Question

（这是最初的标题，表达式树比较DateTime与Nullable DateTime，但正如接受的答案所示，这根本不是问题。）

我正在尝试构建表达式树来比较实体框架查询中的日期。在下面的示例中，＆＃39;结果＆＃39;是已传递给方法的IQueryable，而DateFilter是来自UI的对象，该UI具有可为空的DateTime字段，FirstDate，SecondDate和枚举，DateType和DateMode。

我见过很多类似的问题，常见的线程似乎是使用Expression.Convert来确保日期的类型正确。但是，我仍然做错了，因为当我走到尽头，而不是(System.Nullable'1[System.DateTime]$x.EffectiveDate >= ...我的表达有.Invoke(.Constant<TheClassIAmIn i.e., 'this'+<>c__DisplayClass47_0>('this'+<>c__DisplayClass47_0).resultFunc)( $x, ......

以下是完整代码段：

        var changeInfo = Expression.Parameter(typeof(MyEntity), "x");
        var targetDate = Expression.Property(changeInfo, DateFilter.DateType.ToString());

        var dateFilter = Expression.Parameter(typeof(MyDateFilter), "DateFilter");
        var firstDate = Expression.Property(dateFilter, "FirstDate");
        var secondDate = Expression.Property(dateFilter, "SecondDate");

        //  Note that FirstDate, SecondDate, and ActionDate are nullable,
        //  SubmittedDate and EffectiveDate are not.
        var ge = Expression.GreaterThanOrEqual(Expression.Convert(targetDate, firstDate.Type), firstDate);

        var tree =
            Expression.Lambda<Func<MyEntity, MyDateFilter, bool>>
            (ge, changeInfo, dateFilter);

    var resultFunc = tree.Compile();
            result = result.Where(x => resultFunc(x, MyDateFilter));

Answer 1

此问题与DateTime和Nullable<DateTime>比较没有任何共同之处。

你看到的表达方式是：

Expression<Func<MyEntity, bool>> expr = x => resultFunc(x, MyDateFilter);

是lambda表达式，参数x 调用 resultFunc变量，其中包含Func<MyEntity, MyDateFilter, bool>类型的委托。

你不应该引入MyDateFilter 参数，这会产生不兼容的lambda表达式，不应该编译委托并发出调用表达式。相反，您应该直接构建类型为Expression<Func<MyEntity, bool>>的lambda表达式。 MyDateFilter值将作为Expression.Constant传递：

var changeInfo = Expression.Parameter(typeof(MyEntity), "x");
var targetDate = Expression.Property(changeInfo, DateFilter.DateType.ToString());

// Assuming DateFilter is a field/property/variable of type MyDateFilter    
var dateFilter = Expression.Constant(DateFilter);
var firstDate = Expression.Property(dateFilter, "FirstDate");
var secondDate = Expression.Property(dateFilter, "SecondDate");

var ge = Expression.GreaterThanOrEqual(
    Expression.Convert(targetDate, firstDate.Type),
    firstDate);

var predicate = Expression.Lambda<Func<MyEntity, bool>>(
    ge, changeInfo);

result = result.Where(predicate);

表达式树生成UnWanted Invoke

1 个答案: