.NET Framework Newtonsoft.Json自定义必填属性异常消息方案

方法一：Web API全局异常过滤器 #

针对API端点自动处理反序列化的场景，通过自定义异常过滤器捕获JsonSerializationException并重构异常消息：

1. 实现自定义异常过滤器

public class JsonRequiredExceptionFilter : ExceptionFilterAttribute
{
    public override void OnException(HttpActionExecutedContext context)
    {
        if (context.Exception is not JsonSerializationException jsonEx) return;

        var requiredPropRegex = new Regex(@"Required property '(.*)' not found in JSON.*");
        var match = requiredPropRegex.Match(jsonEx.Message);
        if (!match.Success) return;

        var propName = match.Groups[1].Value;
        // 从Action参数中获取目标DTO类型
        var targetParam = context.ActionContext.ActionDescriptor.GetParameters()
            .FirstOrDefault(p => p.ParameterType.IsClass && !p.ParameterType.IsAbstract);
        var typeName = targetParam?.ParameterType.Name.ToLower() ?? "object";
        var customMessage = $"Unable to create {typeName}, '{propName}' is required.";

        context.Response = context.Request.CreateErrorResponse(
            HttpStatusCode.BadRequest,
            customMessage,
            jsonEx);
    }
}

2. 在Web API配置中注册过滤器

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.Filters.Add(new JsonRequiredExceptionFilter());
        
        // 保留原有Json序列化配置
        config.Formatters.JsonFormatter.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
    }
}

方法二：自定义ContractResolver（通用场景） #

如果需要在所有Newtonsoft.Json反序列化场景生效（包括非Web环境），可以通过重写ContractResolver替换必填属性的验证逻辑：

1. 实现自定义ContractResolver

public class CustomRequiredContractResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var property = base.CreateProperty(member, memberSerialization);

        if (property.Required != Required.Always) return property;

        // 包装原有验证逻辑，替换异常消息
        var originalValidator = property.RequiredValidator;
        property.RequiredValidator = (obj, serializer) =>
        {
            try
            {
                originalValidator?.Invoke(obj, serializer);
            }
            catch (JsonSerializationException ex)
            {
                var typeName = obj.GetType().Name.ToLower();
                var customMessage = $"Unable to create {typeName}, '{property.PropertyName}' is required.";
                throw new JsonSerializationException(customMessage, ex);
            }
        };

        return property;
    }
}

2. 全局配置序列化设置（Web API示例）

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.Formatters.JsonFormatter.SerializerSettings.ContractResolver = new CustomRequiredContractResolver();
    }
}

关键说明 #

• 方法一依赖Web API的异常过滤体系，适合Web场景，可通过Action参数动态识别目标类型生成对应消息。
• 方法二是通用方案，不依赖Web框架，任何使用Newtonsoft.Json的反序列化操作都会触发自定义异常消息。
• 两种方案均保留原始异常的堆栈信息，既对外返回友好提示，也不影响后端调试。

内容的提问来源于stack exchange，提问作者Wouter Vandenputte