Add external validator support

README.md

@@ -68,6 +68,39 @@ class Widget : IValidatableObject
 }
 ```
 
+### Use external validators
+
+External validators can be resolved from the `IServiceProvider` passed to `MiniValidator`. Register one or more `IValidate<T>` services (or `IAsyncValidate<T>` services when calling `TryValidateAsync`):
+
+```csharp
+using System.ComponentModel.DataAnnotations;
+using Microsoft.Extensions.DependencyInjection;
+using MiniValidation;
+
+var services = new ServiceCollection();
+services.AddSingleton<IValidate<Widget>, WidgetValidator>();
+using var serviceProvider = services.BuildServiceProvider();
+
+var widget = new Widget { Name = "" };
+var isValid = MiniValidator.TryValidate(widget, serviceProvider, out var errors);
+
+class Widget
+{
+    public string? Name { get; set; }
+}
+
+class WidgetValidator : IValidate<Widget>
+{
+    public IEnumerable<ValidationResult> Validate(Widget target, ValidationContext validationContext)
+    {
+        if (string.IsNullOrWhiteSpace(target.Name))
+        {
+            yield return new ValidationResult("Name is required.", new[] { nameof(Widget.Name) });
+        }
+    }
+}
+```
+
 ### Console app
 
 ```csharp

src/MiniValidation/IAsyncValidate.cs

@@ -0,0 +1,20 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Threading.Tasks;
+
+namespace MiniValidation;
+
+/// <summary>
+/// Provides a way for an object to be validated asynchronously by an external validator.
+/// </summary>
+/// <typeparam name="TTarget">The type of object to validate.</typeparam>
+public interface IAsyncValidate<in TTarget>
+{
+    /// <summary>
+    /// Determines whether the specified object is valid.
+    /// </summary>
+    /// <param name="target">The object to validate.</param>
+    /// <param name="validationContext">The validation context.</param>
+    /// <returns>A collection that holds failed-validation information.</returns>
+    Task<IEnumerable<ValidationResult>> ValidateAsync(TTarget target, ValidationContext validationContext);
+}

src/MiniValidation/IValidate.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+
+namespace MiniValidation;
+
+/// <summary>
+/// Provides a way for an object to be validated by an external validator.
+/// </summary>
+/// <typeparam name="TTarget">The type of object to validate.</typeparam>
+public interface IValidate<in TTarget>
+{
+    /// <summary>
+    /// Determines whether the specified object is valid.
+    /// </summary>
+    /// <param name="target">The object to validate.</param>
+    /// <param name="validationContext">The validation context.</param>
+    /// <returns>A collection that holds failed-validation information.</returns>
+    IEnumerable<ValidationResult> Validate(TTarget target, ValidationContext validationContext);
+}