拼接linq查询表达式

来源：互联网发布：淘宝投资要多少钱编辑：程序博客网时间：2024/06/03 03:58

拼接linq查询表达式

1 比如在做数据库查询时面对前端可能多样的查询条件是，有时拼接查询条件能很方便的处理这种情况，如下代码所示

public GListResult<Entity.SysRole> GetList(Hashtable ht, int skip, int top)        {            System.Linq.Expressions.Expression<Func<Entity.SysRole, bool>> select = ff => ff.Id != 0;            System.Linq.Expressions.Expression<Func<Entity.SysRole, int>> orderBy = ob => ob.Id;            System.Linq.Expressions.Expression<Func<Entity.SysRole, bool>> subselect = ff => ff.Id == 0;            if (ht != null && ht.Count > 0)            {                if (ht.Contains("Id"))                {                    select = select.And(ff => ff.Id == HQ.Job.Tools.StrHelper.ToInt32(ht["Id"].ToString()));                }                if (ht.Contains("RoleID"))                {                    string[] arrRoleId = ht["RoleID"].ToString().Split(',');                    foreach (string item in arrRoleId)                    {                        subselect=subselect.Or(ff => ff.Id == HQ.Job.Tools.StrHelper.ToInt32(item.ToString()));                    }                    select = select.And(subselect);                }                <pre name="code" class="csharp"><span style="white-space:pre"></span>if (ht.Contains("Name"))                {                    select = select.And(ff => ff.Name == HQ.Job.Tools.StrHelper.Format(ht["Name"].ToString()));                }

} var g = dal.GetAll(select, null, skip, top, orderBy); return g; }

dal.GetAll 的接口如下

GListResult<T> GetAll<TKey>(Expression<Func<T, bool>> query,            Expression<Func<T, T>> select, int skip, int top,            Expression<Func<T, TKey>> order = null, bool isDesOrder = false);

原生的linq暂还不支持select.And select.Or 这种写法，要借助这个扩展类LinqHelper，如下:

using System;using System.Collections.Generic;using System.Linq;using System.Linq.Expressions;using System.Text;using System.Threading.Tasks;namespace HQ.Backstage.Business{    public static class LinqHelper    {        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another)        {            var candidateExpr = Expression.Parameter(typeof(T), "candidate");            var parameterReplacer = new ParameterReplacer(candidateExpr);            var left = parameterReplacer.Replace(one.Body);            var right = parameterReplacer.Replace(another.Body);            var body = Expression.And(left, right);            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);        }        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another)        {            var candidateExpr = Expression.Parameter(typeof(T), "candidate");            var parameterReplacer = new ParameterReplacer(candidateExpr);            var left = parameterReplacer.Replace(one.Body);            var right = parameterReplacer.Replace(another.Body);            var body = Expression.Or(left, right);            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);        }        internal class ParameterReplacer : ExpressionVisitor        {            public ParameterReplacer(ParameterExpression paramExpr)            {                this.ParameterExpression = paramExpr;            }            public ParameterExpression ParameterExpression { get; private set; }            public Expression Replace(Expression expr)            {                return this.Visit(expr);            }            protected override Expression VisitParameter(ParameterExpression p)            {                return this.ParameterExpression;            }        }    }}

这样的话做查询处理就非常方便

1 0