Improve nullable type detection in API modeling

framework/src/Volo.Abp.Core/Volo/Abp/Reflection/TypeHelper.cs

@@ -86,7 +86,7 @@ public static class TypeHelper
         {
             return default;
         }
-        
+
         if (IsPrimitiveExtended(typeof(TProperty), includeEnums: true))
         {
             var conversionType = typeof(TProperty);
@@ -116,6 +116,12 @@ public static class TypeHelper
         return type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>);
     }
 
+    public static bool IsNullableOrNotValueType(Type type)
+    {
+        return (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>)) ||
+               !type.IsValueType;
+    }
+
     public static bool IsNullableEnum(Type type)
     {
         return type.IsGenericType &&

framework/src/Volo.Abp.Http/Volo/Abp/Http/Modeling/PropertyApiDescriptionModel.cs

@@ -42,7 +42,7 @@ public class PropertyApiDescriptionModel
             Type = ApiTypeNameHelper.GetTypeName(propertyInfo.PropertyType),
             TypeSimple = ApiTypeNameHelper.GetSimpleTypeName(propertyInfo.PropertyType),
             IsRequired = customAttributes.OfType<RequiredAttribute>().Any() || propertyInfo.GetCustomAttributesData().Any(attr => attr.AttributeType.Name == "RequiredMemberAttribute"),
-            IsNullable = TypeHelper.IsNullable(propertyInfo.PropertyType),
+            IsNullable = TypeHelper.IsNullableOrNotValueType(propertyInfo.PropertyType),
             Minimum = customAttributes.OfType<RangeAttribute>().Select(x => x.Minimum).FirstOrDefault()?.ToString(),
             Maximum = customAttributes.OfType<RangeAttribute>().Select(x => x.Maximum).FirstOrDefault()?.ToString(),
             MinLength = customAttributes.OfType<MinLengthAttribute>().FirstOrDefault()?.Length ?? customAttributes.OfType<StringLengthAttribute>().FirstOrDefault()?.MinimumLength,