);

    return outer.Provider.CreateQuery(
               Expression.Call(
                   typeof(Queryable), "GroupJoin",
                   new Type[] { outer.ElementType, inner.AsQueryable().ElementType, outerSelectorLambda.Body.Type, resultsSelectorLambda.Body.Type },
                   outer.Expression, inner.AsQueryable().Expression, Expression.Quote(outerSelectorLambda), Expression.Quote(innerSelectorLambda), Expression.Quote(resultsSelectorLambda)));
}

Answer 2

我会采用一种方法来实现这一目标：

获取与LINQ查询等效的表达式。
获取从LINQ查询中提取的Expression的ToString（）。
研究表达式，了解输入参数，类型参数，表达式参数等...

如果提到的方法不明确，请回复我。

如何实现外连接表达式树？

2 个答案: