Question

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

// 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，我通过将表达式中的TransferRequest参数替换为提供的Expression<Func<ApplicationUser, TransferRequest, bool>>并将其转换为Expression<Func<ApplicationUser, bool>>。这在我的单元测试中效果很好，但是EFCore似乎不喜欢它，并且在实际数据库上运行时出现此错误：

System.InvalidOperationException：将子表达式从类型System.Nullable<System.DateTime>重写为类型System.Collections.Generic.IEnumerable<System.Nullable<System.DateTime>> 不允许，因为它会改变操作的含义。如果这是故意的，请覆盖“ VisitUnary”并将其更改为允许这个改写。在 System.Linq.Expressions.ExpressionVisitor.ValidateChildType（Type 之前，之后输入，字符串methodName）在 System.Linq.Expressions.ExpressionVisitor.ValidateUnary（UnaryExpression 之前，UnaryExpression之后）在 System.Linq.Expressions.ExpressionVisitor.VisitUnary（UnaryExpression 节点） System.Linq.Expressions.UnaryExpression.Accept（ExpressionVisitor visitor）at System.Linq.Expressions.ExpressionVisitor.Visit（Expression 节点）...

基本上，如果在上次“等待批准”事件之后发生任何“转发”事件，即当有新的“等待批准”事件之前的“转发”事件发生，则认为该人已“转发”了转移请求'事件不再计数（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“等待批准”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，负责替换参数：

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。我得到

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

此外，以下代码也可以正常工作

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

使用EFCore时尝试在Where（）子句中使用表达式树时出错

编辑

0 个答案: