A universal PredicateBuilder

Posted by: Pete Montgomery on: February 10, 2011

• In: Uncategorized
• 68 Comments

I don’t use the famous Albahari PredicateBuilder because it doesn’t work so well with Entity Framework or the latest version of NHibernate.

This is because it relies on InvocationExpressions, which aren’t supported by many query translators, and the workaround requires an unnatural call to Tomas‘ AsExpandable method in all your queries.

A couple of years ago, Colin Meek on the EF team showed me how to compose predicates without using InvocationExpressions and since then I’ve used my own synthesized version of PredicateBuilder, with the same API as the awesome Albahari original.

The same API also allows a lovely point-free style of composition. Say you have a list of predicates you want to “or” together, use the standard Aggregate function like so:

var predicate = predicates.Aggregate(PredicateBuilder.Or);

If you need a PredicateBuilder that works as you’d expect with Entity Framework, NHibernate… as well as Linq to SQL, here it is. (It ought to work in Silverlight too. Let me know…!)

    /// <summary>
    /// Enables the efficient, dynamic composition of query predicates.
    /// </summary>
    public static class PredicateBuilder
    {
        /// <summary>
        /// Creates a predicate that evaluates to true.
        /// </summary>
        public static Expression<Func<T, bool>> True<T>() { return param => true; }

        /// <summary>
        /// Creates a predicate that evaluates to false.
        /// </summary>
        public static Expression<Func<T, bool>> False<T>() { return param => false; }

        /// <summary>
        /// Creates a predicate expression from the specified lambda expression.
        /// </summary>
        public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }

        /// <summary>
        /// Combines the first predicate with the second using the logical "and".
        /// </summary>
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.AndAlso);
        }

        /// <summary>
        /// Combines the first predicate with the second using the logical "or".
        /// </summary>
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.OrElse);
        }

        /// <summary>
        /// Negates the predicate.
        /// </summary>
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
        {
            var negated = Expression.Not(expression.Body);
            return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
        }

        /// <summary>
        /// Combines the first expression with the second using the specified merge function.
        /// </summary>
        static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // zip parameters (map from parameters of second to parameters of first)
            var map = first.Parameters
                .Select((f, i) => new { f, s = second.Parameters[i] })
                .ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with the parameters in the first
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // create a merged lambda expression with parameters from the first expression
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        class ParameterRebinder : ExpressionVisitor
        {
            readonly Dictionary<ParameterExpression, ParameterExpression> map;

            ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }

            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }

            protected override Expression VisitParameter(ParameterExpression p)
            {
                ParameterExpression replacement;

                if (map.TryGetValue(p, out replacement))
                {
                    p = replacement;
                }

                return base.VisitParameter(p);
            }
        }
    }

You’ll also need the ExpressionVisitor if you’re not on .NET 4.