ExpressionBuilder.cs

/*
 * QUANTCONNECT.COM - Democratizing Finance, Empowering Individuals.
 * Lean Algorithmic Trading Engine v2.0. Copyright 2014 QuantConnect Corporation.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
*/
 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
 
namespace QuantConnect.Util
{
    /// <summary>
    /// Provides methods for constructing expressions at runtime
    /// </summary>
    public static class ExpressionBuilder
    {
        /// <summary>
        /// Constructs a selector of the form: x => x.propertyOrField where x is an instance of 'type'
        /// </summary>
        /// <param name="type">The type of the parameter in the expression</param>
        /// <param name="propertyOrField">The name of the property or field to bind to</param>
        /// <returns>A new lambda expression that represents accessing the property or field on 'type'</returns>
        public static LambdaExpression MakePropertyOrFieldSelector(Type type, string propertyOrField)
        {
            var parameter = Expression.Parameter(type);
            var property = Expression.PropertyOrField(parameter, propertyOrField);
            var lambda = Expression.Lambda(property, parameter);
            return lambda;
        }
 
        /// <summary>
        /// Constructs a selector of the form: x => x.propertyOrField where x is an instance of 'type'
        /// </summary>
        /// <typeparam name="T">The type of the parameter in the expression</typeparam>
        /// <typeparam name="TProperty">The type of the property or field being accessed in the expression</typeparam>
        /// <param name="propertyOrField">The name of the property or field to bind to</param>
        /// <returns>A new lambda expression that represents accessing the property or field on 'type'</returns>
        public static Expression<Func<T, TProperty>> MakePropertyOrFieldSelector<T, TProperty>(string propertyOrField)
        {
            return (Expression<Func<T, TProperty>>) MakePropertyOrFieldSelector(typeof (T), propertyOrField);
        }
 
        /// <summary>
        /// Constructs a lambda expression that accepts two parameters of type <typeparamref name="T"/> and applies
        /// the specified binary comparison and returns the boolean result.
        /// </summary>
        public static Expression<Func<T, T, bool>> MakeBinaryComparisonLambda<T>(ExpressionType type)
        {
            if (!type.IsBinaryComparison())
            {
                throw new ArgumentException($"Provided ExpressionType '{type}' is not a binary comparison.");
            }
 
            var left = Expression.Parameter(typeof(T), "left");
            var right = Expression.Parameter(typeof(T), "right");
            var body = Expression.MakeBinary(type, left, right);
            var lambda = Expression.Lambda<Func<T, T, bool>>(body, left, right);
            return lambda;
        }
 
        /// <summary>
        /// Determines whether or not the specified <paramref name="type"/> is a binary comparison.
        /// </summary>
        public static bool IsBinaryComparison(this ExpressionType type)
        {
            switch (type)
            {
                case ExpressionType.Equal:
                case ExpressionType.NotEqual:
                case ExpressionType.LessThan:
                case ExpressionType.LessThanOrEqual:
                case ExpressionType.GreaterThan:
                case ExpressionType.GreaterThanOrEqual:
                    return true;
 
                default:
                    return false;
            }
        }
 
        /// <summary>
        /// Converts the specified expression into an enumerable of expressions by walking the expression tree
        /// </summary>
        /// <param name="expression">The expression to enumerate</param>
        /// <returns>An enumerable containing all expressions in the input expression</returns>
        public static IEnumerable<Expression> AsEnumerable(this Expression expression)
        {
            var walker = new ExpressionWalker();
            walker.Visit(expression);
            return walker.Expressions;
        }
 
        /// <summary>
        /// Returns all the expressions of the specified type in the given expression tree
        /// </summary>
        /// <typeparam name="T">The type of expression to search for</typeparam>
        /// <param name="expression">The expression to search</param>
        /// <returns>All expressions of the given type in the specified expression</returns>
        public static IEnumerable<T> OfType<T>(this Expression expression)
            where T : Expression
        {
            return expression.AsEnumerable().OfType<T>();
        }
 
        /// <summary>
        /// Returns the single expression of the specified type or throws if none or more than one expression
        /// of the specified type is contained within the expression.
        /// </summary>
        /// <typeparam name="T">The type of expression to search for</typeparam>
        /// <param name="expression">The expression to search</param>
        /// <returns>Expression of the specified type</returns>
        public static T Single<T>(this Expression expression)
            where T : Expression
        {
            return expression.AsEnumerable().OfType<T>().Single();
        }
 
        /// <summary>
        /// Returns the single expression of the specified type or throws if none or more than one expression
        /// of the specified type is contained within the expression.
        /// </summary>
        /// <typeparam name="T">The type of expression to search for</typeparam>
        /// <param name="expressions">The expressions to search</param>
        /// <returns>Expression of the specified type</returns>
        public static T Single<T>(this IEnumerable<Expression> expressions)
            where T : Expression
        {
            return expressions.OfType<T>().Single();
        }
 
        private class ExpressionWalker : ExpressionVisitor
        {
            public readonly HashSet<Expression> Expressions = new HashSet<Expression>();
            public override Expression Visit(Expression node)
            {
                Expressions.Add(node);
                return base.Visit(node);
            }
        }
    }
}