如何使用LINQ查询通用列表？ [英] How to query generic list using LINQ?

问题描述

 foreach（setNames中的T setName）
 {
 List< T> list = objectList.Where（x => x.QualificationID == setName）。ToList（）; 
 redisClient.SetAdd< string>（Qualification：+ setName，list.Select（x => x.ProductID）.ToString（））; 
} 

在第3行中，T不包含'QualificationID'的定义，并且没有扩展方法'QualificationID'接受Type T的第一个参数可能是找到。

类似于第4行，

T不包含'ProductID'的定义，也没有扩展方法'ProductID'接受Type T的第一个参数



我们如何查询泛型类型列表？



我尝试过：

 foreach（setNames中的T setName）
 {
 List< T> list = objectList.Where（x => x.QualificationID == setName）。ToList（）; 
 redisClient.SetAdd< string>（Qualification：+ setName，list.Select（x => x.ProductID）.ToString（））; 
} 

解决方案

当你使用泛型方法时，它会从集合的类型填充空白 - 所以您可以在每个对象上使用的属性和方法取决于集合的类型。

因此Point对象的集合可以使用Point属性：

列表与LT;点和GT; points = GetPoints（）; 
列表<点> leftColumn = points.Where（p = >  pX <  =  10 ）。ToList（）; 

但是字符串集合可以让你使用字符串属性：

 List<串GT; strings = GetStrings（）; 
 List< string> shortStrings = strings.Where（s = >  s.Length <  =  10 ）。ToList（）; 

无论你的集合被声明为什么，你都可以在课堂上使用哪些属性和方法，而不是集合中的项目可能是。因此，如果您使用基类：

 List< object> strings = GetStrings（）; 
 List< string> shortStrings = strings.Where（s = >  s.Length <  =  10 ）。ToList（）; 

您将收到错误，因为长度不是<$的属性c $ c> object class，即使集合中的所有对象都是字符串，并且有一个Length - 系统也不会知道类中的所有对象将始终是字符串：

列表< object> strings =  new  List< object>（）; 
 strings.Add（  Hello！）; 
 strings.Add（ 666 ）; 
 strigns.Add（ new  Point（ 12 ， 42 ））; 
 List< string> shortStrings = strings.Where（s = >  s.Length <  =  10 ）。ToList（）; 

那么看看 setNames 被声明为 - 它控制你可以使用哪些属性使用

foreach (T setName in setNames)
{
 List<T> list = objectList.Where(x => x.QualificationID == setName).ToList();
 redisClient.SetAdd<string>("Qualification:" + setName, list.Select(x => x.ProductID).ToString());
}

In Line 3, T does not contain a definition for 'QualificationID' and no extension method 'QualificationID' accepting a first argument of Type T could be found.
Similarly in Line 4,
T does not contain a definition for 'ProductID' and no extension method 'ProductID' accepting a first argument of Type T could be found

How can we query the list of generic type?

What I have tried:

foreach (T setName in setNames)
{
 List<T> list = objectList.Where(x => x.QualificationID == setName).ToList();
 redisClient.SetAdd<string>("Qualification:" + setName, list.Select(x => x.ProductID).ToString());
}

解决方案

When you use generic methods, it "fills in the blanks" from the type of the collection - so the properties and methods you can use on each object depends on the type of the collection.
So a collection of Point object can use Point propertuies:

List<Point> points = GetPoints();
List<Point> leftColumn = points.Where(p => p.X <= 10).ToList();

But a collection of strings would let you use string properties:

List<string> strings = GetStrings();
List<string> shortStrings = strings.Where(s => s.Length <= 10).ToList();

Whatever your collection is declared as determines what properties and methods you can use inside your class, not what items in the collection may be. So if you use a base class:

List<object> strings = GetStrings();
List<string> shortStrings = strings.Where(s => s.Length <= 10).ToList();

You will get an error because Length is not a property of the object class, even if all the objects in the collection are strings, and have a Length - the system doesn't "know" that all the objects in the class will always be strings:

List<object> strings = new List<object>();
strings.Add("Hello!");
strings.Add(666);
strigns.Add(new Point(12, 42));
List<string> shortStrings = strings.Where(s => s.Length <= 10).ToList();

So look to what setNames is declared as - that controls what properties you can use.

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