C# Natural type for lambdas

06/22/2021 23:05:10 | China Standard Time

C# Language Design Meeting for March 3rd, 2021 提出了 Natural type for lambdas，这种语法可以让 C# 的 lambda 更加接近于 JavaScript，学习和使用成本更低，代码更加简洁。此文章将会简单介绍一下 C# Natural type for lambdas 的一些变化，以及一些使用场景和案例。

## Attributes

为 lambda 表达式添加特性

```csharp
[MyAttribute] x => x;
```

MyAttribute 应用于参数还是整个 lambda ？

通过括号来区分

```csharp
[HttpPost] ([FromBody] Todo todo) => todo
```

HttpPost 用于 lambda 而 FromBody 用于参数。

## Return types

用 : 表示返回类型，返回类型为 Todo

```csharp
(Todo todo) : Todo => todo
```

备选方案

```csharp
() -> int { ... } 
(-> int) { ... }
(return: int) { ... }
```

## Natural type for lambdas

#### ASP.NET 案例

- 在 C# 9 中

```csharp
using System;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;

var app = WebApplication.Create(args);

[HttpGet("/")]
Todo GetTodo() => new(Id: 0, Name: "Play more!", IsComplete: false);
app.MapAction((Func<Todo>)GetTodo);

[HttpPost("/")]
Todo EchoTodo([FromBody] Todo todo) => todo;
app.MapAction((Func<Todo, Todo>)EchoTodo);

[HttpGet("/id/{id?}")]
IResult GetTodoFromId([FromRoute] int? id) =>
	id is null ?
	new StatusCodeResult(404) :
	new JsonResult(new Todo(Id: id.Value, Name: "From id!", IsComplete: false));
app.MapAction((Func<int?, IResult>)GetTodoFromId);

await app.RunAsync();

record Todo(int Id, string Name, bool IsComplete);
```

- 使用自然委托类型

```csharp
using System;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc;

var app = WebApplication.Create(args);

app.MapAction([HttpGet("/")] () => new(Id: 0, Name: "Play more!", IsComplete: false));
app.MapAction([HttpPost("/")] ([FromBody] Todo todo) => todo);

await app.RunAsync();

record Todo(int Id, string Name, bool IsComplete);
```

- 自然委托 lambda 返回类型

```csharp
using System;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc;

var app = WebApplication.Create(args);

app.MapAction([HttpPost("/")] ([FromBody] Todo todo) : Todo => todo);
app.MapAction([HttpGet("/")] () : Todo => new(Id: 0, Name: "Play more!", IsComplete: false));

app.MapAction([HttpGet("/id/{id?}")] ([FromRoute] int? id) : IResult => 
	id is null ?
	new StatusCodeResult(404) :
	new JsonResult(new Todo(Id: id.Value, Name: "From id!", IsComplete: false)));

await app.RunAsync();

record Todo(int Id, string Name, bool IsComplete);
```

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