Lambda 帮助类

using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Text;

namespace Infrastructure
{

    /// <summary>
    /// Lambda extensions
    /// </summary>
    public static class LambdaApi
    {       /// <summary>
            /// 合并表达式 expr1 AND expr2
            /// </summary>
            /// <typeparam name="T"></typeparam>
            /// <param name="expr1"></param>
            /// <param name="expr2"></param>
            /// <returns></returns>
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {
            //return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, expr2.Body), expr1.Parameters);
            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);

            var left = visitor.Replace(expr1.Body);
            var right = visitor.Replace(expr2.Body);
            var body = Expression.And(left, right);
            return Expression.Lambda<Func<T, bool>>(body, newParameter);

        }
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {

            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);

            var left = visitor.Replace(expr1.Body);
            var right = visitor.Replace(expr2.Body);
            var body = Expression.Or(left, right);
            return Expression.Lambda<Func<T, bool>>(body, newParameter);
        }
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expr)
        {
            var candidateExpr = expr.Parameters[0];
            var body = Expression.Not(expr.Body);

            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }
    }
    internal class NewExpressionVisitor : ExpressionVisitor
    {
        public ParameterExpression _NewParameter { get; private set; }
        public NewExpressionVisitor(ParameterExpression param)
        {
            this._NewParameter = param;
        }
        public Expression Replace(Expression exp)
        {
            return this.Visit(exp);
        }
        protected override Expression VisitParameter(ParameterExpression node)
        {
            return this._NewParameter;
        }
    }
}