LINQ表达式树中的Any（）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Expression Trees（这似乎是我需要使用反射和运行时类型时唯一可以做到的方法）。 请注意，在真实代码中（见下文），AdAccountAlert实际上是通过反射和for循环动态变化的。
我的问题 ：我可以生成所有的.Where（）子句。 由于不兼容的类型，whereExpression方法调用被炸毁。 通常我知道该放什么，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，但没有运气。 任何与.Where（）和.ToDictionary（）的帮助将不胜感激。
这是我迄今为止的：
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);
请注意： ToList()之后的所有内容在IQueryable<T>上不起作用，但在IEnumerable<T>上不起作用。 因此，不需要创建表达式树。 这当然不是EF或类似的解释。
如果您要查看编译器为原始查询生成的代码，您会发现它只会在第一次调用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);
请注意它是如何为ToList生成表达式的。 包括它在内的所有内容都只是对扩展方法的普通调用。
如果你没有在你的代码中模仿这个，你实际上会发送一个到Enumerable.ToList的调用给LINQ提供者 - 然后它会尝试转换为SQL并失败。
看起来，在构建whereLambda ，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。