在 LINQ 中使用外部函数

发布于 2024-10-28 05:22:05 字数 222 浏览 2 评论 0原文

是否可以在 linq select 结果中使用外部函数？以及如何将参数传递给它？

sub x
Dim q = From a In contex Select New With {.z=z("MYNAME")}
end sub

function z(name as string)
return( name & "Something...")
end function

原文

is it possible to use external function in linq select resault ?
and how to pass param to it?

sub x
Dim q = From a In contex Select New With {.z=z("MYNAME")}
end sub

function z(name as string)
return( name & "Something...")
end function

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西瑶 2024-11-04 05:22:05

这取决于所选的提供商。使用基于 IEnumerable 查询的普通 LINQ（又名 LINQ to Objects），您可以做到这一点。但是，对于大多数基于表达式树的 LINQ（例如 LINQ to SQL、LINQ to Entities、NHibernate、LLBLGen 等），您不能这样做。这些提供程序将表达式树转换为其他语言，例如 SQL、AD、SharePoint 等。他们只是不知道如何处理您的自定义方法。

您可以通过多种方式解决这个问题。例如，尝试在查询中内联编写方法的代码。这样，提供者就知道如何处理它。另一种选择是让该方法在表达式树之外调用。例如（对不起我的 C#）：

var q = (from a in context select a).AsEnumerable();

// q is an IEnumerable and the z method will be called by .NET instead
// of being translated to SQL.
var q2 = (from a in q select new { z = z("MYNAME") };

It depends upon the choosen provider. With normal LINQ over IEnumerable queries (a.k.a. LINQ to Objects) you can do. However, with most LINQ over Expression trees (such as LINQ to SQL, LINQ to Entities, NHibernate, LLBLGen, etc, etc) you can't. Those providers convert an expression tree to some other language, such as SQL, AD, SharePoint, etc, etc. They just don't know what to do with your custom method.

You can solve this in several ways. For instance, try to write the code of the method inline in the query. This way the provider knows what to do with it. Another option is to let the method call outside of the expression tree. For instance (sorry my C#):

var q = (from a in context select a).AsEnumerable();

// q is an IEnumerable and the z method will be called by .NET instead
// of being translated to SQL.
var q2 = (from a in q select new { z = z("MYNAME") };

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