LINQ中的投影是什么,如.Select() [英] what is a projection in LINQ, as in .Select()
问题描述
我通常进行移动应用程序开发,但并不总是具有.Select.但是,我已经看到了一些用处,但我真的不知道它的作用或作用.就像
I typically do mobile app development, which doesn't always have .Select. However, I've seen this used a bit, but I don't really know what it does or how it's doing whatever it does. It is anything like
from a in list select a // a.Property // new Thing { a.Property}
我之所以问是因为,当我看到使用.Select()的代码时,我对它的工作感到有些困惑.
I'm asking because when I've seen code using .Select(), I was a bit confused by what it was doing.
推荐答案
.Select()
来自LINQ的方法语法,代码from a in list select a
中的select
用于查询语法.两者相同,查询语法会编译为方法语法.
.Select()
is from method syntax for LINQ, select
in your code from a in list select a
is for query syntax. Both are same, query syntax compiles into method syntax.
您可能会看到: LINQ(C#)中的查询语法和方法语法
投影:
投影是指将一个对象转换为一个对象的操作. 新表格通常只包含那些将要 随后使用.通过使用投影,您可以构造一个新类型 从每个对象构建的.您可以投影属性并执行 一个数学函数.您也可以投影原始 对象而不更改它.
Projection refers to the operation of transforming an object into a new form that often consists only of those properties that will be subsequently used. By using projection, you can construct a new type that is built from each object. You can project a property and perform a mathematical function on it. You can also project the original object without changing it.
您可能还会看到: LINQ投影
转换查询结果的过程称为 投影.您可以在任何过滤器之后投影查询结果 已应用于更改集合的类型,即 返回.
The process of transforming the results of a query is called projection. You can project the results of a query after any filters have been applied to change the type of the collection that is returned.
List<string> words = new List<string>() { "an", "apple", "a", "day" };
var query = from word in words
select word.Substring(0, 1);
在上面的示例中,仅选择/投影了每个字符串实例中的第一个字符.
In the above example only first character from each string instance is selected / projected.
您还可以从您的集合中选择一些字段,然后创建一个匿名类型或现有类的实例,该过程称为投影.
You can also select some fields from your collection and create an anonymous type or an instance of existing class, that process is called projection.
from a in list select new { ID = a.Id}
在上面的代码字段中,Id
被投影为匿名类型,而忽略了其他字段.考虑您的列表中有一个定义为MyClass
的对象,如下所示:
In the above code field Id
is projected into an anonymous type ignoring other fields. Consider that your list has an object of type MyClass
defined like:
class MyClass
{
public int Id { get; set; }
public string Name { get; set; }
public string Address { get; set; }
}
现在,您可以将Id
和Name
投影为匿名类型,例如:
Now you can project the Id
and Name
to an anonymous type like:
查询语法:
var result = from a in list
select new
{
ID = a.Id,
Name = a.Name,
};
方法语法
var result = list.Select(r => new { ID = r.Id, Name = r.Name });
您还可以将结果投影到新的班级.考虑您有一个像这样的课程:
You can also project result to a new class. Consider you have a class like:
class TemporaryHolderClass
{
public int Id { get; set; }
public string Name { get; set; }
}
然后您可以执行以下操作:
Then you can do:
查询语法:
var result = from a in list
select new TemporaryHolderClass
{
Id = a.Id,
Name = a.Name,
};
方法语法:
var result = list.Select(r => new TemporaryHolderClass
{
Id = r.Id,
Name = r.Name
});
如果您不打算投影到为LINQ to SQL或Entity Framework生成/创建的类,则还可以投影到同一类.
You can also project to the same class, provided you are not trying to project to classes generated/created for LINQ to SQL or Entity Framework.
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