Blazor添加AddNewtonsoftJson后，JsonConvert反序列化ValidationResult报错

问题场景 #

在Blazor应用中，为实现Patch功能配置了builder.Services.AddControllers().AddNewtonsoftJson()，但出现以下矛盾：

• 接口无验证错误时运行正常
• 返回验证响应时，JsonConvert.DeserializeObject抛出异常，提示无法找到System.ComponentModel.DataAnnotations.ValidationResult的合适构造函数
• 移除.AddNewtonsoftJson()后，验证消息可正常返回，但Patch功能失效

相关代码与异常信息 #

前端响应处理代码

response = await Http.PostAsJsonAsync(Constants.uri, editModel);
var res = response.Content.ReadAsStringAsync().Result;
var validationResponse = JsonConvert.DeserializeObject<MymodelResponse>(res);

异常信息

blazor.webassembly.js:1 crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
  Unhandled exception rendering component: Unable to find a constructor to use for type System.ComponentModel.DataAnnotations.ValidationResult. A class should either have a default constructor, one constructor with arguments or a constructor marked with the JsonConstructor attribute. Path 'validationResult.validationResults[0].memberNames', line 1, position 493.
Newtonsoft.Json.JsonSerializationException: Unable to find a constructor to use for type System.ComponentModel.DataAnnotations.ValidationResult. A class should either have a default constructor, one constructor with arguments or a constructor marked with the JsonConstructor attribute. Path 'validationResult.validationResults[0].memberNames', line 1, position 493.
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateNewObject(JsonReader reader, JsonObjectContract objectContract, JsonProperty containerMember, JsonProperty containerProperty, String id, Boolean& createdFromNonDefaultCreator)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateObject(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)
   at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.CreateValueInternal(JsonReader reader, Type objectType, JsonContract contract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerMember, Object existingValue)

模型定义

public class MymodelResponse : BaseResponse
{
    public int MymodelId { get; set; }
    public string? Phone { get; set; }
    public long CreatedBy { get; set; }
    public DateTime CreatedDate { get; set; }
    public long? ModifiedBy { get; set; }
    public DateTime? ModifiedDate { get; set; }
    public bool IsDeleted { get; set; }
    public string? Notes { get; set; }

    public MymodelResponse(List<ValidationResult> validationResults) : base(validationResults)
    {

    }
}

// BaseResponse 片段
public class BaseResponse
{
    public List<ValidationResult> ValidationResults  { get; set; }
}

解决方案 #

方案1：为ValidationResult添加自定义Json转换器

ValidationResult没有默认构造函数，Newtonsoft.Json无法自动反序列化，需自定义转换器处理：

public class ValidationResultConverter : JsonConverter<ValidationResult>
{
    public override ValidationResult ReadJson(JsonReader reader, Type objectType, ValidationResult existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        var jsonObject = JObject.Load(reader);
        var errorMessage = jsonObject["ErrorMessage"]?.ToString();
        var memberNames = jsonObject["MemberNames"]?.ToObject<IEnumerable<string>>() ?? Enumerable.Empty<string>();
        
        return new ValidationResult(errorMessage, memberNames);
    }

    public override void WriteJson(JsonWriter writer, ValidationResult value, JsonSerializer serializer)
    {
        var jsonObject = new JObject
        {
            ["ErrorMessage"] = value.ErrorMessage,
            ["MemberNames"] = JArray.FromObject(value.MemberNames)
        };
        jsonObject.WriteTo(writer);
    }
}

使用方式：

• 局部反序列化时注册：

var settings = new JsonSerializerSettings();
settings.Converters.Add(new ValidationResultConverter());
var validationResponse = JsonConvert.DeserializeObject<MymodelResponse>(res, settings);

• 全局配置时添加（服务注册阶段）：

builder.Services.AddControllers().AddNewtonsoftJson(options =>
{
    options.SerializerSettings.Converters.Add(new ValidationResultConverter());
});

方案2：自定义DTO替代ValidationResult

创建一个与ValidationResult结构一致的DTO类，避免直接使用框架自带类型：

public class ValidationResultDto
{
    public string? ErrorMessage { get; set; }
    public List<string> MemberNames { get; set; } = new List<string>();

    public ValidationResult ToValidationResult()
    {
        return new ValidationResult(ErrorMessage, MemberNames);
    }
}

修改BaseResponse与MymodelResponse：

public class BaseResponse
{
    public List<ValidationResultDto> ValidationResults { get; set; } = new List<ValidationResultDto>();

    public BaseResponse(List<ValidationResultDto> validationResults)
    {
        ValidationResults = validationResults;
    }

    public List<ValidationResult> GetValidationResults()
    {
        return ValidationResults.Select(dto => dto.ToValidationResult()).ToList();
    }
}

public class MymodelResponse : BaseResponse
{
    // 原有属性...

    public MymodelResponse(List<ValidationResultDto> validationResults) : base(validationResults)
    {
    }
}

方案3：混合使用序列化库

保留AddNewtonsoftJson()支持Patch，前端处理验证响应时改用System.Text.Json（它可以处理无默认构造的类型）：

response = await Http.PostAsJsonAsync(Constants.uri, editModel);
var res = await response.Content.ReadAsStringAsync();
var validationResponse = System.Text.Json.JsonSerializer.Deserialize<MymodelResponse>(res);

若有特殊序列化需求，可配置System.Text.Json选项：

var options = new JsonSerializerOptions
{
    PropertyNameCaseInsensitive = true,
    WriteIndented = true
};
var validationResponse = System.Text.Json.JsonSerializer.Deserialize<MymodelResponse>(res, options);

内容的提问来源于stack exchange，提问作者Mayur Saner