扩展IQueryable并解析属性

Question

I'm fairly new to linq and wanted to delve in a bit deeper by extending IQueryable. I want to create a simple extension method that queries a date range. Something that will be the equivalent to:

IQuerable.Where(x => x.Date > fromDate && x.Date < toDate);

but with some extra processing on the dates. I want to be able to parse a property to perform the query so the method call will be something like:

IQueryable.WhereDateRange(x => x.Date, fromDate, toDate);
IQueryable.WhereDateRange(x => x.AnotherDate, fromDate, toDate);

I've been looking into something like below but I'm not entirely sure what I'm doing

public static IQueryable<T> WhereDateRange<T>(this IQueryable<T> source, Func<T, DateTime> getter, DateTime from, DateTime to) {
//query here
}

Is this possible and if so how do I do it?

Answer 1

Untested, but:

public static IQueryable<T> WhereDateRange<T>(
    this IQueryable<T> source, Expression<Func<T, DateTime>> getter,
    DateTime from, DateTime to)
{
    Expression body = getter.Body;

    var predicate = Expression.Lambda<Func<T, bool>>(
        Expression.And(
            Expression.GreaterThan(body, Expression.Constant(from)),
            Expression.LessThan(body, Expression.Constant(to))
        ),
        getter.Parameters);
    return source.Where(predicate);
}

For info, though; I usually assume that a range is >= start and < end , so I'd use Expression.GreaterThanOrEqual .

Making that more reusable (and noting that I'm using GreaterThanOrEqual here):

public static IQueryable<T> WhereInRange<T, TValue>(
    this IQueryable<T> source, Expression<Func<T, TValue>> selector,
    TValue from, TValue to)
{
    Expression body = selector.Body;

    var predicate = Expression.Lambda<Func<T, bool>>(
        Expression.And(
            Expression.GreaterThanOrEqual(
                body, Expression.Constant(from, typeof(TValue))),
            Expression.LessThan(
                body, Expression.Constant(to, typeof(TValue)))
        ),
        selector.Parameters);
    return source.Where(predicate);
}

(need to specify the TValue on the constants in this case, since null will otherwise cause big problems)