C#, Linq2SQL: Creating a predicate to find elements within a number of ranges
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23-08-2019 - |
Question
Lets say I have something called Stuff in my database, with a property called Id. From the user I get a sequence of selected Range objects (or rather I create them from their input) with the Ids they want. A stripped down version of that struct looks like this:
public struct Range<T> : IEquatable<Range<T>>, IEqualityComparer<Range<T>>
{
public T A;
public T B;
public Range(T a, T b)
{
A = a;
B = b;
}
...
}
So one could for example have gotten:
var selectedRange = new List<Range<int>>
{
new Range(1, 4),
new Range(7,11),
};
I then want to use that to create a predicate to select only things which have a value between those. For example, using the PredicateBuilder, I can for example do that this way:
var predicate = PredicateBuilder.False<Stuff>();
foreach (Range<int> r in selectedRange)
{
int a = r.A;
int b = r.B;
predicate = predicate.Or(ø => ø.Id >= a && ø.Id <= b);
}
and then:
var stuff = datacontext.Stuffs.Where(predicate).ToList();
Which works! What I would like to do now, is to create a generic extension method to create those predicates for me. Kind of like this:
public static Expression<Func<T,bool>> ToPredicate<T>(this IEnumerable<Range<int>> range, Func<T, int> selector)
{
Expression<Func<T, bool>> p = PredicateBuilder.False<T>();
foreach (Range<int> r in range)
{
int a = r.A;
int b = r.B;
p = p.Or(ø => selector(ø) >= a && selector(ø) <= b);
}
return p;
}
Problem here, is that it crashes with a NotSupportedException because of the selector(ø) call: Method 'System.Object DynamicInvoke(System.Object[])' has no supported translation to SQL.
I guess that is understandable. But is there any way to get around this? What I would like to end up with is so that I could just do:
var stuff = datacontext.Stuffs.Where(selectedRange.ToPredicate<Stuff>(ø => ø.Id));
Or even better, create something that returns an IQueryable so that I could just do:
var stuff = datacontext.Stuffs.WhereWithin<Stuff>(selectedRange, ø => ø.Id); // Possibly without having to specify Stuff as type there...
So, any ideas? I would really like to get this working, cause if not I will get A LOT of those foreach blocks of code, creating predicates...
Note 1: Of course, would be nice if I could expand to more than int, like DateTime and such, but not sure how that ends up with using the >= and <= operators... Does CompareTo work with linq-to-sql? If not there is no problem creating two. One for int and one for DateTime, since that is mostly the types this will be used for.
Note 2: It is going to be used for reporting, where the user is going to be able to narrow down what comes out, based on different things. Like, I want this report for those people and those dates.
Solution
The use with generics is problematic, since C# doesn't support operators on generics - meaning you'd have to write the expression manually. And as we've already seen, string works differently. But for the rest, how about something like (untested):
(edited for multiple ranges)
public static IQueryable<TSource> WhereBetween<TSource, TValue>(
this IQueryable<TSource> source,
Expression<Func<TSource, TValue>> selector,
params Range<TValue>[] ranges)
{
return WhereBetween<TSource,TValue>(source, selector,
(IEnumerable<Range<TValue>>) ranges);
}
public static IQueryable<TSource> WhereBetween<TSource, TValue>(
this IQueryable<TSource> source,
Expression<Func<TSource, TValue>> selector,
IEnumerable<Range<TValue>> ranges)
{
var param = Expression.Parameter(typeof(TSource), "x");
var member = Expression.Invoke(selector, param);
Expression body = null;
foreach(var range in ranges)
{
var filter = Expression.AndAlso(
Expression.GreaterThanOrEqual(member,
Expression.Constant(range.A, typeof(TValue))),
Expression.LessThanOrEqual(member,
Expression.Constant(range.B, typeof(TValue))));
body = body == null ? filter : Expression.OrElse(body, filter);
}
return body == null ? source : source.Where(
Expression.Lambda<Func<TSource, bool>>(body, param));
}
Note; the use of Expression.Invoke means it will probably work on LINQ-to-SQL but not EF (at the moment; hopefully fixed in 4.0).
With usage (tested on Northwind):
Range<decimal?> range1 = new Range<decimal?>(0,10),
range2 = new Range<decimal?>(15,20);
var qry = ctx.Orders.WhereBetween(order => order.Freight, range1, range2);
Generating TSQL (re-formatted):
SELECT -- (SNIP)
FROM [dbo].[Orders] AS [t0]
WHERE (([t0].[Freight] >= @p0) AND ([t0].[Freight] <= @p1))
OR (([t0].[Freight] >= @p2) AND ([t0].[Freight] <= @p3))
Just what we wanted ;-p
OTHER TIPS
You are getting that error, because everything for LINQ to SQL needs to be in the form of an Expression. Try this
public static Expression<Func<T,bool>> ToPredicate<T>(
this IEnumerable<Range<int>> range,
Expression<Func<T, int>> selector
) {
Expression<Func<T, bool>> p = PredicateBuilder.False<T>();
Func<T, int> selectorFunc = selector.Compile();
foreach (Range<int> r in range)
{
int a = r.A;
int b = r.B;
p = p.Or(ø => selectorFunc(ø) >= a && selectorFunc(ø) <= b);
}
return p;
}
Notice that I compile the selector before using it. This should work with out a hitch, I have used something like it in the past.