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Generic Predicate Class

CodeWhizFaisal answered on February 22, 2023 Popularity 1/10 Helpfulness 1/10

answer Generic Predicate Class

Generic Predicate Class

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using System.Linq.Expressions;

namespace RSM.DAL.Repositories.Configuration
{
    /// <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);
            }
        }
    }
}

Popularity 1/10 Helpfulness 1/10 Language csharp

Source: Grepper

Tags: class predicate whatever

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Contributed on Feb 22 2023

CodeWhizFaisal

0 Answers Avg Quality 2/10

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