Linq动态条件查询-技术开发专区

Linq动态条件查询

作者：ITPUB论坛编辑： nancy 2008-01-25 00:00

【IT168技术文档】

1，构造表达式树

 private Expression<Func<Blog, bool>> getCondition()
{
Expression<Func<Blog, bool>> expression = blog => true;
if (!String.IsNullOrEmpty(Request["BlogClassID"]))
{
int blogClassID;
if (Int32.TryParse(Request["BlogClassID"], out blogClassID))
{
Expression<Func<Blog, bool>> e2 = blog => blog.BlogClass == null;
var invokedExpr = Expression.Invoke(e, expression.Parameters.Cast<Expression>());
expression = Expression.Lambda<Func<Blog, bool>>(Expression.And(expression.Body, invokedExpr), expression.Parameters);
}
}
return expression;
}

主查询是这个样子：

var result = new DongBlogDataContext().Blogs.Where(getCondition());

因为根据SQL追踪，生成SQL类似：

SELECT [t0].[BlogID], [t0].[ChannelID], [t0].[BlogClassID], [t0].[Title], [t0].[Content], [t0].[Tag], [t0].[CreateDateTime]
FROM [dbo].[Blog] AS [t0]
WHERE [t0].[BlogClassID] IS NULL

这种方法是实质是合并Lamba表达式，也就是这三句：

Expression<Func<Blog, bool>> e = blog => blog.BlogClass == null;
var invokedExpr = Expression.Invoke(e, expression.Parameters.Cast<Expression>());
expression = Expression.Lambda<Func<Blog, bool>>(Expression.And(expression.Body, invokedExpr), expression.Parameters);

如果每个条件合并都这么写会很麻烦，幸好已经有人给写好的辅助类：

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;
public static class PredicateBuilder
{
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>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
return Expression.Lambda<Func<T, bool>>
(Expression.Or (expr1.Body, invokedExpr), expr1.Parameters);
}
public static Expression<Func<T, bool>> And<T> (this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
return Expression.Lambda<Func<T, bool>>
(Expression.And (expr1.Body, invokedExpr), expr1.Parameters);
}
}

这个类可以用于Expression<Func<T, bool>>类型的表达式的合并了。
2，构造Query
同第一种查询更好的写法：

 private IQueryable<Blog> getQuery()
{
IQueryable<Blog> query = new DongBlogDataContext().Blogs;
if (!String.IsNullOrEmpty(Request["BlogClassID"]))
{
int blogClassID;
if (Int32.TryParse(Request["BlogClassID"], out blogClassID))
query = query.Where<Blog>(blog => blog.BlogClass == null);
}
return query.OrderByDescending(blog => blog.CreateDateTime);
}

主查询

var result = getQuery();

生成的SQL和第一个完全相同.

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