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If your LINQ provider supports basic filtering but doesn't support Contains() (looking at you WCF Data Services), here's a workaround

View gist:6893533
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using System;
using System.Linq;
using System.Linq.Expressions;
 
static class Program {
static Expression<Func<T, bool>> In<T, R>(this Expression<Func<T, R>> member, params R[] values) {
var prop = member.Body as MemberExpression;
if (prop == null)
throw new Exception("Expression has to be member");
if (values.Length == 0)
return _ => true;
var body = values.Select(v => Expression.Equal(prop, Expression.Constant(v))).Aggregate(Expression.OrElse);
return Expression.Lambda<Func<T, bool>>(body, member.Parameters[0]);
}
 
// Helps with type inference
static Expression<Func<A, B>> E<A, B>(Expression<Func<A, B>> exp) {
return exp;
}
 
class SomeModel {
public string Id { get; set; }
}
 
static void Main(string[] args) {
var values = new[] { "value1", "value2" };
var exp = E((SomeModel x) => x.Id).In(values);
Console.WriteLine(exp); // x => ((x.Id == "value1") OrElse x.Id == "value2"))
}
}
Owner

Equivalent F# : http://fssnip.net/ks

Nice, but one suggestion:

if (values.Length == 0)
return _ => true;

should be changed to
if (values.Length == 0)
        return _ => false;

I think it makes logical sense for the expressions to return false since the value can never be contained in an empty collection.

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