61 lines
2.3 KiB
Markdown
61 lines
2.3 KiB
Markdown
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---
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title: LINQ
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localeTitle: LINQ
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---
# LINQ（语言集成查询）
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LINQ（语言集成查询）是一种Microsoft编程模型和方法，它基本上将正式查询功能添加到基于Microsoft .NET的编程语言中。 LINQ为操作数据提供了紧凑，富有表现力和可理解的语法。 LINQ的真正价值在于它能够将相同的查询应用于SQL数据库，数据集，列表，字典等。
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## 例
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LINQ可用于过滤，转换，搜索数据以及更多复杂任务。假设我们有以下对象列表：
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```csharp
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var fruits = new List<Fruit>() {
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new Fruit() { Id = 1, Name = "Orange", Color = "Orange", Quantity: 3 },
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new Fruit() { Id = 2, Name = "Strawberry", Color = "Red", Quantity: 12 },
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new Fruit() { Id = 3, Name = "Grape", Color = "Purple", Quantity: 25 },
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new Fruit() { Id = 4, Name = "Pineapple", Color = "Yellow", Quantity: 1 },
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new Fruit() { Id = 5, Name = "Apple", Color = "Red", Quantity: 5 },
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new Fruit() { Id = 6, Name = "Mango", Color = "Yellow", Quantity: 2 }
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}
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```
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然后我们可以做以下事情：
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```csharp
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// Get the name of the first fruit
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var firstName = fruits.Select(f => f.Name).First(); // Orange
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// Count how many fruits are red
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var qntRed = fruits.Where(Color == "Red").Count(); // 2
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// Create a list of yellow fruits
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var yellowFruits = fruits.Where(f => f.Color == "Yellow").ToList(); // { Pineapple, Mango }
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// Orders list by quantity from most to less
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var orderedFruits = fruits.OrderByDescending(f => f.Quantity).ToList(); // {Grape, Strawberry, Orange, Apple, Mango, Pineapple}
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// Sum the quantity of fruits
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var quantity = fruits.Sum(f => f.Quantity); // 53
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// Check if there are any green fruits
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var hasGreen = fruits.Any(f => f.Color == "Green"); // false
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// Group fruits by color into a dictionary
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var fruitsByColor = fruits.GroupBy(g => g.Color).ToDictionary(k => k.Key, v => v.ToList()); // Dictionary of list of fruits by color
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// linq operations can be concatenated and are not performed as long as data is needed
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var logs = new List<Log>;
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if (filterBySeverity)
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logs = logs.Where(p => p.Severity == severity);
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//IQueryable
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if (filterByUser)
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logs = logs.Where(p => p.User == user);
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//IQueryable
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result = logs.ToList();
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//List<log>
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```
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