Add validation filters with FluentValidation support.

Author: imclint21

src/Initium/ApiController.cs

@@ -8,6 +8,7 @@ namespace Initium;
 [Route("api/[controller]")]
 [TypeFilter(typeof(ApiExceptionFilter))]
 [TypeFilter(typeof(ApiResponseFilter))]
+[TypeFilter(typeof(ImplicitValidationFilter))]
 public class ApiController : BaseController;
 
 public class ApiController<TService>(TService service) : ApiController where TService : class

src/Initium/Attributes/ValidateModelAttribute.cs

@@ -0,0 +1,45 @@
+using System.Diagnostics.CodeAnalysis;
+using FluentValidation;
+using Initium.Response;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace Initium.Attributes;
+
+/// <summary>
+/// An attribute that performs model validation using a specified FluentValidation validator.
+/// </summary>
+[AttributeUsage(AttributeTargets.Method)]
+[SuppressMessage("ReSharper", "UnusedType.Global")]
+public class ValidateModelAttribute(Type validatorType) : Attribute, IActionFilter
+{
+	/// <summary>
+	/// Called before the action method executes. Validates the model using the specified validator.
+	/// </summary>
+	/// <param name="context">The context for the action execution.</param>
+	public void OnActionExecuting(ActionExecutingContext context)
+	{
+		var argument = context.ActionArguments.Values.FirstOrDefault();
+		if (argument == null) return;
+
+		if (Activator.CreateInstance(validatorType) is not IValidator validator) return;
+
+		var validationContext = new ValidationContext<object>(argument);
+		var validationResult = validator.Validate(validationContext);
+
+		if (validationResult.IsValid) return;
+		
+		context.Result = ApiResponseBuilder
+			.CreateFromContext(context.HttpContext)
+			.WithMessage("One or more validation errors occurred.")
+			.WithStatusCode(StatusCodes.Status400BadRequest)
+			.WithErrors(validationResult.Errors)
+			.BuildAsJsonResult();
+	}
+
+	/// <summary>
+	/// Called after the action method executes. No operation is performed in this implementation.
+	/// </summary>
+	/// <param name="context">The context for the executed action.</param>
+	public void OnActionExecuted(ActionExecutedContext context) { }
+}

src/Initium/Filters/ImplicitValidationFilter.cs

@@ -0,0 +1,49 @@
+using FluentValidation;
+using Initium.Infrastructure;
+using Initium.Response;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace Initium.Filters;
+
+/// <summary>
+/// A filter that performs implicit validation on actions using requests derived from <see cref="BaseRequestWithValidator{T}"/>.
+/// </summary>
+internal class ImplicitValidationFilter : IActionFilter
+{
+    /// <summary>
+    /// Called before the action executes, to validate the action arguments.
+    /// </summary>
+    /// <param name="context">The context for the action execution.</param>
+    public void OnActionExecuting(ActionExecutingContext context)
+    {
+        var argument = context.ActionArguments.Values.FirstOrDefault();
+        if (argument == null) return;
+
+        var argumentType = argument.GetType();
+        var baseType = argumentType.BaseType;
+
+        if (baseType is not { IsGenericType: true } || baseType.GetGenericTypeDefinition() != typeof(BaseRequestWithValidator<>)) return;
+
+        var validatorType = baseType.GetGenericArguments().FirstOrDefault();
+        if (validatorType == null || Activator.CreateInstance(validatorType) is not IValidator validator) return;
+
+        var validationContext = new ValidationContext<object>(argument);
+        var validationResult = validator.Validate(validationContext);
+
+        if (validationResult.IsValid) return;
+
+        context.Result = ApiResponseBuilder
+            .CreateFromContext(context.HttpContext)
+            .WithMessage("One or more validation errors occurred.")
+            .WithStatusCode(StatusCodes.Status400BadRequest)
+            .WithErrors(validationResult.Errors)
+            .BuildAsJsonResult();
+    }
+
+    /// <summary>
+    /// Called after the action executes. No operation is performed in this implementation.
+    /// </summary>
+    /// <param name="context">The context for the executed action.</param>
+    public void OnActionExecuted(ActionExecutedContext context) { }
+}