dotnet-cqrs/Svrnty.CQRS.Events/Schema/SystemTextJsonSchemaGenerator.cs

using System;
using Svrnty.CQRS.Events.Abstractions.Schema;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;
using Svrnty.CQRS.Events.Abstractions;

namespace Svrnty.CQRS.Events.Schema;

/// <summary>
/// Basic JSON Schema Draft 7 generator using System.Text.Json reflection.
/// </summary>
/// <remarks>
/// <para>
/// This is a simple implementation that generates basic JSON schemas from CLR types.
/// For more advanced features (XML doc comments, complex validation, etc.),
/// consider using NJsonSchema library instead.
/// </para>
/// <para>
/// Supports:
/// - Primitive types (string, number, boolean)
/// - Object types with properties
/// - Nullable types
/// - Required properties (non-nullable reference types)
/// - Basic type descriptions
/// </para>
/// </remarks>
public class SystemTextJsonSchemaGenerator : IJsonSchemaGenerator
{
    private static readonly JsonSerializerOptions _jsonOptions = new()
    {
        WriteIndented = true
    };

    /// <inheritdoc />
    public Task<string> GenerateSchemaAsync(
        Type type,
        CancellationToken cancellationToken = default)
    {
        if (type == null)
            throw new ArgumentNullException(nameof(type));

        var schema = GenerateSchemaObject(type, new HashSet<Type>());
        var json = JsonSerializer.Serialize(schema, _jsonOptions);
        return Task.FromResult(json);
    }

    /// <inheritdoc />
    public Task<bool> ValidateAsync(
        string jsonData,
        string jsonSchema,
        CancellationToken cancellationToken = default)
    {
        // Basic implementation: Just check if JSON is parseable
        // For proper validation, use NJsonSchema or similar library
        try
        {
            using var doc = JsonDocument.Parse(jsonData);
            return Task.FromResult(true);
        }
        catch
        {
            return Task.FromResult(false);
        }
    }

    /// <inheritdoc />
    public Task<IReadOnlyList<string>> GetValidationErrorsAsync(
        string jsonData,
        string jsonSchema,
        CancellationToken cancellationToken = default)
    {
        // Basic implementation: Just check JSON parsing errors
        var errors = new List<string>();

        try
        {
            using var doc = JsonDocument.Parse(jsonData);
        }
        catch (JsonException ex)
        {
            errors.Add($"Invalid JSON: {ex.Message}");
        }

        return Task.FromResult<IReadOnlyList<string>>(errors);
    }

    private Dictionary<string, object> GenerateSchemaObject(Type type, HashSet<Type> visitedTypes)
    {
        // Prevent infinite recursion for circular references
        if (visitedTypes.Contains(type))
        {
            return new Dictionary<string, object>
            {
                ["$ref"] = $"#/definitions/{type.Name}"
            };
        }

        visitedTypes.Add(type);

        var schema = new Dictionary<string, object>
        {
            ["$schema"] = "http://json-schema.org/draft-07/schema#",
            ["type"] = GetJsonType(type)
        };

        // Add title from type name
        schema["title"] = type.Name;

        // Handle nullable types
        var underlyingType = Nullable.GetUnderlyingType(type);
        if (underlyingType != null)
        {
            type = underlyingType;
            schema["type"] = new[] { GetJsonType(type), "null" };
        }

        // Handle complex types (objects)
        if (IsComplexType(type))
        {
            var properties = new Dictionary<string, object>();
            var required = new List<string>();

            foreach (var prop in type.GetProperties(BindingFlags.Public | BindingFlags.Instance))
            {
                var propName = GetJsonPropertyName(prop);
                var propSchema = new Dictionary<string, object>
                {
                    ["type"] = GetJsonType(prop.PropertyType)
                };

                // Check if property is required (non-nullable reference type)
                if (!IsNullable(prop))
                {
                    required.Add(propName);
                }

                // Handle nested complex types
                if (IsComplexType(prop.PropertyType))
                {
                    propSchema = GenerateSchemaObject(prop.PropertyType, new HashSet<Type>(visitedTypes));
                }

                properties[propName] = propSchema;
            }

            schema["properties"] = properties;

            if (required.Any())
            {
                schema["required"] = required;
            }
        }

        return schema;
    }

    private static string GetJsonType(Type type)
    {
        var underlyingType = Nullable.GetUnderlyingType(type) ?? type;

        if (underlyingType == typeof(string))
            return "string";
        if (underlyingType == typeof(int) || underlyingType == typeof(long) ||
            underlyingType == typeof(short) || underlyingType == typeof(byte))
            return "integer";
        if (underlyingType == typeof(double) || underlyingType == typeof(float) ||
            underlyingType == typeof(decimal))
            return "number";
        if (underlyingType == typeof(bool))
            return "boolean";
        if (underlyingType == typeof(DateTime) || underlyingType == typeof(DateTimeOffset))
            return "string"; // ISO 8601 format
        if (underlyingType == typeof(Guid))
            return "string"; // UUID format

        // Arrays and lists
        if (type.IsArray || (type.IsGenericType &&
            (type.GetGenericTypeDefinition() == typeof(List<>) ||
             type.GetGenericTypeDefinition() == typeof(IEnumerable<>) ||
             type.GetGenericTypeDefinition() == typeof(IList<>) ||
             type.GetGenericTypeDefinition() == typeof(IReadOnlyList<>))))
        {
            return "array";
        }

        return "object";
    }

    private static bool IsComplexType(Type type)
    {
        var underlyingType = Nullable.GetUnderlyingType(type) ?? type;

        return underlyingType.IsClass &&
               underlyingType != typeof(string) &&
               underlyingType != typeof(DateTime) &&
               underlyingType != typeof(DateTimeOffset) &&
               underlyingType != typeof(Guid) &&
               !underlyingType.IsArray;
    }

    private static bool IsNullable(PropertyInfo property)
    {
        // Check for nullable value types (e.g., int?)
        if (Nullable.GetUnderlyingType(property.PropertyType) != null)
            return true;

        // Check for nullable reference types (C# 8.0+)
        var nullabilityContext = new NullabilityInfoContext();
        var nullabilityInfo = nullabilityContext.Create(property);
        return nullabilityInfo.WriteState == NullabilityState.Nullable;
    }

    private static string GetJsonPropertyName(PropertyInfo property)
    {
        // Use JsonPropertyName attribute if present
        var attr = property.GetCustomAttribute<System.Text.Json.Serialization.JsonPropertyNameAttribute>();
        if (attr != null)
            return attr.Name;

        // Default: camelCase
        var name = property.Name;
        return char.ToLowerInvariant(name[0]) + name.Substring(1);
    }
}