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基础知识

LINQ

在本页

  • 概述
  • 使集合可查询
  • 支持的聚合阶段
  • $project
  • $match
  • $limit
  • $skip
  • $unwind
  • $group
  • $sort
  • $lookup
  • $vectorSearch
  • 位运算符
  • $bitAnd
  • $bitOr
  • $bitNot
  • $bitXor
  • 不支持的聚合阶段
  • 支持的方法
  • 查看翻译查询

概述

在本指南中，您可以了解如何使用LINQ与 MongoDB .NET/C# 驱动程序。LINQ 允许您使用语言关键字和运算符构造针对对象强类型集合的查询。.NET/C# 驱动程序自动将 LINQ 查询转换为聚合操作。

重要

在 MongoDB .NET/C# 驱动程序中，LINQ3 是唯一的 LINQ 提供程序。如果您已手动配置项目使用 LINQ2，则无法编译。

本指南中的示例使用restaurants 集合，该集合位于 sample_restaurants 数据库中，该数据库包含在Atlas 样本数据集中提供。要了解如何创建免费的 MongoDB Atlas 集群并加载数据集，请参阅快速入门.

以下 Restaurant、Address 和 GradeEntry 类模型化了这个集合中的文档。

public class Restaurant
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    [BsonElement("restaurant_id")]
    public string RestaurantId { get; set; }
    public string Cuisine { get; set; }
    public Address Address { get; set; }
    public string Borough { get; set; }
    public List<GradeEntry> Grades { get; set; }
}
public class Address
{
    public string Building { get; set; }
    [BsonElement("coord")]
    public double[] Coordinates { get; set; }
    public string Street { get; set; }
    [BsonElement("zipcode")]
    public string ZipCode { get; set; }
}
public class GradeEntry
{
    public DateTime Date { get; set; }
    public string Grade { get; set; }
    public float Score { get; set; }
}

注意：

restaurants 集合中的文档使用蛇形命名约定。本指南中的示例使用 ConventionPack 将集合中的字段反序列化为 Pascal 大小写，并将它们映射到 Restaurant 类的属性。

要了解更多关于自定义序列化的信息，请参阅 自定义序列化。

使集合可查询

要使用 LINQ 查询您的集合，您必须首先创建一个链接到集合的 IQueryable 对象。要创建对象，使用以下方式调用 AsQueryable() 方法

var restaurantsDatabase = client.GetDatabase("sample_restaurants");
var restaurantsCollection = restaurantsDatabase.GetCollection<Restaurant>("restaurants");
var queryableCollection = restaurantsCollection.AsQueryable();

一旦您有了可查询对象，您就可以使用 方法语法 编写查询。某些管道阶段也支持 查询理解语法，其类似于 SQL 查询语法。

选择方法语法 或 查询语法 选项卡，了解如何使用 LINQ 编写查询

var query = queryableCollection
    .Where(r => r.Name == "The Movable Feast")
    .Select(r => new { r.Name, r.Address });
var query = from r in queryableCollection
    where r.Name == "The Movable Feast"
    select new { r.Name, r.Address };

您可以根据前面的示例按如下方式打印结果

foreach (var restaurant in query)
{
    Console.WriteLine(restaurant.ToJson());
}
{ "name" : "The Movable Feast", "address" : { "building" : "284", "coord" : [-73.982923900000003, 40.6580753], "street" : "Prospect Park West", "zipcode" : "11215" } }

提示

访问查询结果

您还可以通过使用 ToList() 或 ToCursor() 方法来访问查询结果

var results = query.ToList();
var results = query.ToCursor();

支持的聚合阶段

您可以使用LINQ创建一个聚合管道。.NET/C# 驱动程序自动将每个LINQ语句转换为相应的聚合管道阶段。在本节中，您可以了解哪些聚合管道阶段是受支持的。

要了解有关聚合管道阶段更多信息，请参阅服务器手册中的聚合阶段页面。

$project

$project 聚合阶段返回只包含指定字段的文档。

选择方法语法或查询语法选项卡，了解如何使用LINQ生成 $project 阶段。

var query = queryableCollection
    .Select(r => new { r.Name, r.Address });
var query = from r in queryableCollection
    select new { r.Name, r.Address };

上一个示例的结果包含以下文档

{ "name" : "The Movable Feast", "address" : { "building" : "284", "coord" : [-73.982923900000003, 40.6580753], "street" : "Prospect Park West", "zipcode" : "11215" } }

注意：

排除_id字段

如果您在LINQ投影中不包括 _id 字段，.NET/C# 驱动程序将自动将其排除在结果之外。

$match

聚合阶段 $match 返回符合指定条件的文档。

选择 方法语法 或 查询语法 选项卡，查看如何使用 LINQ 生成 $match 阶段。

var query = queryableCollection
    .Where(r => r.Name == "The Movable Feast");
var query = from r in queryableCollection
    where r.Name == "The Movable Feast"
    select r;

上一个示例的结果包含以下文档

// Results Truncated
{ "_id" : ObjectId(...), "name" : "The Movable Feast", "restaurant_id" : "40361606", "cuisine" : "American", "address" : {...}, "borough" : "Brooklyn", "grades" : [...] }

$limit

聚合阶段 $limit 限制了查询返回的文档数量。以下示例展示了如何使用 LINQ 生成 $limit 阶段。

var query = queryableCollection
    .Where(r => r.Cuisine == "Italian")
    .Select(r => new {r.Name, r.Cuisine})
    .Take(5);

前一个示例的结果包含以下文档

{ "name" : "Philadelhia Grille Express", "cuisine" : "Italian" }
{ "name" : "Isle Of Capri Resturant", "cuisine" : "Italian" }
{ "name" : "Marchis Restaurant", "cuisine" : "Italian" }
{ "name" : "Crystal Room", "cuisine" : "Italian" }
{ "name" : "Forlinis Restaurant", "cuisine" : "Italian" }

$skip

聚合阶段 $skip 跳过查询返回的指定数量的文档，然后返回剩余的结果。以下示例展示了如何使用 LINQ 生成 $skip 阶段。

var query = queryableCollection
    .Where(r => r.Cuisine == "Italian")
    .Select(r => new {r.Name, r.Cuisine})
    .Skip(2);

前一个示例跳过了符合条件的前两家餐厅，并返回了其余结果。结果包含以下文档

// Results Truncated
{ "name" : "Marchis Restaurant", "cuisine" : "Italian" }
{ "name" : "Crystal Room", "cuisine" : "Italian" }
{ "name" : "Forlinis Restaurant", "cuisine" : "Italian" }
...

$unwind

《$unwind》聚合阶段会将指定的数组字段解构，并为该数组中的每个元素返回一个文档。

选择方法语法或查询语法选项卡，以了解如何使用LINQ生成《$unwind》阶段。

var query = queryableCollection
    .Where(r => r.Name == "The Movable Feast")
    .SelectMany(r => r.Grades);
var query = from r in queryableCollection
    where r.Name == "The Movable Feast"
    from grade in r.Grades
    select grade;

前面示例中的查询找到名称字段值为"The Movable Feast"的文档。然后，对于该文档的《Grades》数组中的每个元素，查询返回一个新的文档。结果包含以下文档

{ "date" : ISODate("2014-11-19T00:00:00Z"), "grade" : "A", "score" : 11 }
{ "date" : ISODate("2013-11-14T00:00:00Z"), "grade" : "A", "score" : 2 }
{ "date" : ISODate("2012-12-05T00:00:00Z"), "grade" : "A", "score" : 13 }
{ "date" : ISODate("2012-05-17T00:00:00Z"), "grade" : "A", "score" : 11 }

$group

《$group》聚合阶段根据指定的标准将文档分离成组。

选择方法语法或查询语法选项卡，以了解如何使用LINQ生成《$group》阶段。

var query = queryableCollection
    .GroupBy(r => r.Cuisine)
    .Select(g => new { Cuisine = g.Key, Count = g.Count() });
var query = from r in queryableCollection
    group r by r.Cuisine into g
    select new {Cuisine = g.Key, Count = g.Count()};

前面示例根据每个文档的《Cuisine》字段值对其进行分组，然后计算具有每个《Cuisine》值多少个文档。结果包含以下文档

// Results Truncated
{ "cuisine" : "Caribbean", "count" : 657 }
{ "cuisine" : "Café/Coffee/Tea", "count" : 1214 }
{ "cuisine" : "Iranian", "count" : 2 }
{ "cuisine" : "Nuts/Confectionary", "count" : 6 }
{ "cuisine" : "Middle Eastern", "count" : 168 }
...

注意：

结果顺序

前面的查询不一定总是以相同的顺序返回结果。运行此示例可能返回的结果顺序与上述所示不同。

$sort

《$sort》聚合阶段按照您指定的顺序返回查询结果。

选择方法语法或查询语法选项卡，查看如何使用LINQ生成一个《$sort》阶段。

var query = queryableCollection
    .OrderBy(r => r.Name)
    .ThenByDescending(r => r.RestaurantId);
var query = from r in queryableCollection
    orderby r.Name, r.RestaurantId descending
    select r;

前面的示例按名称字段字母顺序排序查询结果，并在名称相同的情况下按《RestaurantId》字段降序排序。以下是返回结果中包含的文档的子集。

// Results Truncated
...
{ "_id" : ObjectId(...), "name" : "Aba Turkish Restaurant", "restaurant_id" : "41548686", "cuisine" : "Turkish", "address" : {...}, "borough" : "Manhattan", "grades" : [...] }
{ "_id" : ObjectId(...), "name" : "Abace Sushi", "restaurant_id" : "50006214", "cuisine" : "Japanese", "address" : { ... }, "borough" : "Manhattan", "grades" : [...] }
{ "_id" : ObjectId(...), "name" : "Abacky Potluck", "restaurant_id" : "50011222", "cuisine" : "Asian", "address" : { ... }, "borough" : "Manhattan", "grades" : [...] }
{ "_id" : ObjectId(...), "name" : "Abaleh", "restaurant_id" : "50009096", "cuisine" : "Mediterranean", "address" : { ... }, "borough" : "Manhattan", "grades" : [...] }
...

《$lookup》聚合阶段将一个集合的文档与同一数据库中另一个集合的文档进行连接。在《$lookup》阶段，为每个输入文档添加一个新数组字段。新数组字段包含来自“连接”集合的匹配文档。

注意：

要执行查找，必须使用《AsQueryable()`》方法使两个集合可查询。

有关如何使集合可查询的信息，请参阅使集合可查询。

考虑在《sample_restaurants》数据库中名为《reviews》的第二个集合，它包含餐厅评论。您可以使用《$lookup》阶段将该集合的文档与具有相同《name》值的《restaurants》集合中的文档进行连接。

以下《Review》类模拟了《reviews》集合中的文档。

public class Review
{
    public ObjectId Id { get; set; }
    [BsonElement("restaurant_name")]
    public string RestaurantName { get; set; }
    public string Reviewer { get; set; }
    [BsonElement("review_text")]
    public string ReviewText { get; set; }
}

选择《Method Syntax》或《Query Syntax》选项卡，查看如何使用LINQ生成一个《$lookup》阶段。

var query = queryableCollection
    .GroupJoin(reviewCollection,
        restaurant => restaurant.Name,
        review => review.RestaurantName,
        (restaurant, reviews) =>
            new { Restaurant = restaurant, Reviews = reviews }
    );
var query = from restaurant in queryableCollection
    join rv in reviewCollection on restaurant.Name equals rv.RestaurantName into reviews
    select new { restaurant, reviews };

前面的示例返回《restaurants》集合中的所有文档。每个餐厅文档都有一个新增的字段名为《reviews》，其中包含该餐厅的所有评论。如果评论文档中的《name》字段值与餐厅文档中的《name》字段值匹配，则评论与餐厅匹配。

以下显示了返回结果的一个子集。

// Results Truncated
{
   "restaurant": {
      "_id": ObjectId("..."),
      "name": "The Movable Feast",
      "restaurant_id": "40361606",
      "cuisine": "American",
      "address": { ... },
      "borough": "Brooklyn",
      "grades": [ ... ]
   },
   "reviews": [
      {
         "_id": ObjectId("..."),
         "restaurant_name": "The Movable Feast",
         "reviewer": "Lazlo Cravensworth",
         "review_text": "Great restaurant! 12/10 stars!"
      },
      {
         "_id": ObjectId("..."),
         "restaurant_name": "The Movable Feast",
         "reviewer": "Michael Scarn",
         "review_text": "It really was a feast"
      }
   ]
}

向量搜索

$vectorSearch 聚合阶段在指定字段中执行向量上的近似最近邻搜索。在您可以对数据进行向量搜索之前，您的集合必须定义 Atlas 向量搜索索引。

提示

要获取以下示例中使用的示例数据集，请参阅 快速入门。要创建以下示例中使用的样本 Atlas 向量搜索索引，请参阅 Atlas 手册中的 创建 Atlas 向量搜索索引。

考虑 sample_mflix 数据库中的 embedded_movies 集合。您可以使用 $vectorSearch 阶段在集合中文档的 plot_embedding 字段上执行语义搜索。

以下 EmbeddedMovie 类表示 embedded_movies 集合中的文档

[BsonIgnoreExtraElements]
public class EmbeddedMovie
{
   [BsonIgnoreIfDefault]
   public string Title { get; set; }
   public string Plot { get; set; }
   [BsonElement("plot_embedding")]
   public double[] Embedding { get; set; }
}

以下示例显示了如何生成一个 $vectorSearch 阶段，使用字符串 "time travel" 的向量嵌入搜索 plot_embedding 字段

// Defines vector embeddings for the string "time travel"
var vector = new[] 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// Specifies that the vector search will consider the 150 nearest neighbors
// in the specified index
var options = new VectorSearchOptions<EmbeddedMovie>()
{
   IndexName = "vector_index",
   NumberOfCandidates = 150
};
// Builds aggregation pipeline and specifies that the $vectorSearch stage
// returns 10 results
var results = queryableCollection
   .VectorSearch(m => m.Embedding, vector, 10, options)
   .Select(m => new { m.Title, m.Plot });

前面示例的结果包含以下文档

{ "_id" : ObjectId("573a13a0f29313caabd04a4f"), "plot" : "A reporter, learning of time travelers visiting 20th century disasters, tries to change the history they know by averting upcoming disasters.", "title" : "Thrill Seekers" }
{ "_id" : ObjectId("573a13d8f29313caabda6557"), "plot" : "At the age of 21, Tim discovers he can travel in time and change what happens and has happened in his own life. His decision to make his world a better place by getting a girlfriend turns out not to be as easy as you might think.", "title" : "About Time" }
{ "_id" : ObjectId("573a13a5f29313caabd13b4b"), "plot" : "Hoping to alter the events of the past, a 19th century inventor instead travels 800,000 years into the future, where he finds humankind divided into two warring races.", "title" : "The Time Machine" }
{ "_id" : ObjectId("573a13aef29313caabd2e2d7"), "plot" : "After using his mother's newly built time machine, Dolf gets stuck involuntary in the year 1212. He ends up in a children's crusade where he confronts his new friends with modern techniques...", "title" : "Crusade in Jeans" }
{ "_id" : ObjectId("573a1399f29313caabceec0e"), "plot" : "An officer for a security agency that regulates time travel, must fend for his life against a shady politician who has a tie to his past.", "title" : "Timecop" }
{ "_id" : ObjectId("573a1399f29313caabcee36f"), "plot" : "A time-travel experiment in which a robot probe is sent from the year 2073 to the year 1973 goes terribly wrong thrusting one of the project scientists, a man named Nicholas Sinclair into a...", "title" : "A.P.E.X." }
{ "_id" : ObjectId("573a13c6f29313caabd715d3"), "plot" : "Agent J travels in time to M.I.B.'s early days in 1969 to stop an alien from assassinating his friend Agent K and changing history.", "title" : "Men in Black 3" }
{ "_id" : ObjectId("573a13d4f29313caabd98c13"), "plot" : "Bound by a shared destiny, a teen bursting with scientific curiosity and a former boy-genius inventor embark on a mission to unearth the secrets of a place somewhere in time and space that exists in their collective memory.", "title" : "Tomorrowland" }
{ "_id" : ObjectId("573a13b6f29313caabd477fa"), "plot" : "With the help of his uncle, a man travels to the future to try and bring his girlfriend back to life.", "title" : "Love Story 2050" }
{ "_id" : ObjectId("573a13e5f29313caabdc40c9"), "plot" : "A dimension-traveling wizard gets stuck in the 21st century because cell-phone radiation interferes with his magic. With his home world on the brink of war, he seeks help from a jaded ...", "title" : "The Portal" }

有关 Atlas 向量搜索、Atlas 向量搜索索引以及如何将其集成到您的应用程序中的更多信息，请参阅 Atlas 手册中的 Atlas 向量搜索概述。有关使用 .NET/C# 驱动程序运行 Atlas 向量搜索查询的更多示例，请参阅 Atlas 手册中的 运行向量搜索查询 并从下拉菜单中选择 C#。

位运算符

本节介绍了.NET/C#驱动程序支持的位运算符，您可以在聚合管道中使用它。您可以在同一阶段使用多个位运算符。使用位运算符时，以下指南适用

  • 所有操作数必须是类型int或long。

  • $bitAnd、$bitOr和$bitXor接受两个或更多操作数。$bitNot接受一个操作数。

  • 位运算从左到右进行评估。

本节中的示例使用名为ingredients的集合中的以下文档

{ "_id" : 1, "name" : "watermelon", "is_available" : 1, "is_cheap" : 1 },
{ "_id" : 2, "name" : "onions", "is_available" : 1, "is_cheap" : 0 },
{ "_id" : 3, "name" : "eggs", "is_available" : 0, "is_cheap" : 0 },
{ "_id" : 4, "name" : "potatoes", "is_available" : 1, "is_cheap" : 1 },
{ "_id" : 5, "name" : "pasta", "is_available" : 0, "is_cheap" : 1 },
{ "_id" : 6, "name" : "cheese", "is_available" : 1 }

"is_available"字段表示一个配料是否可用。如果此字段值为0，则配料不可用。如果值为1，则配料可用。

"is_cheap"字段表示一个配料是否便宜。如果此字段值为0，则配料不便宜。如果值为1，则配料便宜。

以下Ingredient类表示名为ingredients的集合中的文档

public class Ingredient 
{
    public int Id { get; set; }
    public string Name { get; set; }
    [BsonElement("is_available")]
    public int? IsAvailable { get; set; }
    [BsonElement("is_cheap")]
    public int? IsCheap { get; set; }
}

注意：

缺少或未定义的操作数

如果传递给任何位运算符的操作数是 可空 int 或 long，并且包含缺失或未定义的值，则整个表达式求值为 null。如果操作数是类型为非可空的 int 或 long 并包含缺失或未定义的值，.NET/C# 驱动程序将抛出错误。

$bitAnd

$bitAnd 聚合操作符对给定的参数执行位与运算。您可以通过使用 & 字符将两个或多个子句连接起来来使用 $bitAnd 操作符。

以下示例显示了如何使用 LINQ 创建 $bitAnd 阶段。该代码检索具有 Name 字段值为 "watermelon" 的文档。然后在此文档中对 IsAvailable 和 IsCheap 字段的值执行位与运算。

var query = queryableCollection
    .Where(i => i.Name == "watermelon")
    .Select(i => i.IsAvailable & i.IsCheap);

前面的代码返回 1，这是对 IsAvailable 字段（值为 1）和 IsCheap 字段（值为 1）执行 AND 运算的结果。

以下示例对集合中的所有文档执行相同的位与运算

var query = queryableCollection
    .Select(i => i.IsAvailable & i.IsCheap);
1
0
0
1
0
null

空值结果来自具有 Name 字段值为 "cheese" 的文档。此文档缺少 IsCheap 字段，因此表达式求值为 null。

$bitOr

$bitOr 聚合运算符对给定的参数执行位或操作。您可以通过使用 | 字符将两个或多个子句连接起来来使用 $bitOr 运算符。

以下示例展示了如何使用 LINQ 创建 $bitOr 阶段。该代码检索具有 "onions" 值的 Name 字段的文档。然后在该文档中对 IsAvailable 和 IsCheap 字段的值执行位或操作。

var query = queryableCollection
    .Where(i => i.Name == "onions")
    .Select(i => i.IsAvailable | i.IsCheap);

前面的代码返回 1，这是对 IsAvailable 字段（值为 1）和 IsCheap 字段（值为 0）的位或操作的结果。

$bitNot

$bitNot 聚合运算符对给定的参数执行位非操作。您可以通过在操作数前加上 ~ 字符来使用 $bitNot 运算符。$bitNot 只接受一个参数。以下示例展示了如何使用 LINQ 创建 $bitNot 阶段

var query = queryableCollection
    .Select(i => ~i.IsCheap);
-2
-1
-1
-2
-2
null

$bitXor

《$bitXor》聚合运算符对给定的参数执行位异或操作。您可以使用《$bitXor》运算符通过用^字符连接两个或多个子句来使用。

以下示例展示了如何使用LINQ创建《$bitXor》阶段。该代码检索具有《Name》字段值为“watermelon”或“onions”的文档。然后对这些文档中《IsAvailable》和《IsCheap》字段的值执行位异或操作。

var query = queryableCollection
    .Where(i => i.Name == "watermelon" || i.Name == "onions")
    .Select(i => i.IsAvailable ^ i.IsCheap);

结果包含以下值

0
1

不支持的聚合阶段

MongoDB .NET/C# 驱动程序的LINQ实现不支持以下聚合阶段

  • $redact

  • $geoNear

  • $out

有关如何使用Builders创建包含《$out》阶段的聚合管道的信息，请参阅将管道结果写入集合部分。

支持的方法

以下是一些MongoDB .NET/C# LINQ驱动程序实现所支持的方法

方法名称
描述

Any

确定是否存在任何文档符合指定的标准

Average

计算指定字段的平均值

Count

返回一个表示符合指定标准的文档数量的Int32

LongCount

返回一个表示符合指定标准的文档数量的Int64

Distinct

返回符合指定标准的唯一文档

First

返回第一个匹配的文档，如果没有找到则抛出异常

FirstOrDefault

返回第一个匹配的文档，如果没有找到则返回null

GroupBy

根据指定的标准对文档进行分组

GroupJoin

在同一数据库中的另一个集合上执行左外连接

Max

返回具有最大指定值的文档

OfType

返回符合指定类型的文档

OrderBy、OrderByDescending

返回按指定顺序排序的结果

ThenBy、ThenByDescending

允许指定次要排序

Select

根据指定的标准选择文档

SelectMany

将序列中的每个元素投影并合并生成的序列到一个文档中

Single

返回唯一的匹配文档，如果没有找到或找到多个则抛出异常

SingleOrDefault

返回单个匹配文档，如果没有找到则返回null

Skip

跳过指定数量的文档并返回剩余的结果

Sum

返回指定字段中值的总和

Take

指定要返回的结果数量

Where

返回所有符合指定标准的文档

查看已翻译的查询

当你运行LINQ查询时，.NET/C#驱动程序会自动将你的查询翻译成使用MongoDB查询API编写的聚合管道。你可以通过使用ToString()方法或LoggedStages属性来查看已翻译的查询。

要查看非标量操作的已翻译查询，请使用ToString()方法。非标量操作是返回查询对象的操作，例如

  • Where

  • Select

  • SelectMany

  • GroupJoin

以下示例在LINQ查询上调用ToString()方法并打印已翻译的查询

 var queryableCollection = _restaurantsCollection.AsQueryable();
 var query = queryableCollection
      .Where(r => r.Name == "The Movable Feast");
 var queryTranslated = query.ToString();
 Console.WriteLine(queryTranslated);
sample_restaurants.restaurants.Aggregate([{ "$match" : { "name" : "The Movable Feast" } }])

要获取标量操作的已翻译查询，请使用LoggedStages属性。标量操作是返回标量结果而不是查询对象的操作，例如

  • First

  • Sum

  • Count

  • 最小值

  • Max

要使用LoggedStages属性获取已翻译的查询，你必须直接在执行之后、在执行任何其他相同查询对象上的查询之前保存已翻译的查询。

以下示例在执行标量操作的LINQ查询上使用LoggedStages属性，然后打印已翻译的查询

var queryableCollection = _restaurantsCollection.AsQueryable();
var query = queryableCollection
    .Where(r => r.Name == "The Movable Feast");
var result = query.FirstOrDefault();
var queryTranslated = query.LoggedStages;
Console.WriteLine(queryTranslated.ToJson());
[{ "$match" : { "name" : "The Movable Feast" } }, { "$limit" : NumberLong(1) }]

重要

LoggedStages不是线程安全的。从多个线程执行查询并访问相关的LoggedStages属性可能会导致不可预测的结果。

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