Add AlsoValidatePropertyAttribute type

Author: Sergio0694

CommunityToolkit.Mvvm/ComponentModel/Attributes/AlsoValidatePropertyAttribute.cs

@@ -0,0 +1,44 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+
+namespace CommunityToolkit.Mvvm.ComponentModel;
+
+/// <summary>
+/// An attribute that can be used to support <see cref="ObservablePropertyAttribute"/> in generated properties, when applied to
+/// fields contained in a type that is inheriting from <see cref="ObservableValidator"/> and using any validation attributes.
+/// When this attribute is used, the generated property setter will also call <see cref="ObservableValidator.ValidateProperty(object?, string)"/>.
+/// This allows generated properties to opt-in into validation behavior without having to fallback into a full explicit observable property.
+/// <para>
+/// This attribute can be used as follows:
+/// <code>
+/// partial class MyViewModel : ObservableValidator
+/// {
+///     [ObservableProperty]
+///     [AlsoValidateProperty]
+///     [Required]
+///     [MinLength(2)]
+///     private string username;
+/// }
+/// </code>
+/// </para>
+/// And with this, code analogous to this will be generated:
+/// <code>
+/// partial class MyViewModel
+/// {
+///     [Required]
+///     [MinLength(2)]
+///     public string Username
+///     {
+///         get => username;
+///         set => SetProperty(ref username, value, validate: true);
+///     }
+/// }
+/// </code>
+/// </summary>
+[AttributeUsage(AttributeTargets.Field, AllowMultiple = false, Inherited = false)]
+public sealed class AlsoValidatePropertyAttribute : Attribute
+{
+}