如何在VS2008中的Language Integrated Query动态条件查询
1,构造表达式树
private Expression<Func<Blog, bool>> getCondition()
{
Expression<Func<Blog, bool>> expression = blog => true;
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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>());
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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
Expression<Func<Blog, bool>> e = blog => blog.BlogClass == null;
var invokedExpr = Expression.Invoke(e, expression.Parameters.Cast<Expression>());
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expression = Expression.Lambda<Func<Blog, bool>>(Expression.And(expression.Body, invokedExpr), expression.Parameters);
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);
}
}
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);
}
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return query.OrderByDescending(blog => blog.CreateDateTime);
}
var result = getQuery();
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这种方法是实质是合并Lamba表达式,也就是这三句:
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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:http://www.albahari.com/expressions/
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这个类可以用于Expression<Func<T, bool>>类型的表达式的合并了。具体用法参看http://www.albahari.com/expressions/或http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1745163&SiteID=1
2,构造Query
同第一种查询更好的写法:
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主查询
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生成的SQL和第一个完全相同。
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