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分享一个 .NET EF 6 扩展 Where 的方法

news 来源：原创 2024/9/23 3:19:50

前言

Entity Framework 6（EF 6）中的 Where 方法用于筛选数据库中的数据并返回符合条件的结果，但 Where 方法只能进行简单的筛选条件，例如相等、大于、小于等简单条件，如果需要处理更复杂的逻辑条件，则需要使用其他方法或手动处理，增加了代码复杂性。

本文分享一个扩展 .NET EF 6 Where 的方法。

代码 & 步骤

写 Where 的表达扩展类，留意注释

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Linq.Expressions;namespace ERP.DAL.Repository.Extensions
{/// <summary>/// Linq 表达式扩展类/// </summary>public static class ExpressionExtensions{/// <summary>/// 添加And条件/// </summary>/// <typeparam name="T"></typeparam>/// <param name="first"></param>/// <param name="second"></param>/// <returns></returns>public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second){return first.AndAlso<T>(second, Expression.AndAlso);}/// <summary>/// 添加Or条件/// </summary>/// <typeparam name="T"></typeparam>/// <param name="first"></param>/// <param name="second"></param>/// <returns></returns>public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second){return first.AndAlso<T>(second, Expression.OrElse);}/// <summary>/// 合并表达式以及参数/// </summary>/// <typeparam name="T"></typeparam>/// <param name="expr1"></param>/// <param name="expr2"></param>/// <param name="func"></param>/// <returns></returns>private static Expression<Func<T, bool>> AndAlso<T>(this Expression<Func<T, bool>> expr1,Expression<Func<T, bool>> expr2,Func<Expression, Expression, BinaryExpression> func){var parameter = Expression.Parameter(typeof(T));var leftVisitor = new ReplaceExpressionVisitor(expr1.Parameters[0], parameter);var left = leftVisitor.Visit(expr1.Body);var rightVisitor = new ReplaceExpressionVisitor(expr2.Parameters[0], parameter);var right = rightVisitor.Visit(expr2.Body);return Expression.Lambda<Func<T, bool>>(func(left, right), parameter);}/// <summary>/// 继承表达式树的访问者或重写者。/// </summary>private class ReplaceExpressionVisitor : ExpressionVisitor{private readonly Expression _oldValue;private readonly Expression _newValue;public ReplaceExpressionVisitor(Expression oldValue, Expression newValue){_oldValue = oldValue;_newValue = newValue;}public override Expression Visit(Expression node){if (node == _oldValue)return _newValue;return base.Visit(node);}}}
}

使用

Expression<Func<hospital, bool>> filter = u => u.status != -1;
filter = filter.And(c => c.name.Contains("第三医院"));
filter = filter.Or(c => c.name.Contains("第二医院"));
List<hospital> list = new List<hospital>();
list = list.AsQueryable().Where(filter).ToList();