Support runtime property attributes via TypeDescriptor

Directory.Packages.props

@@ -3,6 +3,7 @@
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
   </PropertyGroup>
   <ItemGroup>
+    <PackageVersion Include="PolySharp" Version="1.13.2" />
     <PackageVersion Include="System.ComponentModel.Annotations" Version="5.0.0" />
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="1.1.1" />
     <PackageVersion Include="Swashbuckle.AspNetCore" Version="6.4.0" />
@@ -13,4 +14,4 @@
     <PackageVersion Include="BenchmarkDotNet" Version="0.13.2" />
     <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="7.0.0" />
   </ItemGroup>
-</Project>
+</Project>

src/MiniValidation/MiniValidation.csproj

@@ -15,6 +15,10 @@
   <ItemGroup>
     <None Include="../../README.md" Pack="true" PackagePath="\" />
     <PackageReference Include="Microsoft.SourceLink.GitHub" PrivateAssets="All" />
+    <PackageReference Include="PolySharp">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
 </Project>

src/MiniValidation/TypeDetailsCache.cs

@@ -1,7 +1,9 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.ComponentModel.DataAnnotations;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
 
@@ -198,6 +200,11 @@ private static (ValidationAttribute[]?, DisplayAttribute?, SkipRecursionAttribut
             ? paramAttributes.Concat(propertyAttributes)
             : propertyAttributes;
 
+        if (TryGetAttributesViaTypeDescriptor(property, out var typeDescriptorAttributes))
+        {
+            customAttributes = customAttributes.Concat(typeDescriptorAttributes.Cast<Attribute>());
+        }
+
         foreach (var attr in customAttributes)
         {
             if (attr is ValidationAttribute validationAttr)
@@ -218,6 +225,23 @@ private static (ValidationAttribute[]?, DisplayAttribute?, SkipRecursionAttribut
         return new(validationAttributes?.ToArray(), displayAttribute, skipRecursionAttribute);
     }
 
+    private static bool TryGetAttributesViaTypeDescriptor(PropertyInfo property, [NotNullWhen(true)] out IEnumerable<Attribute>? typeDescriptorAttributes)
+    {
+        var attributes = TypeDescriptor.GetProperties(property.ReflectedType!)
+            .Cast<PropertyDescriptor>()
+            .FirstOrDefault(x => x.Name == property.Name)
+            ?.Attributes;
+
+        if (attributes is { Count: > 0 } tdps)
+        {
+            typeDescriptorAttributes = tdps.Cast<Attribute>();
+            return true;
+        }
+
+        typeDescriptorAttributes = null;
+        return false;
+    }
+
     private static Type? GetEnumerableType(Type type)
     {
         if (type.IsInterface && type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>))

tests/MiniValidation.UnitTests/TestTypes.cs

@@ -238,3 +238,8 @@ class ClassWithUri
     [Required]
     public Uri? BaseAddress { get; set; }
 }
+
+class TestTypeForTypeDescriptor
+{
+    public string? PropertyToBeRequired { get; set; }
+}

tests/MiniValidation.UnitTests/TryValidate.cs

@@ -1,4 +1,3 @@
-using System.Runtime.CompilerServices;
 using Microsoft.Extensions.DependencyInjection;
 
 namespace MiniValidation.UnitTests;
@@ -396,4 +395,19 @@ public async Task TryValidateAsync_With_ServiceProvider()
         Assert.Equal(1, errors.Count);
         Assert.Equal(nameof(IServiceProvider), errors.Keys.First());
     }
+
+    [Fact]
+    public async Task TryValidateAsync_With_Attribute_Attached_Via_TypeDescriptor()
+    {
+        var thingToValidate = new TestTypeForTypeDescriptor();
+
+        typeof(TestTypeForTypeDescriptor).AttachAttribute(
+            nameof(TestTypeForTypeDescriptor.PropertyToBeRequired), 
+            _ => new RequiredAttribute());
+
+        var (isValid, errors) = await MiniValidator.TryValidateAsync(thingToValidate);
+
+        Assert.False(isValid);
+        Assert.Equal(1, errors.Count);
+    }
 }

tests/MiniValidation.UnitTests/TypeDescriptorExtensions.cs

@@ -0,0 +1,100 @@
+using System.ComponentModel;
+
+namespace MiniValidation.UnitTests
+{
+    internal static class TypeDescriptorExtensions
+    {
+        public static void AttachAttribute(this Type type, string propertyName, Func<PropertyDescriptor, Attribute> attributeFactory)
+        {
+            var ctd = new PropertyOverridingTypeDescriptor(TypeDescriptor.GetProvider(type).GetTypeDescriptor(type)!);
+
+            foreach (PropertyDescriptor pd in TypeDescriptor.GetProperties(type))
+            {
+                if (pd.Name == propertyName)
+                {
+                    var pdWithAttribute = TypeDescriptor.CreateProperty(
+                        type,
+                        pd,
+                        attributeFactory(pd));
+
+                    ctd.OverrideProperty(pdWithAttribute);
+                }
+            }
+
+            TypeDescriptor.AddProvider(new TypeDescriptorOverridingProvider(ctd), type);
+        }
+    }
+
+    // From https://stackoverflow.com/questions/12143650/how-to-add-property-level-attribute-to-the-typedescriptor-at-runtime
+    internal class PropertyOverridingTypeDescriptor : CustomTypeDescriptor
+    {
+        private readonly Dictionary<string, PropertyDescriptor> overridePds = new Dictionary<string, PropertyDescriptor>();
+
+        public PropertyOverridingTypeDescriptor(ICustomTypeDescriptor parent)
+            : base(parent)
+        { }
+
+        public void OverrideProperty(PropertyDescriptor pd)
+        {
+            overridePds[pd.Name] = pd;
+        }
+
+        public override object? GetPropertyOwner(PropertyDescriptor? pd)
+        {
+            var propertyOwner = base.GetPropertyOwner(pd);
+
+            if (propertyOwner == null)
+            {
+                return this;
+            }
+
+            return propertyOwner;
+        }
+
+        public override PropertyDescriptorCollection GetProperties()
+        {
+            return GetPropertiesImpl(base.GetProperties());
+        }
+
+        public override PropertyDescriptorCollection GetProperties(Attribute[]? attributes)
+        {
+            return GetPropertiesImpl(base.GetProperties(attributes));
+        }
+
+        private PropertyDescriptorCollection GetPropertiesImpl(PropertyDescriptorCollection pdc)
+        {
+            var pdl = new List<PropertyDescriptor>(pdc.Count + 1);
+
+            foreach (PropertyDescriptor pd in pdc)
+            {
+                if (overridePds.ContainsKey(pd.Name))
+                {
+                    pdl.Add(overridePds[pd.Name]);
+                }
+                else
+                {
+                    pdl.Add(pd);
+                }
+            }
+
+            var ret = new PropertyDescriptorCollection(pdl.ToArray());
+
+            return ret;
+        }
+    }
+
+    internal class TypeDescriptorOverridingProvider : TypeDescriptionProvider
+    {
+        private readonly ICustomTypeDescriptor ctd;
+
+        public TypeDescriptorOverridingProvider(ICustomTypeDescriptor ctd)
+        {
+            this.ctd = ctd;
+        }
+
+        public override ICustomTypeDescriptor? GetTypeDescriptor(Type objectType, object? instance)
+        {
+            return ctd;
+        }
+    }
+}