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：
这个类可以用于Expression<Func<T, bool>>类型的表达式的合并了。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); } }
2，构造Query
同第一种查询更好的写法：
主查询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); }
生成的SQL和第一个完全相同.var result = getQuery();
