Enhance enum parsing and nullable context handling

ReflectorNet/src/Convertor/Json/EnumJsonConverter.cs

@@ -63,8 +63,8 @@ public override bool CanConvert(Type typeToConvert)
                 if (!Enum.TryParse(underlyingType, stringValue, ignoreCase: true, out var enumValue))
                     throw new JsonException($"Unable to convert '{stringValue}' to enum {underlyingType.Name}. Valid values are: {string.Join(", ", Enum.GetNames(underlyingType))}");
 
-                if (Enum.IsDefined(underlyingType, enumValue))
-                    return enumValue;
+                if (Enum.IsDefined(underlyingType, enumValue!))
+                    return enumValue!;
 
                 throw new JsonException($"Unable to convert '{stringValue}' to enum {underlyingType.Name}. Valid values are: {string.Join(", ", Enum.GetNames(underlyingType))}");
             }

ReflectorNet/src/Reflector/MethodWrapper.cs

@@ -416,7 +416,7 @@ public virtual bool VerifyParameters(IReadOnlyDictionary<string, object?>? named
             {
                 if (Enum.TryParse(parameterType, stringValue, ignoreCase: true, out var result))
                 {
-                    if (Enum.IsDefined(parameterType, result))
+                    if (Enum.IsDefined(parameterType, result!))
                     {
                         return result;
                     }

ReflectorNet/src/Utils/MethodUtils.cs

@@ -16,17 +16,18 @@ namespace com.IvanMurzak.ReflectorNet.Utils
     /// </summary>
     public static class MethodUtils
     {
-#if NET5_0_OR_GREATER
         // Cache the NullableContextAttribute type for performance
         // This is a compiler-generated internal attribute, so we must access it by name
-        private static readonly Type? NullableContextAttributeType = typeof(System.Runtime.CompilerServices.NullableContextAttribute);
-        // Type.GetType("System.Runtime.CompilerServices.NullableContextAttribute, System.Private.CoreLib");
+#if NET8_0_OR_GREATER
+        private static readonly Type NullableContextAttributeType = typeof(System.Runtime.CompilerServices.NullableContextAttribute);
+#elif NET5_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER // Maybe some environment would support it
+        private static readonly Type? NullableContextAttributeType = Type.GetType("System.Runtime.CompilerServices.NullableContextAttribute, System.Private.CoreLib");
+#endif
 
         // NullableContext attribute values
         private const byte NullableContextOblivious = 0; // No annotation - treat as non-nullable
         private const byte NullableContextNotNull = 1;   // Non-nullable
         private const byte NullableContextNullable = 2;  // Nullable
-#endif
 
         /// <summary>
         /// Determines whether the return type of a method is nullable.
@@ -77,7 +78,7 @@ public static bool IsReturnTypeNullable(MethodInfo methodInfo)
                 ? returnType.GetGenericArguments()[0]
                 : returnType;
 
-#if NET5_0_OR_GREATER
+#if NET5_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
             // First, check if this is a method on a generic type - we need to inspect the generic definition
             // to correctly determine nullability for generic type parameters (T vs T?)
             MethodInfo? methodToInspect = null;

ReflectorNet/src/Utils/TypeUtils.cs

@@ -10,7 +10,6 @@
 using System.ComponentModel;
 using System.Linq;
 using System.Reflection;
-using System.Text.Json.Schema;
 using com.IvanMurzak.ReflectorNet.Model;
 
 namespace com.IvanMurzak.ReflectorNet.Utils
@@ -132,7 +131,8 @@ public static bool IsDictionary(Type type)
             var propertyInfo = type.GetProperty(propertyName);
             return propertyInfo != null ? GetPropertyDescription(propertyInfo) : null;
         }
-        public static string? GetPropertyDescription(JsonSchemaExporterContext context)
+#if NETSTANDARD2_1_OR_GREATER || NET8_0_OR_GREATER
+        public static string? GetPropertyDescription(System.Text.Json.Schema.JsonSchemaExporterContext context)
         {
             if (context.PropertyInfo == null || context.PropertyInfo.DeclaringType == null)
                 return null;
@@ -169,6 +169,7 @@ public static bool IsDictionary(Type type)
 
             return GetDescription(memberInfo);
         }
+#endif
 
         private static string ToPascalCase(string camelCase)
         {