LINQ中的投影是什么，如.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:

投影操作-MSDN

您可能还会看到: LINQ投影

来自MSDN的示例

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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