Deserialize json in a "TryParse" way
C#.NetJsonC# Problem Overview
When I send a request to a service (that I do not own), it may respond either with the JSON data requested, or with an error that looks like this:
{
"error": {
"status": "error message",
"code": "999"
}
}
In both cases the HTTP response code is 200 OK, so I cannot use that to determine whether there is an error or not - I have to deserialize the response to check. So I have something that looks like this:
bool TryParseResponseToError(string jsonResponse, out Error error)
{
// Check expected error keywords presence
// before try clause to avoid catch performance drawbacks
if (jsonResponse.Contains("error") &&
jsonResponse.Contains("status") &&
jsonResponse.Contains("code"))
{
try
{
error = new JsonSerializer<Error>().DeserializeFromString(jsonResponse);
return true;
}
catch
{
// The JSON response seemed to be an error, but failed to deserialize.
// Or, it may be a successful JSON response: do nothing.
}
}
error = null;
return false;
}
Here, I have an empty catch clause that may be in the standard execution path, which is a bad smell... Well, more than a bad smell: it stinks.
Do you know a better way to "TryParse" the response in order to avoid a catch in the standard execution path ?
[EDIT]
Thanks to Yuval Itzchakov's answer I improved my method like that :
bool TryParseResponse(string jsonResponse, out Error error)
{
// Check expected error keywords presence :
if (!jsonResponse.Contains("error") ||
!jsonResponse.Contains("status") ||
!jsonResponse.Contains("code"))
{
error = null;
return false;
}
// Check json schema :
const string errorJsonSchema =
@"{
'type': 'object',
'properties': {
'error': {'type':'object'},
'status': {'type': 'string'},
'code': {'type': 'string'}
},
'additionalProperties': false
}";
JsonSchema schema = JsonSchema.Parse(errorJsonSchema);
JObject jsonObject = JObject.Parse(jsonResponse);
if (!jsonObject.IsValid(schema))
{
error = null;
return false;
}
// Try to deserialize :
try
{
error = new JsonSerializer<Error>.DeserializeFromString(jsonResponse);
return true;
}
catch
{
// The JSON response seemed to be an error, but failed to deserialize.
// This case should not occur...
error = null;
return false;
}
}
I kept the catch clause... just in case.
C# Solutions
Solution 1 - C#
@Victor LG's answer using Newtonsoft is close, but it doesn't technically avoid the a catch as the original poster requested. It just moves it elsewhere. Also, though it creates a settings instance to enable catching missing members, those settings aren't passed to the DeserializeObject call so they are actually ignored.
Here's a "catch free" version of his extension method that also includes the missing members flag. The key to avoiding the catch is setting the Error
property of the settings object to a lambda which then sets a flag to indicate failure and clears the error so it doesn't cause an exception.
public static bool TryParseJson<T>(this string @this, out T result)
{
bool success = true;
var settings = new JsonSerializerSettings
{
Error = (sender, args) => { success = false; args.ErrorContext.Handled = true; },
MissingMemberHandling = MissingMemberHandling.Error
};
result = JsonConvert.DeserializeObject<T>(@this, settings);
return success;
}
Here's an example to use it:
if(value.TryParseJson(out MyType result))
{
// Do something with result…
}
Solution 2 - C#
With Json.NET
you can validate your json against a schema:
string schemaJson = @"{
'status': {'type': 'string'},
'error': {'type': 'string'},
'code': {'type': 'string'}
}";
JsonSchema schema = JsonSchema.Parse(schemaJson);
JObject jobj = JObject.Parse(yourJsonHere);
if (jobj.IsValid(schema))
{
// Do stuff
}
And then use that inside a TryParse method.
public static T TryParseJson<T>(this string json, string schema) where T : new()
{
JsonSchema parsedSchema = JsonSchema.Parse(schema);
JObject jObject = JObject.Parse(json);
return jObject.IsValid(parsedSchema) ?
JsonConvert.DeserializeObject<T>(json) : default(T);
}
Then do:
var myType = myJsonString.TryParseJson<AwsomeType>(schema);
Update:
Please note that schema validation is no longer part of the main Newtonsoft.Json package, you'll need to add the Newtonsoft.Json.Schema package.
Update 2:
As noted in the comments, "JSONSchema" have a pricing model, meaning it isn't free. You can find all the information here
Solution 3 - C#
A slightly modified version of @Yuval's answer.
static T TryParse<T>(string jsonData) where T : new()
{
JSchemaGenerator generator = new JSchemaGenerator();
JSchema parsedSchema = generator.Generate(typeof(T));
JObject jObject = JObject.Parse(jsonData);
return jObject.IsValid(parsedSchema) ?
JsonConvert.DeserializeObject<T>(jsonData) : default(T);
}
This can be used when you don't have the schema as text readily available for any type.
Solution 4 - C#
Just to provide an example of the try/catch approach (it may be useful to somebody).
public static bool TryParseJson<T>(this string obj, out T result)
{
try
{
// Validate missing fields of object
JsonSerializerSettings settings = new JsonSerializerSettings();
settings.MissingMemberHandling = MissingMemberHandling.Error;
result = JsonConvert.DeserializeObject<T>(obj, settings);
return true;
}
catch (Exception)
{
result = default(T);
return false;
}
}
Then, it can be used like this:
var result = default(MyObject);
bool isValidObject = jsonString.TryParseJson<MyObject>(out result);
if(isValidObject)
{
// Do something
}
Solution 5 - C#
You may deserialize JSON to a dynamic
, and check whether the root element is error
. Note that you probably don't have to check for the presence of status
and code
, like you actually do, unless the server also sends valid non-error responses inside a error
node.
Aside that, I don't think you can do better than a try/catch
.
What actually stinks is that the server sends an HTTP 200 to indicate an error. try/catch
appears simply as checking of inputs.
Solution 6 - C#
To test whether a text is valid JSON regardless of schema, you could also do a check on the number of quotation marks:" in your string response, as shown below :
// Invalid JSON
var responseContent = "asgdg";
// var responseContent = "{ \"ip\" = \"11.161.195.10\" }";
// Valid JSON, uncomment to test these
// var responseContent = "{ \"ip\": \"11.161.195.10\", \"city\": \"York\", \"region\": \"Ontartio\", \"country\": \"IN\", \"loc\": \"-43.7334,79.3329\", \"postal\": \"M1C\", \"org\": \"AS577 Bell Afgh\", \"readme\": \"https://ipinfo.io/missingauth\"}";
// var responseContent = "\"asfasf\"";
// var responseContent = "{}";
int count = 0;
foreach (char c in responseContent)
if (c == '\"') count++; // Escape character needed to display quotation
if (count >= 2 || responseContent == "{}")
{
// Valid Json
try {
JToken parsedJson = JToken.Parse(responseContent);
Console.WriteLine("RESPONSE: Json- " + parsedJson.ToString(Formatting.Indented));
}
catch(Exception ex){
Console.WriteLine("RESPONSE: InvalidJson- " + responseContent);
}
}
else
Console.WriteLine("RESPONSE: InvalidJson- " + responseContent);