c# – LINQ Expression Tree Any()里面的Where()

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()的帮助都将受到赞赏.

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,subscribersTupleParameter);

  //Generates:
  //  .GroupJoin(dataContext.AlertSubscriptions,//    a => new { a.AdAccountId,typeof(AdAccount).Name },//    s => new { s.EntityId,s.EntityType },//    (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);

解决方法

请注意：Tolist()之后的所有内容都不适用于IQueryable< T>但是在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.

例：

以下代码：

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

由编译器翻译为：

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之前的所有内容生成表达式.包含它之后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,你实际上会将一个Enumerable.ToList调用发送到LINQ提供程序 – 然后它会尝试转换为sql并失败.

原文地址：https://www.jb51.cc/csharp/99198.html