C# 通用 LINQ 查询谓词？
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Generic LINQ query predicate?
提问by Codewerks
Not sure if this is possible or if I'm expressing correctly what I'm looking for, but I have the following piece of code in my library repeatedly and would like to practice some DRY. I have set of SQL Server tables that I'm querying based on a simple user-supplied search field ala Google. I'm using LINQ to compose the final query based on what's in the search string. I'm looking for a way to use generics and passed in lambda functions to create a reusable routine out of this:
不确定这是否可行，或者我是否正确表达了我正在寻找的内容，但是我的库中反复有以下代码段，并且想练习一些 DRY。我有一组 SQL Server 表，我根据用户提供的简单搜索字段 ala Google 查询这些表。我正在使用 LINQ 根据搜索字符串中的内容编写最终查询。我正在寻找一种方法来使用泛型并传入 lambda 函数来创建一个可重用的例程：
string[] arrayOfQueryTerms = getsTheArray();
var somequery = from q in dataContext.MyTable
select q;
if (arrayOfQueryTerms.Length == 1)
{
somequery = somequery.Where<MyTableEntity>(
e => e.FieldName.StartsWith(arrayOfQueryTerms[0]));
}
else
{
foreach(string queryTerm in arrayOfQueryTerms)
{
if (!String.IsNullOrEmpty(queryTerm))
{
somequery = somequery
.Where<MyTableEntity>(
e => e.FieldName.Contains(queryTerm));
}
}
}
I was hoping to create a generic method with signature that looks something like:
我希望创建一个带有签名的通用方法，如下所示：
private IQueryable<T> getQuery(
T MyTableEntity, string[] arrayOfQueryTerms, Func<T, bool> predicate)
I'm using the same search strategy across all my tables, so the only thing that really differs from usage to usage is the MyTable & MyTableEntity searched and the FieldName searched. Does this make sense? Is there a way with LINQ to dynamically pass in the name of the field to query in the where clause? Or can I pass in this as a predicate lambda?
我在我的所有表中使用相同的搜索策略，因此唯一不同的用法是搜索的 MyTable 和 MyTableEntity 以及搜索的 FieldName。这有意义吗？LINQ 有没有办法动态传入字段名称以在 where 子句中查询？或者我可以将其作为谓词 lambda 传入？
e => e.FieldName.Contains(queryTerm)
I realize there a million and a half ways to do this in SQL, probably easier, but I'd love to keep everything in the LINQ family for this one. Also, I feel that generics should be handy for a problem like this. Any ideas?
我意识到在 SQL 中有一百万种半方法可以做到这一点，可能更容易，但我很想为这个保留 LINQ 系列中的所有内容。另外，我觉得泛型应该很方便解决这样的问题。有任何想法吗？
采纳答案by Erik Forbes
It sounds like you're looking for Dynamic Linq. Take a look here. This allows you to pass strings as arguments to the query methods, like:
听起来您正在寻找 Dynamic Linq。看看这里。这允许您将字符串作为参数传递给查询方法，例如：
var query = dataSource.Where("CategoryID == 2 && UnitPrice > 3")
.OrderBy("SupplierID");
Edit: Another set of posts on this subject, using C# 4's Dynamic support: Part 1and Part 2.
回答by Quintin Robinson
What it sounds like is you want basically a conditional predicate builder..
听起来您基本上想要一个条件谓词构建器..
I hope you can mold this into something you are looking for, good luck!
我希望你能把它塑造成你正在寻找的东西，祝你好运！
回答by Mark Cidade
You might want to look at expression trees:
您可能想查看表达式树：
IQueryable<T> getQuery<T>(T myTableEntity, string[] arrayOfQueryTerms, Expression<Func<T, bool>> predicate)
{ var fieldOrProperty = getMemberInfo(predicate);
/* ... */
}
MemberInfo getmemberInfo<T>(Expression<Func<T,bool> expr)
{ var memberExpr = expr as MemberExpression;
if (memberExpr != null) return memberExpr.Member;
throw new ArgumentException();
}
var q = getQuery<FooTable>(foo, new[]{"Bar","Baz"}, x=>x.FieldName);
回答by user17060
I recently had to do this same thing. You will need Dynamic Linqhereis a way to keep this strongly typed.
我最近不得不做同样的事情。您将需要Dynamic Linq，这是一种保持强类型的方法。
