Add parameter binding documentation

src/.vuepress/config.js

@@ -96,7 +96,7 @@ module.exports = {
         {
           title: "Guide",
           collapsable: true,
-          children: ["", "routing" ],
+          children: ["", "routing", "parameter-binding" ],
         },
       ],
     },

src/guide/README.md

@@ -5,6 +5,6 @@ Level
 
 Beginner 🍃
 
-Intermidate 🍃🍃
+Intermediate 🍃🍃
 
 Advance 🍃🍃🍃

src/guide/parameter-binding.md

@@ -0,0 +1,150 @@
+# Parameter Binding
+
+**Level: Intermediate** 🍃🍃
+
+Starting in .NET 6, route handlers (the Delegate parameters in [EndpointRouteBuilderExtensions](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.builder.endpointroutebuilderextensions) methods) can accept more than just an `HttpContext` as a parameter. Route handlers can now accept any number of parameters with various types bound from different sources. This guide describes the conventions that determine how each parameter is populated.
+
+## Conventions
+
+```cs
+app.MapPut("/todos/{id}", async (TodoDb db, TodoItem updateTodo, int id) =>
+{
+    // ...
+});
+```
+
+The route handler above accepts three parameters:
+
+1. `TodoDb db` which comes from the service provider.
+2. `TodoItem updateTodo` which is read as JSON from the request body.
+3. `int id` which is read from the `{id}` segment of the route.
+
+These were all determined by convention but could be specified explicitly with attributes as follows:
+
+```cs
+using Microsoft.AspNetCore.Mvc;
+
+// ...
+
+app.MapPut("/todos/{id}", async (
+    [FromService] TodoDb db,
+    [FromBody] TodoItem updateTodo,
+    [FromRoute(Name = "id")] int nameDoesNotMatter) =>
+{
+    // ...
+});
+```
+
+Parameters sources are determined using the following rules applied in order:
+
+1. [Parameter attributes](#attributes) take precedence over conventions if they are present.
+2. Any [well-known types](#well-known-types) are bound from the the [HttpContext](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.http.httpcontext) or one of its properties.
+3. `string` parameters are bound from `HttpContext.RouteValues[{ParameterName}]` or `HttpContext.Query[{ParameterName}]` depending on whether `{ParameterName}` is part of the route pattern.
+4. Types with public static [BindAsync](#bindasync) methods are bound using `BindAsync`.
+5. Types with public static [TryParse](#tryparse) methods are bound by calling `TryParse` with `HttpContext.RouteValues[{ParameterName}]` or `HttpContext.Query[{ParameterName}]` depending on whether `{ParameterName}` is part of the route pattern. This includes most built-in numeric types, enums, `DateTime`, `TimeSpan` and more.
+6. Types registered as a service are bound from request services.
+7. Any remaining types are bound from the request body as JSON.
+
+## Attributes
+
+### `[FromRoute]`
+
+[FromRouteAttribute](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.fromrouteattribute) implements `Microsoft.AspNetCore.Http.Metadata.IFromRouteMetadata`. Any attribute implementing this interface is equivalent. This will bind the parameter from `HttpRequest.RouteValues[{ParameterName}]`. If the parameter is not a string, the parameter type's [TryParse](#tryparse) method will be called to convert the string to the parameter type.
+
+If the `Name` property is provided (e.g. `[FromRoute(Name = "id")]`), the name specified using the property is used instead of the parameter name.
+
+### `[FromQuery]`
+
+[FromQueryAttribute](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.fromqueryattribute) implements `Microsoft.AspNetCore.Http.Metadata.IFromQueryMetadata`. Any attribute implementing this interface is equivalent. This will bind the parameter from `HttpRequest.Query[{ParameterName}]`. If the parameter is not a string, the parameter type's [TryParse](#tryparse) method will be called to convert the string to the parameter type.
+
+If the `Name` property is provided (e.g. `[FromQuery(Name = "page")]`), the name specified using the property is used instead of the parameter name.
+
+### `[FromHeader]`
+
+[FromHeaderAttribute](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.fromheaderattribute) implements `Microsoft.AspNetCore.Http.Metadata.IFromHeaderMetadata`. Any attribute implementing this interface is equivalent. This will bind the parameter from `HttpRequest.Headers[{ParameterName}]`. If the parameter is not a string, the parameter type's [TryParse](#tryparse) method will be called to convert the string to the parameter type.
+
+If the `Name` property is provided (e.g. `[FromHeader(Name = "X-My-Custom-Header")]`), the name specified using the property is used instead of the parameter name.
+
+### `[FromServices]`
+
+[FromServicesAttribute](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.fromservicesattribute) implements `Microsoft.AspNetCore.Http.Metadata.IFromServiceMetadata`. Any attribute implementing this interface is equivalent. This will bind the parameter from request services as described in the [Services section](#services) of this doc.
+
+### `[FromBody]`
+
+[FromBodyAttribute](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.mvc.frombodyattribute) implements `Microsoft.AspNetCore.Http.Metadata.IFromBodyMetadata`. Any attribute implementing this interface is equivalent. This will bind the parameter from the request body as JSON as described in the [JSON Request Body section](#json-request-body) of this doc.
+
+If the `EmptyBodyBehavior` property is set to `EmptyBodyBehavior.Allow` (e.g. `[FromBody(EmptyBodyBehavior = EmptyBodyBehavior.Allow)]`), the parameter will be set to `null` or its `default` value if the request body is empty. This corresponds to the `IFromBodyMetadata.AllowEmpty` being true.
+
+## Well-Known Types
+
+- HttpContext
+- HttpRequest (`HttpContext.Request`)
+- HttpResponse (`HttpContext.Response`)
+- ClaimsPrincipal (`HttpContext.User`)
+- CancellationToken (`HttpContext.RequestAborted`)
+
+## BindAsync
+
+If the parameter type, one of its parent/ancestor types or any of its implemented interfaces define a public static `BindAsync` method with one of the following signatures, the parameter will be bound using `BindAsync` assuming no parameter attribute specified another source.
+
+```cs
+public static ValueTask<{ParameterType}> BindAsync(HttpContext context, ParameterInfo parameter)
+{
+    // ...
+}
+
+// Or
+
+public static ValueTask<{ParameterType}> BindAsync(HttpContext context)
+{
+    // ...
+}
+```
+
+The return value can be either `ValueTask<{ParameterType}>` or `ValueTask<{ParameterType}?>` for both overloads. Whether returning a `null` value is allowed is determined by the nullability of parameter type. If the parameter type is non-nullable, the route handler will not be called and a bad request will be logged.
+
+In the case where both overloads are defined anywhere in the parameter type's hierarchy, the `BindAsync` method with the `ParameterInfo` argument will be called.
+
+If there is more than one `BindAsync` method with the same signature, the method from the most derived type will be called. `BindAsync` methods on interfaces are chosen last. A parameter type implementing more than one interface defining matching `BindAsync` methods is an error.
+
+## TryParse
+
+If the parameter type, one of its parent/ancestor types or any of its implemented interfaces define a public static `TryParse` method with one of the following signatures, the parameter will be bound using `TryParse` using the `string` from the source specified in the [Conventions section](#conventions) of this document.
+
+```cs
+public static bool TryParse(string? value, IFormatProvider formatProvider, out {ParameterType} result)
+{
+    // ...
+}
+
+// Or
+
+public static bool TryParse(string? value, out {ParameterType} result)
+{
+    // ...
+}
+```
+
+The out parameter can be either `out {ParameterType}` or `out {ParameterType}?` for both overloads. Whether providing a `null` value is allowed is determined by the nullability of parameter type. If the parameter type is non-nullable, the route handler will not be called and a bad request will be logged.
+
+In the case where both overloads are defined anywhere in the parameter type's hierarchy, the `TryParse` method with the `IFormatProvider` argument will be called.
+
+If there is more than one `TryParse` method with the same signature, the method from the most derived type will be called. `TryParse` methods on interfaces are chosen last. A parameter type implementing more than one interface defining matching `TryParse` methods is an error.
+
+## Services
+
+Service parameters are resolved from `HttpContext.RequestServices.GetService(typeof({ParameterType}))`.
+
+Whether or not a given parameter type is a service is determined using `IServiceProviderIsService` unless the parameter is explicitly attributed with `[FromServices]`. Given the `[FromServices]` attribute, the parameter type is assumed to exist.
+
+For non-nullable parameters, the parameter type must be resolvable as a service for the route handler to be called. If the service does not exist, an exception will be thrown when the endpoint is hit. For Nullable
+
+[IServiceProviderIsService](https://docs.microsoft.com/dotnet/api/microsoft.extensions.dependencyinjection.iserviceproviderisservice.isservice) is a new interface introduced in .NET 6 that automatically implemented by the default service provider and some third-party containers. If `IServiceProviderIsService` itself is not available as a service, the `[FromServices]` attribute must be used to resolve parameters from services.
+
+## JSON Request Body
+
+JSON request bodies are read using the [ReadFromJsonAsync](https://docs.microsoft.com/dotnet/api/system.net.http.json.httpcontentjsonextensions.readfromjsonasync#System_Net_Http_Json_HttpContentJsonExtensions_ReadFromJsonAsync__1_System_Net_Http_HttpContent_System_Text_Json_JsonSerializerOptions_System_Threading_CancellationToken_) extension method. This can be configured like all other calls to `ReadFromJsonAsync` using the [options pattern](https://docs.microsoft.com/aspnet/core/fundamentals/configuration/#configure-options-with-a-delegate-1) to configure [JsonOptions](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.http.json.jsonoptions).
+
+For non-nullable parameters, empty request bodies are disallowed by default. If a request matching the route pattern has an empty body, the route handler will not be called and a bad request will be logged.
+
+Empty request bodies are always allowed when the parameter is nullable even if `EmptyBodyBehavior.Disallow` is set via the `[FromBody]` attribute.