c# - EF Core 的 ConstantExpression 中的正确集合

Question

我尝试实现自己的表达式序列化器/反序列化器，以便通过服务传递它(我想实现我自己的 EF Core 服务端点)。所以，现在我对 LambdaExpressions 中的集合有问题。例如，

var dataQuery = testDb.Users.Include(e => e.EmployeeInfo).Include(f => f.Notifications).Where(s => tstList.Contains(s.Id)).Select(e => e.FullName);
var tstEspressionBase = dataQuery.Expression;
var tstEspression = new ReflectionLocalValculationVisitor().Visit(tstEspressionBase);

这里

public class ReflectionLocalValculationVisitor : ExpressionVisitor
{
    protected override Expression VisitMember(MemberExpression memberExpression)
    {
        var expression = Visit(memberExpression.Expression);

        if (expression is ConstantExpression)
        {
            object container = ((ConstantExpression)expression).Value;
            var member = memberExpression.Member;
            if (member is FieldInfo)
            {
                object value = ((FieldInfo)member).GetValue(container);
                return Expression.Constant(value);
            }
            if (member is PropertyInfo)
            {
                object value = ((PropertyInfo)member).GetValue(container, null);
                return Expression.Constant(value);
            }
        }
        return base.VisitMember(memberExpression);
    }
}

var tstList = new List<Guid>()
{
   new Guid("D45E1A1A-F546-48DB-77BA-08D7775C6A93"),
   new Guid("5B21C782-9B95-48F2-77BD-08D7775C6A93")
};

使用此代码执行

var providerAsync = testDb.GetService<IAsyncQueryProvider>();
var toListAsyncMethodInfo = typeof(EntityFrameworkQueryableExtensions).GetMethod(nameof(EntityFrameworkQueryableExtensions.ToListAsync)).MakeGenericMethod(typeof(string));
var s3 = await toListAsyncMethodInfo.InvokeAsync(null, new object[] { providerAsync.CreateQuery(tstEspression), default(CancellationToken) }).ConfigureAwait(false);

给了我正确的结果。

因此，在使用 Newtonsoft Json 进行序列化/反序列化后，我在 Lambda 中的集合出现问题。来自Where方法:

Error Message: The LINQ expression 'DbSet .Where(u => List { d45e1a1a-f546-48db-77ba-08d7775c6a93, 5b21c782-9b95-48f2-77bd-08d7775c6a93, }.Contains(s.Id))' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync(). See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

我试图实现这个“建议”，但没有效果(见下面的代码):

var asEnumerableMethod = typeof(Enumerable).GetMethod(nameof(Enumerable.AsEnumerable)).MakeGenericMethod(GenericTypes.Select(e => e.FromNode()).ToArray());
var asEnumerabled = asEnumerableMethod.Invoke(null, new object[] { Value });

这里Value对象是 List<Guid>由 JSON.NET 反序列化后生成。因此，我比较了 Value 的实现接口(interface)在ConstantExpression代表List<guid>序列化之前和反序列化之后 - 都实现 8 个接口(interface)。

所以，也许有人遇到了同样的问题。

谢谢。

附注我不知道为什么 EF Core 给我 Where(u => ...而不是Where(s => ... ，因为在这个表达式的 DebugView 模式下我看到正确的 Where(s => ...代表。

让我们看看序列化/(反序列化和恢复)表达式(来自 DebugView 的数据):

.Call System.Linq.Queryable.Select(
    .Call System.Linq.Queryable.Where(
        .Call Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.Include(
            .Call Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.Include(
                .Constant<Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[EFCoreDataModel.DataClasses.Users.Base.User]>(Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[EFCoreDataModel.DataClasses.Users.Base.User]),
                '(.Lambda #Lambda1<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,EFCoreDataModel.DataClasses.Users.Employ.EmployeeInfo]>))
            ,
            '(.Lambda #Lambda2<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Collections.Generic.ICollection`1[EFCoreDataModel.DataClasses.Notifications.Notification]]>))
        ,
        '(.Lambda #Lambda3<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Boolean]>)),
    '(.Lambda #Lambda4<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.String]>))

.Lambda #Lambda1<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,EFCoreDataModel.DataClasses.Users.Employ.EmployeeInfo]>(EFCoreDataModel.DataClasses.Users.Base.User $e)
{
    $e.EmployeeInfo
}

.Lambda #Lambda2<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Collections.Generic.ICollection`1[EFCoreDataModel.DataClasses.Notifications.Notification]]>(EFCoreDataModel.DataClasses.Users.Base.User $f)
{
    $f.Notifications
}

.Lambda #Lambda3<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Boolean]>(EFCoreDataModel.DataClasses.Users.Base.User $s)
{
    .Call .Constant<System.Collections.Generic.List`1[System.Guid]>(System.Collections.Generic.List`1[System.Guid]).Contains($s.Id)
}

.Lambda #Lambda4<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.String]>(EFCoreDataModel.DataClasses.Users.Base.User $e)
{
    $e.FullName
}

原始表达式(来自 DebugView):

.Call System.Linq.Queryable.Select(
    .Call System.Linq.Queryable.Where(
        .Call Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.Include(
            .Call Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.Include(
                .Constant<Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[EFCoreDataModel.DataClasses.Users.Base.User]>(Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1[EFCoreDataModel.DataClasses.Users.Base.User]),
                '(.Lambda #Lambda1<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,EFCoreDataModel.DataClasses.Users.Employ.EmployeeInfo]>))
            ,
            '(.Lambda #Lambda2<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Collections.Generic.ICollection`1[EFCoreDataModel.DataClasses.Notifications.Notification]]>))
        ,
        '(.Lambda #Lambda3<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Boolean]>)),
    '(.Lambda #Lambda4<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.String]>))

.Lambda #Lambda1<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,EFCoreDataModel.DataClasses.Users.Employ.EmployeeInfo]>(EFCoreDataModel.DataClasses.Users.Base.User $e)
{
    $e.EmployeeInfo
}

.Lambda #Lambda2<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Collections.Generic.ICollection`1[EFCoreDataModel.DataClasses.Notifications.Notification]]>(EFCoreDataModel.DataClasses.Users.Base.User $f)
{
    $f.Notifications
}

.Lambda #Lambda3<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.Boolean]>(EFCoreDataModel.DataClasses.Users.Base.User $s)
{
    .Call .Constant<System.Collections.Generic.List`1[System.Guid]>(System.Collections.Generic.List`1[System.Guid]).Contains($s.Id)
}

.Lambda #Lambda4<System.Func`2[EFCoreDataModel.DataClasses.Users.Base.User,System.String]>(EFCoreDataModel.DataClasses.Users.Base.User $e)
{
    $e.FullName
}

所以，它们是平等的。并且序列化/反序列化没有u Lambda 中的参数，可能只是“s”。

Answer 1

是的，当您使用自己的自定义集合作为 where 表达式中的常量时，EF Core 无法正常运行。它们必须在表达式中完全可计算，即使这样，EF Core 也很难正确翻译它们。也许您想简化您的 list ？我的意思是，您为每个用户创建一个表达式，将每个项目与其进行比较。

我已经实现了一个用于集合的小型谓词构建器，它可能会对您有帮助吗？它负责所有 ParameterExpression 映射。

var whereExpression = CollectionConstantPredicateBuilder<UserEntity>
    // Here begins the scope of ALL users.
    .CreateFromCollection(tstList)
    // Each expression of one user is combined with OrElse (||)
    .DefinePredicatePerItem(consecutiveItemBinaryExpressionFactory: Expression.OrElse,
        // Pre-Conditions, you may check for null or empty list or you simply configures
        // comparisonValuesBehaviourFlags. Do NOT use method call on 'yourTstList'.
        sourceAndItemPredicate: (user, yourTstList) => true)
    // Here begins the scope of ALL items of 'yourTstList' in ONE user.
    // The resulted expression of the hole collection that belongs to one user
    // is combined with the previous expression from ONE user by AndAlso (&&).
    .ThenCreateFromCollection(parentBinaryExpressionFactory: Expression.AndAlso,
        comparisonValuesFactory: yourTstList => yourTstList,
        // If the collection of the comparison values is null or empty,
        // the boolean expression branch for each each item won't be
        // created. Instead an expression is used that leads to false.
        // Ergo, if 'yourTstList' does not contain the user id, the user
        // won't be queried.
        comparisonValuesBehaviourFlags: ComparisonValuesBehaviourFlags.NullOrEmptyLeadsToFalse)
    // Each expression of one item is combined with OrElse (||).
    // So one user's ID can be the one or the other 'oneTstItem'.
    .DefinePredicatePerItem(Expression.OrElse,
        sourceAndItemPredicate: (user, oneTstItem) => user.Id == oneTstItem)
    .BuildLambdaExpression();

dbContext.Users.AsQueryable().Where(whereExpression).

你可以在我的 pre-release package (0.1.7-alpha.68) 中找到 CollectionConstantPredicateBuilder上NuGet .