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   源码介绍

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优选方案是：PredicateExtensions  很方便，很强大

调用示例：

public static void Test3()  
        {  
            DbDataContext db = new DbDataContext();  
            Expression<Func<TemplateDocument, bool>> expressleft =  
                                 PredicateExtensions.True<TemplateDocument>();  
            expressleft = expressleft.Or(t => t.CategoriesID > 3);  
 
            Expression<Func<TemplateDocument, bool>> expressright =   
                PredicateExtensions.False<TemplateDocument>();  
                                    expressright = expressright.Or(t => t.CategoriesID < 5);  
 
            expressleft = expressleft.And(expressright);  
            var sql = db.GetCommand(db.TemplateDocument.Where(expressleft).Select(t => new {   
                                    t.TemplateID, t.TemplateName, t.CategoriesID })).CommandText;  
            Console.WriteLine(sql);  
 
        } 

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

/// <summary>
/// 构造函数使用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混合时写在AND后的OR有效
/// 构造函数使用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混合时写在OR后面的AND有效
/// </summary>
public static class PredicateExtensions
{
    public static Expression<Func<T, bool>> True<T>() { return f => true; }

    public static Expression<Func<T, bool>> False<T>() { return f => false; }

    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
    {
        var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());

        return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
    }

    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
    {
        var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());

        return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
    }
}