Question

我正在尝试生成以下LINQ查询：

//Query the database for all AdAccountAlerts that haven't had notifications sent out
//Then get the entity (AdAccount) the alert pertains to, and find all accounts that
//are subscribing to alerts on that entity.
var x = dataContext.Alerts.Where(a => a.NotificationsSent == null)
  .OfType<AdAccountAlert>()
  .ToList()
  .GroupJoin(dataContext.AlertSubscriptions,
    a => new Tuple<int, string>(a.AdAccountId, typeof(AdAccount).Name),
    s => new Tuple<int, string>(s.EntityId, s.EntityType),
    (Alert, Subscribers) => new Tuple<AdAccountAlert, IEnumerable<AlertSubscription>> (Alert, Subscribers))
  .Where(s => s.Item2.Any())
  .ToDictionary(kvp => (Alert)kvp.Item1, kvp => kvp.Item2.Select(s => s.Username));

使用表达式树（当我需要使用反射和运行时类型时，这似乎是我能够做到这一点的唯一方法）。请注意，在实际代码中（见下文），AdAccountAlert实际上是通过反射和for循环动态的。

我的问题：我可以生成.Where（）子句之前的所有内容。由于类型不兼容，whereExpression方法调用会爆炸。通常我知道要放在那里，但Any（）方法调用让我感到困惑。我尝试了所有我能想到的类型而且没有运气。任何有关.Where（）和.ToDictionary（）的帮助都将不胜感激。

这是我到目前为止所拥有的：

var alertTypes = AppDomain.CurrentDomain.GetAssemblies()
  .Single(a => a.FullName.StartsWith("Alerts.Entities"))
  .GetTypes()
  .Where(t => typeof(Alert).IsAssignableFrom(t) && !t.IsAbstract && !t.IsInterface);

var alertSubscribers = new Dictionary<Alert, IEnumerable<string>>();

//Using tuples for joins to keep everything strongly-typed
var subscribableType = typeof(Tuple<int, string>);
var doubleTuple = Type.GetType("System.Tuple`2, mscorlib", true);

foreach (var alertType in alertTypes)
{
  Type foreignKeyType = GetForeignKeyType(alertType);
  if (foreignKeyType == null)
    continue;

  IQueryable<Alert> unnotifiedAlerts = dataContext.Alerts.Where(a => a.NotificationsSent == null);

  //Generates: .OfType<alertType>()
  MethodCallExpression alertsOfType = Expression.Call(typeof(Enumerable).GetMethod("OfType").MakeGenericMethod(alertType), unnotifiedAlerts.Expression);

  //Generates: .ToList(), which is required for joins on Tuples
  MethodCallExpression unnotifiedAlertsList = Expression.Call(typeof(Enumerable).GetMethod("ToList").MakeGenericMethod(alertType), alertsOfType);

  //Generates: a => new { a.{EntityId}, EntityType = typeof(AdAccount).Name }
  ParameterExpression alertParameter = Expression.Parameter(alertType, "a");
  MemberExpression adAccountId = Expression.Property(alertParameter, alertType.GetProperty(alertType.GetForeignKeyId()));
  NewExpression outerJoinObject = Expression.New(subscribableType.GetConstructor(new Type[] { typeof(int), typeof(string)}), adAccountId, Expression.Constant(foreignKeyType.Name));
  LambdaExpression outerSelector = Expression.Lambda(outerJoinObject, alertParameter);

  //Generates: s => new { s.EntityId, s.EntityType }
  Type alertSubscriptionType = typeof(AlertSubscription);
  ParameterExpression subscriptionParameter = Expression.Parameter(alertSubscriptionType, "s");
  MemberExpression entityId = Expression.Property(subscriptionParameter, alertSubscriptionType.GetProperty("EntityId"));
  MemberExpression entityType = Expression.Property(subscriptionParameter, alertSubscriptionType.GetProperty("EntityType"));
  NewExpression innerJoinObject = Expression.New(subscribableType.GetConstructor(new Type[] { typeof(int), typeof(string) }), entityId, entityType);
  LambdaExpression innerSelector = Expression.Lambda(innerJoinObject, subscriptionParameter);

  //Generates: (Alert, Subscribers) => new Tuple<Alert, IEnumerable<AlertSubscription>>(Alert, Subscribers)
  var joinResultType = doubleTuple.MakeGenericType(new Type[] { alertType, typeof(IEnumerable<AlertSubscription>) });
  ParameterExpression alertTupleParameter = Expression.Parameter(alertType, "Alert");
  ParameterExpression subscribersTupleParameter = Expression.Parameter(typeof(IEnumerable<AlertSubscription>), "Subscribers");
  NewExpression joinResultObject = Expression.New(
    joinResultType.GetConstructor(new Type[] { alertType, typeof(IEnumerable<AlertSubscription>) }),
    alertTupleParameter,
    subscribersTupleParameter);

  LambdaExpression resultsSelector = Expression.Lambda(joinResultObject, alertTupleParameter, subscribersTupleParameter);

  //Generates:
  //  .GroupJoin(dataContext.AlertSubscriptions,
  //    a => new { a.AdAccountId, typeof(AdAccount).Name },
  //    s => new { s.EntityId, s.EntityType },
  //    (Alert, Subscribers) => new Tuple<Alert, IEnumerable<AlertSubscription>>(Alert, Subscribers))
  IQueryable<AlertSubscription> alertSubscriptions = dataContext.AlertSubscriptions.AsQueryable();
  MethodCallExpression joinExpression = Expression.Call(typeof(Enumerable),
    "GroupJoin",
    new Type[]
    {
      alertType,
      alertSubscriptions.ElementType,
      outerSelector.Body.Type,
      resultsSelector.ReturnType
    },
    unnotifiedAlertsList,
    alertSubscriptions.Expression,
    outerSelector,
    innerSelector,
    resultsSelector);

  //Generates: .Where(s => s.Item2.Any())
  ParameterExpression subscribersParameter = Expression.Parameter(resultsSelector.ReturnType, "s");
  MemberExpression tupleSubscribers = Expression.Property(subscribersParameter, resultsSelector.ReturnType.GetProperty("Item2"));
  MethodCallExpression hasSubscribers = Expression.Call(typeof(Enumerable),
    "Any",
    new Type[] { alertSubscriptions.ElementType },
    tupleSubscribers);
  LambdaExpression whereLambda = Expression.Lambda(hasSubscribers, subscriptionParameter);
  MethodCallExpression whereExpression = Expression.Call(typeof(Enumerable),
    "Where",
    new Type[] { joinResultType },
    joinExpression,
    whereLambda);

Answer 1

请注意：ToList()之后的所有内容都不会在IQueryable<T>上，而是在IEnumerable<T>上。因此，不需要创建表达式树。它当然不是EF或类似的解释。

如果您查看编译器为原始查询生成的代码，您会看到它仅在第一次调用ToList之前生成表达式树。

示例：

以下代码：

var query = new List<int>().AsQueryable();
query.Where(x => x > 0).ToList().FirstOrDefault(x => x > 10);

由编译器翻译成：

IQueryable<int> query = new List<int>().AsQueryable<int>();
IQueryable<int> arg_4D_0 = query;
ParameterExpression parameterExpression = Expression.Parameter(typeof(int), "x");
arg_4D_0.Where(Expression.Lambda<Func<int, bool>>(Expression.GreaterThan(parameterExpression, Expression.Constant(0, typeof(int))), new ParameterExpression[]
{
    parameterExpression
})).ToList<int>().FirstOrDefault((int x) => x > 10);

请注意它如何为ToList以外的所有内容生成表达式。包括它在内的所有内容都只是对扩展方法的正常调用。

如果你没有在你的代码中模仿这个，你实际上会将Enumerable.ToList的调用发送到LINQ提供程序 - 然后它会尝试转换为SQL并失败。

Answer 2

看起来，在构建whereLambda时，您的第二个参数应该是subscribersParameter而不是subscriptionParameter。至少，这将是你的例外的原因。

LINQ表达式树Where（）里面的Where（）

2 个答案: