using System.Linq.Expressions;
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namespace DocumentServiceAPI.Utility
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{
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/// <summary>
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/// Lambda表达式拼接扩展类
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/// </summary>
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public static class UtilitySearch
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{
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/// <summary>
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/// Lambda表达式拼接
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/// </summary>
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/// <typeparam name="T"></typeparam>
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/// <param name="first"></param>
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/// <param name="second"></param>
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/// <param name="merge"></param>
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/// <returns></returns>
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public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
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{
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// 构建参数映射（从第二个参数到第一个参数）
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var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
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// 将第二个lambda表达式中的参数替换为第一个lambda表达式的参数
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var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
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// 将lambda表达式体的组合应用于第一个表达式中的参数
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return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
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}
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/// <summary>
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/// and扩展
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/// </summary>
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/// <typeparam name="T"></typeparam>
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/// <param name="first"></param>
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/// <param name="second"></param>
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/// <returns></returns>
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public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
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{
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return first.Compose(second, Expression.AndAlso);
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}
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/// <summary>
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/// or扩展
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/// </summary>
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/// <typeparam name="T"></typeparam>
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/// <param name="first"></param>
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/// <param name="second"></param>
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/// <returns></returns>
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public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
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{
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return first.Compose(second, Expression.OrElse);
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}
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}
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/// <summary>
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///
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/// </summary>
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public class ParameterRebinder : ExpressionVisitor
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{
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private readonly Dictionary<ParameterExpression, ParameterExpression> map;
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/// <summary>
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///
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/// </summary>
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/// <param name="map"></param>
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public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
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{
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this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
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}
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/// <summary>
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///
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/// </summary>
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/// <param name="map"></param>
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/// <param name="exp"></param>
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/// <returns></returns>
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public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
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{
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return new ParameterRebinder(map).Visit(exp);
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}
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/// <summary>
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///
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/// </summary>
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/// <param name="p"></param>
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/// <returns></returns>
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protected override Expression VisitParameter(ParameterExpression p)
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{
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ParameterExpression replacement;
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if (map.TryGetValue(p, out replacement))
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{
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p = replacement;
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}
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return base.VisitParameter(p);
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}
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}
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}
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