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;
}
var result =
new DongBlogDataContext().Blogs.Where(getCondition());
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：http://www.albahari.com/expressions/
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。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：
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。
园子里相关讨论还有
http://www.cnblogs.com/neuhawk/archive/2007/07/07/809585.html
http://www.cnblogs.com/blusehuang/archive/2007/07/13/816970.html
还可参考：
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1706161&SiteID=1
本文转自冬冬博客园博客，原文链接：http://www.cnblogs.com/yuandong/archive/2007/07/17/820888.html
，如需转载请自行联系原作者
