Linq動態條件

不少狀況下，咱們開發程序，須要動態拼接SQL查詢語句;

好比  select top 1 * from User where age= 18  and  name = 'renruiquan'

其中紅色的代碼，是咱們須要根據查詢條件是否爲空，來判，要不要加在查詢的SQL裏;

換成Linq裏就不能這麼直接的去拼接了，好在國外的大神有給咱們解決方案。下面直接上代碼：

（新手同窗不須要關心代碼具體是怎麼實現的，只須要知道怎麼調用就好。固然，你能研究一下，給本身充電，也是再好不過了）

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;


namespace JQ.GameClient.Common
{
    /// <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);
        }

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

            /// <summary>
            /// Initializes a new instance of the <see cref="ParameterRebinder"/> class.
            /// </summary>
            /// <param name="map">The map.</param>
            ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }

            /// <summary>
            /// Replaces the parameters.
            /// </summary>
            /// <param name="map">The map.</param>
            /// <param name="exp">The exp.</param>
            /// <returns>Expression</returns>
            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }

            /// <summary>
            /// Visits the parameter.
            /// </summary>
            /// <param name="p">The p.</param>
            /// <returns>Expression</returns>
            protected override Expression VisitParameter(ParameterExpression p)
            {
                ParameterExpression replacement;

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

                return base.VisitParameter(p);
            }
        }
    }

}

代碼調用：

            //動態構造查詢條件
            var predicate = PredicateBuilder.True<UserInfo>();
　　　　　　　//查詢用戶狀態爲1的數據
            predicate = predicate.And(x => x.status == 1);

            if (!string.IsNullOrEmpty(name))
            {
　　　　　　　　　//若是查詢的用戶名不爲空，則拼接表達式
                predicate = predicate.And(x => x.name == name);
            }

            var list = bll.GetList<UserInfo>(predicate,o=>o.orderno);

其中UserInfo爲用戶表的實體類。

list即爲查詢的結果。