JsonSchema.Net验证异常：值应为object实际为string问题排查

C#类定义 #

class PaymentInitiationSchema
{
    [Required]
    [JsonPropertyName("type")]
    public string Type { get; set; }

    [Required]
    [JsonPropertyName("actions")]
    public string[] Actions { get; set; }

    [Required]
    [JsonPropertyName("locations")]
    public string[] Locations { get; set; }

    [Required]
    [JsonPropertyName("instructedAmount")]
    public InstructedAmount InstructedAmount { get; set; }

    [Required]
    [JsonPropertyName("creditorName")]
    public string CreditorName { get; set; }

    [Required]
    [JsonPropertyName("creditorAccount")]
    public CreditorAccount CreditorAccount { get; set; }

    [Required]
    [JsonPropertyName("reemitanceInformationUnstructured")]
    public string ReemitanceInformationUnstructured { get; set; }
}

class InstructedAmount
{
    [Required]
    [JsonPropertyName("currency")]
    public string Currency { get; set; }

    [Required]
    [JsonPropertyName("amount")]
    public decimal Amount { get; set; }
}

class CreditorAccount
{
    [Required]
    [JsonPropertyName("iban")]
    public string Iban { get; set; }
}

生成的JSON Schema #

{
   "type":"object",
   "properties":{
      "type":{
         "type":"string"
      },
      "actions":{
         "$ref":"#/$defs/array"
      },
      "locations":{
         "$ref":"#/$defs/array"
      },
      "instructedAmount":{
         "type":"object",
         "properties":{
            "currency":{
               "type":"string"
            },
            "amount":{
               "type":"number"
            }
         },
         "required":[
            "currency",
            "amount"
         ]
      },
      "creditorName":{
         "type":"string"
      },
      "creditorAccount":{
         "type":"object",
         "properties":{
            "iban":{
               "type":"string"
            }
         },
         "required":[
            "iban"
         ]
      },
      "reemitanceInformationUnstructured":{
         "type":"string"
      }
   },
   "required":[
      "type",
      "actions",
      "locations",
      "instructedAmount",
      "creditorName",
      "creditorAccount",
      "reemitanceInformationUnstructured"
   ],
   "$defs":{
      "array":{
         "type":"array",
         "items":{
            "type":"string"
         }
      }
   }
}

验证函数 #

static bool IsValid(string requestedAuthorizationDetails, JsonSchema authorizationDetailsSchema)
{
    try
    {
        JsonDocument.Parse(requestedAuthorizationDetails);
    }
    catch
    {
        return false;
    }

    var result = authorizationDetailsSchema.Validate(requestedAuthorizationDetails, new ValidationOptions
    {
        OutputFormat = OutputFormat.Detailed
    });

    Console.WriteLine(result.Message + " at " + result.SchemaLocation.Source);

    return result.IsValid;
}

问题现象 #

调用上述验证函数时，始终返回false，报错信息：

Value is "string" but should be "object" at #/type

曾怀疑Schema顶层type与字段type冲突，但移除字段type后错误仍存在。

解决方案 #

问题出在验证函数的参数传递方式：

JsonSchema.Validate()方法接收string类型参数时，不会自动将其解析为JSON对象，而是直接把这个字符串当作待验证的值。你的Schema要求顶层是object类型，但传入的是string类型的JSON文本，自然会触发“值类型不匹配”的错误——哪怕你提前用JsonDocument.Parse检查了格式，也改变不了Validate方法对参数的处理逻辑。

修正后的验证函数 #

将JSON字符串解析为JsonElement或JsonNode后再传入Validate方法：

方式一：使用JsonElement

static bool IsValid(string requestedAuthorizationDetails, JsonSchema authorizationDetailsSchema)
{
    if (!JsonDocument.TryParse(requestedAuthorizationDetails, out var doc))
    {
        return false;
    }

    var result = authorizationDetailsSchema.Validate(doc.RootElement, new ValidationOptions
    {
        OutputFormat = OutputFormat.Detailed
    });

    Console.WriteLine($"{result.Message} at {result.SchemaLocation.Source}");

    return result.IsValid;
}

方式二：使用JsonNode

static bool IsValid(string requestedAuthorizationDetails, JsonSchema authorizationDetailsSchema)
{
    JsonNode? node;
    try
    {
        node = JsonNode.Parse(requestedAuthorizationDetails);
    }
    catch
    {
        return false;
    }

    if (node == null) return false;

    var result = authorizationDetailsSchema.Validate(node, new ValidationOptions
    {
        OutputFormat = OutputFormat.Detailed
    });

    Console.WriteLine($"{result.Message} at {result.SchemaLocation.Source}");

    return result.IsValid;
}

这样Validate方法会验证解析后的JSON对象，而非原始字符串，就能匹配Schema的类型要求，解决错误。

内容的提问来源于stack exchange，提问作者Szyszka947