[英]How to reuse where clauses in Linq To Sql queries
I have users searching records of type Record. 我有用户搜索记录类型的记录。 They type a search term in a textbox and then I search records by matching several fields with the search term.
他们在文本框中键入搜索词,然后通过将多个字段与搜索词匹配来搜索记录。
My query looks like: 我的查询如下:
var results = from record in DataContext.Records
where
record.Field1.ToLower().Contains(term) ||
record.Field2.ToLower().Contains(term) ||
record.Field3.ToLower().Contains(term)
select record;
I have a number of queries that all use the same filter and thus I would like to extract the filtering so it can be reused. 我有许多查询都使用相同的过滤器,因此我想提取过滤,以便可以重复使用。 Something like:
就像是:
var filter = new Func<Record, string, bool>(
(record, term) =>
record.Field1.ToLower().Contains(term) ||
record.Field2.ToLower().Contains(term) ||
record.Field3.ToLower().Contains(term)
);
var results = from record in DataContext.Records
where filter(record, term)
select record;
However, it does not work because: 但是,它不起作用,因为:
Method 'System.Object DynamicInvoke(System.Object[])' has no supported translation to SQL. 方法'System.Object DynamicInvoke(System.Object [])'没有支持的SQL转换。
How can I reuse my where condition across queries? 如何跨查询重用where where条件?
You need to build an expression instead of a function: 您需要构建表达式而不是函数:
Expression<Func<Record, bool>> filter =
record => record.Field1.ToLower().Contains(term); // rest omitted
The lambda expression remains the same, but you need to return it into a variable of type Expression<Func<Record, bool>>
-- that will make the C# compiler compile it as an expression instead of a delegate, allowing it to be passed to LINQ to SQL. lambda表达式保持不变,但是你需要将它返回到
Expression<Func<Record, bool>>
类型的变量中 - 这将使C#编译器将其编译为表达式而不是委托,允许它被传递到LINQ to SQL。
However, you won't be able to use an expression variable with a C#-syntax where clause: you'll need to use the Where extension method: 但是,您将无法将表达式变量与C#-syntax where子句一起使用:您需要使用Where扩展方法:
var results = DataContext.Records.Where(filter);
Edited to add: If you want to be able to create filters on different terms, you just need a method to produce an expression from a term: 编辑添加:如果您希望能够在不同的条件上创建过滤器,您只需要一个方法来从术语生成表达式:
private static Expression<Func<Record, bool>> Filter(string term)
{
return r => r.Field1.ToLower().Contains(term);
}
var results = DataContext.Records.Where(Filter(term));
If you prefer to keep filter
as a lambda as you have at the moment, you can do so, but the generics get a bit nested: 如果您希望将
filter
保留为目前的lambda,则可以这样做,但泛型有点嵌套:
Func<string, Expression<Func<Record, bool>>> filter =
term => (r => r.Field1.ToLower().Contains(term));
var results = DataContext.Records.Where(filter(term));
Regardless, the important thing is that what goes in the Where clause must be an Expression<Func<Record, bool>>
-- but as shown above you can make the expression depend on term
by building a suitable expression on the fly. 无论如何,重要的是Where子句中的内容必须是
Expression<Func<Record, bool>>
- 但是如上所示,您可以通过动态构建合适的表达式使表达式依赖于term
。 Which is exactly what LINQ to SQL would be doing if you spelled out the filter longhand in the Where clause. 如果你在Where子句中详细说明了过滤器,这正是LINQ to SQL所要做的。
Use a CompiledQuery ! 使用CompiledQuery !
var filter = CompiledQuery.Compile(
(DatabaseDataContext dc, Record record, string term) =>
record.Field1.ToLower().Contains(term) ||
record.Field2.ToLower().Contains(term) ||
record.Field3.ToLower().Contains(term)
);
var results = from record in DataContext.Records
where filter(DataContext, record, term)
select record;
For more information, see How to: Store and Reuse Queries . 有关更多信息,请参见如何:存储和重用查询 。
In addition to the Expression<Func<Record, bool>>
issue that others have pointed out, I suggest looking into PredicateBuilder . 除了其他人指出的
Expression<Func<Record, bool>>
问题之外,我建议调查PredicateBuilder 。 It's very good for dynamically combining lambda expressions. 这对于动态组合lambda表达式非常有用。
I think you need to make it an Expression<Func<Record, bool>>
. 我想你需要把它变成
Expression<Func<Record, bool>>
。 Otherwise it's trying to translate the actual C# method call to SQL rather than the description of it. 否则,它试图将实际的C#方法调用转换为SQL而不是它的描述。 This is not a guarantee that this version will work;
这不能保证此版本能够正常运行; I'm not sure which string functions are translatable to SQL.
我不确定哪些字符串函数可以转换为SQL。
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