前面介绍了Linq的三个方面应用:Linq to SQL, Linq to XML和Linq to Object,这篇介绍一下动态Linq的实现方式及应用场景。 命名空间: System.Linq; System.Linq.Expressions; 应用Linq的时候,我们都知道只需要Lambda表达式就行,但有些场景仅仅只使用Data
前面介绍了Linq的三个方面应用:Linq to SQL, Linq to XML和Linq to Object,这篇介绍一下动态Linq的实现方式及应用场景。
命名空间:
System.Linq;
System.Linq.Expressions;
应用Linq的时候,我们都知道只需要Lambda表达式就行,但有些场景仅仅只使用Data Model的字段名操作是不够的或者不方便的。
场景1:假设我们需要拼接Where条件进行查询,一种方式可以拼接IQueryable的表达式。但我想像写SQL语句那样直接拼接一个Where条件就行,Linq要怎么实现?
场景2:假设我们想要一个列表,这个列表可以按每个表头来排序,我们把表头当作参数传给排序函数,在函数内部该怎么实现呢?可以逐一枚举对比,针对不同的列写不同的Linq语句,但代码很冗余。用传统方式根据动态字段名怎么实现?
下面来说说Linq的另一种应用方式: 动态Linq,使用Linq.Expressions. 场景1, 我只想用Where拼接(表名参数)就完成操作,下面看看实现。下面所有的Demo只是应用于Linq to SQL, 如果是Entity Framework会有差异.
DataClasses1DataContext dbContext = new DataClasses1DataContext();
public string dynamicLinq(int id = 50)
{
IQueryable
ParameterExpression paraExpr = Expression.Parameter(typeof(DataListForDemo), "data");
MemberExpression propExpr = Expression.Property(paraExpr, typeof(DataListForDemo).GetProperty("ID"));
BinaryExpression filter = Expression.LessThan(propExpr, Expression.Constant(id));
LambdaExpression lambdaWhere = Expression.Lambda(filter, paraExpr);
MethodCallExpression Where = Expression.Call(typeof(Queryable),
"Where",
new Type[] { typeof(DataListForDemo) },
Expression.Constant(dLinq),
lambdaWhere
);
var data0 = dLinq.AsQueryable().Provider.CreateQuery(Where);
DbCommand comm = dbContext.GetCommand(data0);
dbContext.Log(comm.CommandText);
return comm.CommandText;
}
上面是各种Linq.Expression的类, 用ParameterExpression定义参数也就是要操作的实体对象, 用PropertyExpression定义属性也就是要操作的字段, 用BinaryExpression定义条件查询的表达式也就是Where条件, 用LambdaExpression定义Lambda表达式也就是IQueryable对象, 最后一步就是来完成调用. Call方法是来定义你的表达式方法, 如: Where, Select, OrderBy, GroupBy, All, Any, Equal等等方法, 有哪一种动态需求就写哪一种方法, 这个在MSDN上面没有太多实例, 不过网上可以查到很多.
上面返回的是生成的SQL语句, SQL语句是这样的:
SELECT [t0].[ID], [t0].[col1], [t0].[col2], [t0].[col3], [t0].[col5], [t0].[col4]
FROM [dbo].[DataListForDemo] AS [t0]
WHERE [t0].[ID]
对照生成的SQL语句和Expression的表达式就很容易理解Linq是怎么实现的和怎么工作的. 那么有些人会问, IQueayable和IEnuerable的对象都会带有Linq的表达式而并不是单独的方法通过传参数实现, 要实现这种方式那么就得提一下什么是扩展方法以及扩展方法怎么实现. 在C#里面要扩展某个对象的方法可以override基类方法, 但是像string, iqueryable等这种对象怎么扩展它们的方法呢? Override基类当然不行, 这时就要用this关键字,也是this的另一种应用方式.
使用扩展方法, 首先写一个静态类, 在静态类里面定义一个静态方法, 静态方法里面第一个参数以this开始, 第一个参数也就是你要扩展的系统对象.
如:
public static class DynamicQueryable
{
//扩展IQueryable对象的方法
public static IQueryable Where(this IQueryable source, string predicate, params object[] values)
{
if (source == null) throw new ArgumentNullException("source");
if (predicate == null) throw new ArgumentNullException("predicate");
LambdaExpression lambda = DynamicExpression.ParseLambda(source.ElementType, typeof(bool), predicate, values);
return source.Provider.CreateQuery(
Expression.Call(
typeof(Queryable), "Where",
new Type[] { source.ElementType },
source.Expression, Expression.Quote(lambda)));
}
public static LambdaExpression ParseLambda(Type itType, Type resultType, string expression, params object[] values)
{
return ParseLambda(new ParameterExpression[] { Expression.Parameter(itType, "") }, resultType, expression, values);
}
}
上面就是扩展IQueryabler方法, 所以IQueryable类型的所有对象都有了动态Where的功能, 我不知道为什么Microsoft团队没有把这个功能加上, 而只是提供了Expressions类, 加上这些动态表达之后Linq功能会非常强壮.
来看看调用:
public string SelectDynamic(int id = 0)
{
DataListForDemo model = dbContext.DataListForDemos.Where("ID = " + id.ToString()).SingleOrDefault();
return model.ID.ToString();
}
现在很明显的一个变化是Where里面可以只写一个拼接的where条件了, 而且是一个字符串, 这就是大多数程序员想到的东东吧!
Linq里面所有已经存在的方法都可以通过这种方式扩展和实现动态化, 更多的实现方式可以Google, 建议使用Google, 英文文章有的写得非常透彻,而且资源丰富.

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