Linq多条件查询优化---使用扩展方法

写在算法前：最近在做SQL多条件查询，感觉自己写查询逻辑太过麻烦。比如，假如有一个查询条件，就需要分析两种情况（空条件和一个条件）；两个条件就要四中情况；三个条件，8种情况.......

这样是不是也太麻烦了?!下面这种方法可以为你简化很多步骤，一起来看看吧。。。

1，首先你要自定义两个方法（这里封装到类里）：

public static class MyHelper

{

//查询方法

public List<User> AjaxSeniorSearchUser(string userName, string nickName, string ddlClasses)

{

List<User> eqList = new List<User>();

Expression<Func<User, bool>> predicate = p => 1 == 1;

if (userName == "" && nickName == "" && ddlClasses == "0")

{

predicate = predicate.And(p => 1 == 1);

}

else

{

if (userName != "")

{

predicate = predicate.And(p => p.UserName.Contains(userName.Trim()));

}

if (nickName != "")

{

predicate = predicate.And(p => p.NickName.Contains(nickName.Trim()));

}

if (ddlClasses != "0")

{

int tempClass=int.Parse(ddlClasses);

predicate = predicate.And(p => p.ClassID ==tempClass );

}

}

return SeachBodyFunction(eqList, predicate);

}

//查询的主方法

private List<User> SeachBodyFunction(List<User> eqList, Expression<Func<User, bool>> predicate)

{

var User = db.User.Where(predicate).Join(db.User, u => u.TeacherID, r => r.ID, (u, r) => new

{

ID = u.ID,

UserName = u.UserName,

NickName = u.NickName,

//ClassName = u.ClassName,

Sex = u.Sex,

TeacherID = u.ID,

TeacherName = r.NickName,

PracticeCompany = u.PracticeCompany,

Flow = u.Flow

}).ToList();

foreach (var eq in User)

{

User UserFo = new User();

UserFo.ID=eq.ID;

UserFo.UserName=eq.UserName;

UserFo.NickName=eq.NickName;

UserFo.SexName = EnumTool.GetSex(eq.Sex.Value);

UserFo.TeacherID=eq.TeacherID;

UserFo.TeacherName=eq.TeacherName;

UserFo.PracticeCompany=eq.PracticeCompany;

UserFo.Flow = eq.Flow;

eqList.Add(UserFo);

}

return eqList;

}

},

2，然后要自定义一个扩展方法：

namespace MyProject.Tool

{

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)

{

return first.Compose(second, Expression.And);

}

}

3，主函数调用示例：

首先别忘记引用Tool,

using MyProject.Tool;

public ActionResult StudentIndex(string userName, string nickName, string ddlClasses)//这里实在MVC下的示例，如果是别的，换成相应的调用函数就行了

{

List<User> userList=AjaxSeniorSearchUser(userName, nickName, ddlClasses);//至此，你已经得到查询出来的集合了

return View(userList);

}

完结----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

下面的(你可以完全不用看了)

4，前台页面显示示例

<table>

@foreach (var item in Model)

{

<tr>

<td>

<input name='checkItem' type="checkbox" value="@item.ID" /></td>

<td>

@Html.DisplayFor(modelItem => item.UserName)

</td>

<td>

@Html.DisplayFor(modelItem => item.NickName)

</td>

&n
4000
bsp; <td>

@Html.DisplayFor(modelItem => item.ClassName)

</td>

<td>

@Html.DisplayFor(modelItem => item.SexName)

</td>

<td>

@Html.DisplayFor(modelItem => item.PracticeCompany)

</td>

<td>

@Html.DisplayFor(modelItem => item.TeacherName)

</td>

<td>

@Html.DisplayFor(modelItem => item.Flow)

</td>

</tr>

}

</table>