PredicateBuilder

ParameterRebinder.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace System.Linq.Expressions
{
    public sealed class ParameterRebinder : ExpressionVisitor
    {
        private readonly Dictionary<ParameterExpression, ParameterExpression> _map;

        public 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);
        }
    }
}

PredicateBuilder`1.cs

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;
 
public static class PredicateBuilder
{
  public static Expression<Func<T, bool>> True<T> ()  { return f => true;  }
  public static Expression<Func<T, bool>> False<T> () { return f => false; }
 
  public static Expression<Func<T, bool>> Or<T> (this Expression<Func<T, bool>> expr1,
                                                      Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters);
  }
 
  public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
                                                       Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.AndAlso (expr1.Body, invokedExpr), expr1.Parameters);
  }
}

PredicateBuilder`2.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace System.Linq.Expressions
{
    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)
        {
            UnaryExpression 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>
        private 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)
            Dictionary<ParameterExpression, ParameterExpression> 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
            Expression 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);
        }
    }
}