[dev-v5] Extend DefaultValues with option to set default value on a base class

src/Core/Infrastructure/DefaultValues.cs

@@ -105,16 +105,48 @@ public class DefaultValues
             return null;
         }
 
-        // Try exact type first
-        if (!_componentCache.TryGetValue(componentType, out var properties))
+        var inheritanceChain = new List<Type>();
+
+        for (var currentType = componentType; currentType is not null && currentType != typeof(object); currentType = currentType.BaseType)
+        {
+            inheritanceChain.Add(currentType);
+
+            if (currentType == typeof(FluentComponentBase))
+            {
+                break;
+            }
+        }
+
+        ConcurrentDictionary<string, object?>? mergedProperties = null;
+
+        for (var i = inheritanceChain.Count - 1; i >= 0; i--)
         {
-            // Fallback: check open generic type definition
-            if (componentType.IsGenericType)
+            if (TryGetRegisteredProperties(inheritanceChain[i], out var properties))
             {
-                _componentCache.TryGetValue(componentType.GetGenericTypeDefinition(), out properties);
+                mergedProperties ??= new ConcurrentDictionary<string, object?>(StringComparer.Ordinal);
+                foreach (var property in properties)
+                {
+                    mergedProperties[property.Key] = property.Value;
+                }
             }
         }
 
-        return properties;
+        return mergedProperties;
+    }
+
+    private bool TryGetRegisteredProperties(Type componentType, [NotNullWhen(true)] out ConcurrentDictionary<string, object?>? properties)
+    {
+        if (_componentCache.TryGetValue(componentType, out properties))
+        {
+            return true;
+        }
+
+        if (componentType.IsGenericType)
+        {
+            return _componentCache.TryGetValue(componentType.GetGenericTypeDefinition(), out properties);
+        }
+
+        properties = null;
+        return false;
     }
 }

src/Core/Infrastructure/DefaultValuesComponentBuilder.cs

@@ -50,6 +50,13 @@ public void Set<TValue>(Expression<Func<TComponent, TValue>> parameterSelector,
         var propertyName = propertyInfo?.Name
                         ?? throw new ArgumentException($"The parameter selector '{parameterSelector}' does not resolve to a public property on the component '{typeof(TComponent)}'.", nameof(parameterSelector));
 
+        if (!propertyInfo.CanWrite)
+        {
+            throw new ArgumentException($"The property '{propertyName}' on component '{typeof(TComponent)}' is read-only and cannot be used for default values.", nameof(parameterSelector));
+        }
+
+        ValidateValueCompatibility(propertyInfo, value, propertyName);
+
         // Add the value to the dictionary, using the property name as the key.
         _values.AddOrUpdate(propertyName, AddFunction, UpdateFunction);
 
@@ -68,4 +75,32 @@ public void Set<TValue>(Expression<Func<TComponent, TValue>> parameterSelector,
             return value;
         }
     }
+
+    private static void ValidateValueCompatibility(PropertyInfo propertyInfo, object? value, string propertyName)
+    {
+        var propertyType = propertyInfo.PropertyType;
+        var nullableUnderlyingType = Nullable.GetUnderlyingType(propertyType);
+        var targetType = nullableUnderlyingType ?? propertyType;
+
+        if (value is null)
+        {
+            if (propertyType.IsValueType && nullableUnderlyingType is null)
+            {
+                throw new ArgumentException($"Default value for '{propertyName}' on component '{typeof(TComponent)}' cannot be null because the property type '{propertyType}' is non-nullable.", nameof(value));
+            }
+
+            var nullabilityInfo = new NullabilityInfoContext().Create(propertyInfo);
+            if (nullabilityInfo.WriteState == NullabilityState.NotNull)
+            {
+                throw new ArgumentException($"Default value for '{propertyName}' on component '{typeof(TComponent)}' cannot be null because the property is non-nullable.", nameof(value));
+            }
+
+            return;
+        }
+
+        if (!targetType.IsInstanceOfType(value))
+        {
+            throw new ArgumentException($"Default value for '{propertyName}' on component '{typeof(TComponent)}' must be assignable to '{targetType}', but was '{value.GetType()}'.", nameof(value));
+        }
+    }
 }