Resolved #487: Validation document.

docs/en/FluentValidation.md

@@ -0,0 +1,3 @@
+# FluentValidation Integration
+
+TODO

docs/en/Logging.md

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+# Logging
+
+ABP Framework doesn't implement any logging infrastructure. It uses the [ASP.NET Core's logging system](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/logging).
+
+> .NET Core's logging system is actually independent from the ASP.NET Core. It is usable in any type of application.

docs/en/Validation.md

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-## Validation
+# Validation
 
-TODO
+Validation system is used to validate the user input or client request for a particular controller action or service method.
+
+ABP is compatible with the ASP.NET Core Model Validation system and everything written in [its documentation](https://docs.microsoft.com/en-us/aspnet/core/mvc/models/validation) is already valid for ABP based applications. So, this document mostly focuses on the ABP features rather than repeating the Microsoft documentation.
+
+In addition, ABP adds the following benefits:
+
+* Defines `IValidationEnabled` to add automatic validation to an arbitrary class. Since all the [application services](Application-Services.md) inherently implements it, they are also validated automatically.
+* Automatically localize the validation errors.
+* Provides extensible services to validate a method call or an object state.
+* Provides [FluentValidation](https://fluentvalidation.net/) integration.
+
+## Validating DTOs
+
+This section briefly introduces the validation system. For details, see the [ASP.NET Core validation documentation](https://docs.microsoft.com/en-us/aspnet/core/mvc/models/validation).
+
+### Data Annotation Attributes
+
+Using data annotations is a simple way to implement the formal validation for a [DTO](Data-Transfer-Objects.md) in a declarative way. Example:
+
+````csharp
+public class CreateBookDto
+{
+    [Required]
+    [StringLength(100)]
+    public string Name { get; set; }
+
+    [Required]
+    [StringLength(1000)]
+    public string Description { get; set; }
+
+    [Range(0, 999.99)]
+    public decimal Price { get; set; }
+}
+````
+
+When you use this class as a parameter to an [application service](Application-Services.md) or a controller, it is automatically validated and a localized validation exception is thrown ([and handled](Exception-Handling.md) by the ABP framework).
+
+### IValidatableObject
+
+`IValidatableObject` can be implemented by a DTO to perform custom validation logic. `CreateBookDto`  in the following example implements this interface and checks if the `Name` is equals to the `Description` and returns a validation error in this case.
+
+````csharp
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+
+namespace Acme.BookStore
+{
+    public class CreateBookDto : IValidatableObject
+    {
+        [Required]
+        [StringLength(100)]
+        public string Name { get; set; }
+
+        [Required]
+        [StringLength(1000)]
+        public string Description { get; set; }
+
+        [Range(0, 999.99)]
+        public decimal Price { get; set; }
+
+        public IEnumerable<ValidationResult> Validate(
+            ValidationContext validationContext)
+        {
+            if (Name == Description)
+            {
+                yield return new ValidationResult(
+                    "Name and Description can not be the same!",
+                    new[] { "Name", "Description" }
+                );
+            }
+        }
+    }
+}
+````
+
+#### Resolving a Service
+
+If you need to resolve a service from the [dependency injection system](Dependency-Injection.md), you can use the `ValidationContext` object. Example:
+
+````csharp
+var myService = validationContext.GetRequiredService<IMyService>();
+````
+
+> While resolving services in the `Validate` method allows any possibility, it is not a good practice to implement your domain validation logic in DTOs. Keep DTOs simple. Their purpose is to transfer data (DTO: Data Transfer Object).
+
+## Validation Infrastructure
+
+This section explains a few additional services provided by the ABP framework.
+
+### IValidationEnabled Interface
+
+`IValidationEnabled` is an empty marker interface that can be implemented by any class (registered to and resolved from the [DI](Dependency-Injection.md)) to let the ABP framework perform the validation system for the methods of the class. Example:
+
+````csharp
+using System.Threading.Tasks;
+using Volo.Abp.DependencyInjection;
+using Volo.Abp.Validation;
+
+namespace Acme.BookStore
+{
+    public class MyService : ITransientDependency, IValidationEnabled
+    {
+        public virtual async Task DoItAsync(MyInput input)
+        {
+            //...
+        }
+    }
+}
+````
+
+> ABP framework uses the [dynamic proxying / interception](Dynamic-Proxying-Interceptors.md) system to perform the validation. In order to make it working, your method should be **virtual** or your service should be injected and used over an **interface** (like `IMyService`).
+
+### AbpValidationException
+
+Once ABP determines a validation error, it throws an exception of type `AbpValidationException`. Your application code can throw `AbpValidationException`, but most of the times it is not needed.
+
+* `ValidationErrors` property of the `AbpValidationException` contains the validation error list.
+* Log level of the `AbpValidationException` is set to `Warning`. It logs all the validation errors to the [logging system](Logging.md).
+* `AbpValidationException` is automatically caught by the ABP framework and converted to a usable error into with HTTP 400 status code. See the [exception handling](Exception-Handling.md) document for more.
+
+## FluentValidation Integration
+
+Volo.Abp.FluentValidation package integrates the FluentValidation library to the validation system. See the [FluentValidation Integration document](FluentValidation.md) for more.

docs/en/docs-nav.json

@@ -87,7 +87,14 @@
           "path": "Exception-Handling.md"
         },
         {
-          "text": "Validation"
+          "text": "Validation",
+          "path": "Validation.md",
+          "items": [
+            {
+              "text": "FluentValidation Integration",
+              "path": "FluentValidation.md"
+            }
+          ]
         },
         {
           "text": "Authorization",