c# - 使用 EFCore 时尝试在 Where() 子句中使用表达式树时出错

Question

我在尝试Expression<Func<ApplicationUser, bool>>通过组合其他表达式来组合时遇到错误。目的是在 EFCore 查询的 where() 子句中使用它。例子：

// Only get users that have been forwarded the request...
var request = GetSomeRequestToFilterOn();
var filter = HasRequestBeenForwaredToUserExpression().ReplaceParameter(request);
var results = _context.ApplicationUsers.Where(filter).ToList();

在上面的示例中，我将表达式中的 TransferRequest 参数转换为提供的参数Expression<Func<ApplicationUser, TransferRequest, bool>>。Expression<Func<ApplicationUser, bool>>这在我的单元测试中效果很好，但 EFCore 似乎不喜欢它，并且在针对实际数据库运行时出现此错误：

System.InvalidOperationException：不允许将子表达式从一个类型重写System.Nullable<System.DateTime>为另一个类型，因为它会改变操作的含义。System.Collections.Generic.IEnumerable<System.Nullable<System.DateTime>>如果这是故意的，请覆盖“VisitUnary”并将其更改为允许此重写。在 System.Linq.Expressions.ExpressionVisitor.ValidateChildType(Type before, Type after, String methodName) 在 System.Linq.Expressions.ExpressionVisitor.ValidateUnary(UnaryExpression before, UnaryExpression after) 在 System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression 节点) 在 System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor) 在 System.Linq.Expressions.ExpressionVisitor.Visit(Expression node) 在...

基本上，如果在最后一个“等待批准”事件之后有任何“转发”事件，即当有一个新的“等待批准”事件时，之前的“转发”事件不存在，则认为一个人已被“转发”转移请求'不再计数（但仍然存在）。

这是我的过滤器。

internal static Expression<Func<ApplicationUser, TransferRequest, bool>> HasRequestBeenForwaredToUserExpression()
{
    //only "Forwarded" events after the last "Awaiting Approval" event count.
    //so we need to get the last "Awaiting Approval" event from the request.
    var projection = LastAwaitingApprovalEvent();

    var userParam = Expression.Parameter(typeof(ApplicationUser));
    var requestParam = Expression.Parameter(typeof(TransferRequest));
    var requestToEvent = projection.Body.ReplaceParameter(projection.Parameters[0], requestParam);

    Expression<Func<ApplicationUser, TransferRequest, TransferRequestEvent, bool>> condition =
       (user, rqst, evt) => evt != null && rqst.TransferRequestEvents
            .Any(e2 =>
                Equals(e2.EventType, EventType.Forwarded) &&
                Equals(e2.User, user) &&
                e2.EventDateTime > evt.EventDateTime);

    var body = condition.Body
            .ReplaceParameter(condition.Parameters[0], userParam)
            .ReplaceParameter(condition.Parameters[1], requestParam)
            .ReplaceParameter(condition.Parameters[2], requestToEvent);
    return Expression.Lambda<Func<ApplicationUser, TransferRequest, bool>>(body, userParam, requestParam);
}

此方法获取最后一个“Awaiting Approval”事件

internal static Expression<Func<TransferRequest, TransferRequestEvent>> LastAwaitingApprovalEvent()
{
    return t => t.TransferRequestEvents
        .OrderBy(e => e.EventDateTime).ThenBy(e => e.Id)
        .LastOrDefault(e => Equals(e.EventType, EventType.AwaitingReview));
}

我有一个 ExpressionUtility 类来完成替换参数的工作：

public class ExpressionUtility
{
    public static Expression ReplaceParameter(this Expression expression,
        ParameterExpression toReplace,
        Expression newExpression)
    {
        return new ParameterReplaceVisitor(toReplace, newExpression)
            .Visit(expression);
    }

    public static Expression<Func<TArg2, TReturn>> ReplaceParameter<TArg1, TArg2, TReturn>(this Expression<Func<TArg1, TArg2, TReturn>> source, TArg1 arg1)
    {
        var t1Param = Expression.Constant(arg1);
        var t2Param = Expression.Parameter(typeof(TArg2));
        var body = source.Body
                    .ReplaceParameter(source.Parameters[0], t1Param)
                    .ReplaceParameter(source.Parameters[1], t2Param);
        return Expression.Lambda<Func<TArg2, TReturn>>(body, t2Param);
    }

    public static Expression<Func<TArg1, TReturn>> ReplaceParameter<TArg1, TArg2, TReturn>(this Expression<Func<TArg1, TArg2, TReturn>> source, TArg2 arg2)
    {
        var t1Param = Expression.Parameter(typeof(TArg1));
        var t2Param = Expression.Constant(arg2);
        var body = source.Body
                    .ReplaceParameter(source.Parameters[0], t1Param)
                    .ReplaceParameter(source.Parameters[1], t2Param);
        return Expression.Lambda<Func<TArg1, TReturn>>(body, t1Param);
    }
}

public class ParameterReplaceVisitor : ExpressionVisitor
{
    private ParameterExpression from;
    private Expression to;
    public ParameterReplaceVisitor(ParameterExpression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    protected override Expression VisitParameter(ParameterExpression node)
    {
        return node == from ? to : base.VisitParameter(node);
    }
}

编辑

当我将最终表达式的 ToString 打印到控制台时。我明白了

{Param_0 => ((值(Debugging.Models.TransferRequest).TransferRequestEvents.OrderBy(e => e.EventDateTime).ThenBy(e => e.Id).LastOrDefault(e => Equals(e.EventType, EventType. AwaitingReview)) != null) AndAlso value(Debugging.Models.TransferRequest).TransferRequestEvents.Any(e2 => ((Equals(e2.EventType, EventType.Forwarded) AndAlso Equals(e2.User, Param_0)) AndAlso (e2. EventDateTime > value(Debugging.Models.TransferRequest).TransferRequestEvents.OrderBy(e => e.EventDateTime).ThenBy(e => e.Id).LastOrDefault(e => Equals(e.EventType, EventType.AwaitingReview))。事件日期时间））））}

此外，下面的代码工作得很好

Expression<Func<string, ApplicationUser, bool>> filter = (s, u) => u.FirstName.Contains(s);
var whereClause= filter.ReplaceParameter("mark");
_context.AppicationUsers.Where(whereClause).ToList();

标签： c#entity-frameworklinqexpression