I need to change an existing model property type from string to List<string>.
Because my database already has several instances of such model with the string property, I'd like to find a way to read the property:
If it is valorized as string, return a new List with the item
If it's valorized as a List, return as is
I am on .NET Core 6 and I believe my two options are
Add a decorator to the property
Modify the SQL query and try to parse string as array
What would be the best way to accomplish this?
On .NET you can define a method that handles custom deserialization/serialization.
The V3 SDK uses Newtonsoft, so I'll use that as an example, you can implement JsonConverter (https://www.newtonsoft.com/json/help/html/CustomJsonConverter.htm).
public class MyConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(List<string>);
}
// Used for serializing, if true, override WriteJson
public override bool CanWrite { get { return false; }}
public override object ReadJson(
JsonReader reader,
Type objectType,
object existingValue,
JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
{
return null;
}
JToken itemsJToken = JToken.Load(reader);
if (itemsJToken.Type == JTokenType.String)
{
return new List<string>(){ itemsJToken.Value<string>() };
}
else // assuming it can only be an array
{
return itemsJToken.ToObject<List<string>>(serializer);
}
}
public override void WriteJson(
JsonWriter writer,
object value,
JsonSerializer serializer)
{
throw new NotImplementedException("Unnecessary because CanWrite is false. The type will skip the converter.");
}
}
public class MyType
{
/* other properties */
[JsonConverter(typeof(MyConverter))]
public List<string> Items {get;set;}
}
Related
EDIT: Clarify question:
I have overridden the JsonConverter for a base type (by applying [JsonConverter(typeof(TConverter))] to the superclass), but when deserializing the sub-type directly I want to use STANDARD serialization (i.e. no custom converter) for deserializing my derived object. How do I specify STANDARD serialization for use in the deserialize method, as if I had NOT overridden the JsonConverter?
I am using elastic search and can't call JsonConvert.DeserializeObject with my custom implementation of JsonConverter, and have to rely on the attribute for Elastic to use my converter.
However, using this converter as attribute seems to affect all sub classes as well, but I just want them to use the standard converter, so that I don't have to implement JsonConverter for each of many implementations.
This is my classes/logic as I would like it to look:
[Route("test")]
[HttpPost]
public HttpResponseMessage Test([FromBody] JToken json)
{
var res = json.ToObject<Product>(); // I want an object of ProductImpl type here
return Request.CreateResponse(res);
}
[JsonConverter(typeof(JsonProductConverted))]
public abstract class Product
{
}
public class ProductImpl : Product
{
}
public class JsonProductConverted : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JObject json = JObject.Load(reader);
//var type = GetTypeFromId((int) json["typeId"]); // Construct type from field in
var type = typeof(ProductImpl);
// var res = JsonConvert.DeserializeObject(json.ToString(), type, DEFAULT_JSONCONVERTER_HERE);
var res = DeserializeToObjectWithStandardJsonConverter(json, type);
return res;
}
public override bool CanConvert(Type objectType)
{
return false;
}
}
If I don't supply the default JsonConverter, or similar it will just use the JsonProductConverted converter, which creates an infinite loop.
Since you have added [JsonConverter(typeof(JsonProductConverted))] directly to your Product type, you could add a dummy converter to ProductImpl that returns false from CanRead and CanWrite:
[JsonConverter(typeof(NoConverter))]
public class ProductImpl : Product
{
}
public class NoConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return false;
}
public override bool CanRead { get { return false; } }
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override bool CanWrite { get { return false; } }
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
This overrides the base class's converter and then falls back on default serialization for both reading and writing
Sample .Net fiddle.
Another option would be to use serializer.Populate(). This avoids the call to the converter for the object itself:
public class JsonProductConverted : JsonTypeInferringConverterBase
{
protected override Type InferType(Type objectType, JObject json)
{
//var type = GetTypeFromId((int) json["typeId"]); // Construct type from field in
return typeof(ProductImpl);
}
public override bool CanConvert(Type objectType)
{
return false;
}
}
public abstract class JsonTypeInferringConverterBase : JsonConverter
{
public override bool CanWrite { get { return false; } }
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
protected abstract Type InferType(Type objectType, JObject json);
protected virtual object CreateObject(Type actualType, JsonSerializer serializer, JObject json)
{
var contract = (JsonObjectContract)serializer.ContractResolver.ResolveContract(actualType);
return contract.DefaultCreator();
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var json = JObject.Load(reader);
var actualType = InferType(objectType, json);
// Construct object (or reuse existingValue if compatible)
if (existingValue == null || !actualType.IsAssignableFrom(existingValue.GetType()))
{
existingValue = CreateObject(actualType, serializer, json);
}
// Populate object.
using (var subReader = json.CreateReader())
{
serializer.Populate(subReader, existingValue);
}
return existingValue;
}
}
Note that the concrete objects must have parameterless constructors for this to work. If not, you can override protected virtual object CreateObject(Type actualType, JsonSerializer serializer, JObject json) and manually invoke a parameterized constructor by deserializing select properties inside the JObject json.
Sample fiddle #2.
I try to create custom JSON converter by using code for some answered question.
public class SingleValueArrayConverter<T> : JsonConverter
{
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.StartObject)
{
T instance = serializer.Deserialize<T>(reader);
return new List<T>() { instance };
}
else if (reader.TokenType == JsonToken.StartArray)
{
return serializer.Deserialize<List<T>>(reader);
}
return null;
}
public override bool CanConvert(Type objectType)
{
return true;
}
public override bool CanWrite
{
get { return false; }
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
This is my sample model class.
public class Foo
{
[JsonProperty("type")]
public string Type;
}
I use the following code to execute. It causes error about stack overflow.
var converter = new SingleValueArrayConverter<Foo>();
var test1 = "[{ \"type\": \"test\" }]";
var result1 = JsonConvert.DeserializeObject<List<Foo>>(test1, converter);
var test2 = "{ \"type\": \"test\" }";
var result2 = JsonConvert.DeserializeObject<List<Foo>>(test2, converter);
The easiest way to convert any JSON to .NET object is using "ToObject" method of JToken object.
public class FooConverter : JsonConverter
{
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
var token = JToken.Load(reader);
if (token.Type == JTokenType.Array)
{
return token.ToObject<List<Foo>>();
}
var item = token.ToObject<Foo>();
return new List<Foo> { item };
}
public override bool CanConvert(Type objectType)
{
return true;
}
public override bool CanWrite
{
get { return false; }
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
PS. JToken object is the base class of any JSON data so we can convert JToken object to any appropriated type.
JToken - abstract base class
JContainer - abstract base class of JTokens that can contain other JTokens
JArray - represents an JSON array (contains an ordered list of JTokens)
JObject - represents an JSON object (contains a collection of JProperties)
JProperty - represents a JSON property (a name/JToken pair inside a JObject)
JValue - represents a primitive JSON value (string, number, boolean, null)
Assuming a json string like the following:
string json = '{"string_property":"foo_bar", ... other objects here ...}';
I was wondering if there's a way to run a transformation on the parsed object so that instead of getting foo_bar, I'll get foo bar after running the following method (can be anything really)
public string Transform(string s) {
return s.Replace("_"," ");
}
I can manually alter my poco after deserializing, but wondered what would be a "cleaner" approach?
You can transform your string properties as you deserialize your root object by using a custom JsonConverter targeted at all string type values:
public class ReplacingStringConverter : JsonConverter
{
readonly string oldValue;
readonly string newValue;
public ReplacingStringConverter(string oldValue, string newValue)
{
if (string.IsNullOrEmpty(oldValue))
throw new ArgumentException("string.IsNullOrEmpty(oldValue)");
if (newValue == null)
throw new ArgumentNullException("newValue");
this.oldValue = oldValue;
this.newValue = newValue;
}
public override bool CanConvert(Type objectType)
{
return objectType == typeof(string);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var s = (string)JToken.Load(reader);
return s.Replace(oldValue, newValue);
}
public override bool CanWrite { get { return false; } }
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Then use it like:
var settings = new JsonSerializerSettings { Converters = new[] { new ReplacingStringConverter("_", "") } };
var result = JsonConvert.DeserializeObject<RootObject>(json, settings);
Note however that if individual string-type properties have their own converters applied directly with [JsonConverter(Type)], those converters will be used in preference to the ReplacingStringConverter in the Converters list.
I've ended up doing the following:
First, create a converter that only reads and all it does is url decode the string.
public class UrlDecoderConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(string);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
if (reader.TokenType == JsonToken.Null)
return null;
var s = (string)JToken.Load(reader);
return HttpUtility.UrlDecode(s);
}
public override bool CanWrite => false;
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Then, simply add the following to the POCO properties that need to be decoded:
[JsonConverter(typeof(UrlDecoderConverter))]
public string url { get; set; }
I've got some json with huge integers, in the order of a few hundred digits. I'd like to parse those as BouncyCastle's BigInteger (https://github.com/onovotny/BouncyCastle-PCL/blob/pcl/crypto/src/math/BigInteger.cs).
{
"bigNumber":12093812947635091350945141034598534526723049126743245...
}
So I've implemented a converter, using a contract resolver in the default settings.
internal class BigIntegerConverter : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteRawValue(value.ToString());
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JToken jToken = JToken.Load(reader);
return new BigInteger(jToken.ToString());
}
public override bool CanConvert(Type objectType)
{
return (objectType == typeof(BigInteger));
}
}
public class BigIntegerContractResolver : DefaultContractResolver
{
private static readonly JsonConverter bigIntegerConverter = new BigIntegerConverter();
private static Type type = typeof(BigInteger);
protected override JsonConverter ResolveContractConverter(Type objectType)
{
if (objectType == type)
{
return bigIntegerConverter;
}
return base.ResolveContractConverter(objectType);
}
}
JsonConvert.DefaultSettings = () => new JsonSerializerSettings
{
ContractResolver = new BigIntegerContractResolver()
};
The writer works as it should, writing a (large) integer value instead of the class BigInteger with all its properties etc. However, the reading fails. Neither ReadJson nor CanConvert appear to be invoked.
I get the following exception:
JsonReaderException: JSON integer 340597435091750914358634185762341897561435984635897436598435643875643189576413589743659817456... is too large or small for an Int64.
How do I get Json.NET to parse this number as a string instead of an integer?
Ideally I don't want to have to parse the json string myself first, to add quotes.
If your large number isn't quoted, Json.Net will deserialize it as a System.Numerics.BigInteger. This happens inside the JsonTextReader, well before the converter gets a chance to handle it. So if you want your result type to be Org.BouncyCastle.Math.BigInteger, you'll need to convert from System.Numerics.BigInteger. (Seems a little backwards, I know. The other alternative is to create your own JsonTextReader, but that is probably going to be more trouble than it is worth -- most of the useful bits of the existing reader are in private or internal methods, so subclassing it is not practical.)
I was able to get this converter to work:
class BigIntegerConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return (objectType == typeof(Org.BouncyCastle.Math.BigInteger));
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
System.Numerics.BigInteger big = (System.Numerics.BigInteger)reader.Value;
return new Org.BouncyCastle.Math.BigInteger(big.ToString());
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteRawValue(value.ToString());
}
}
Here is the test program I used. Note that I did not use a resolver. JsonSerializerSettings has a Converters collection, so I just added the BigIntegerConverter to that.
class Program
{
static void Main(string[] args)
{
string json = #"
{
""bigNumber"": 12093812947635091350945141034598534526723049126743245
}";
JsonConvert.DefaultSettings = () => new JsonSerializerSettings
{
Converters = new List<JsonConverter> { new BigIntegerConverter() }
};
Foo foo = JsonConvert.DeserializeObject<Foo>(json);
Console.WriteLine(foo.BigNumber.ToString());
}
}
class Foo
{
public Org.BouncyCastle.Math.BigInteger BigNumber { get; set; }
}
Output:
12093812947635091350945141034598534526723049126743245
You can try creating object that handles the output of the json like this:
public class YourModel
{
[JsonConverter(typeof(CustomConverter<BigInteger>))]
public BigInteger YourProperty{ get; set; }
}
And now it can become more generic for every type you need:
public class CustomConverter<T> : JsonConverter
{
public override bool CanConvert(Type objectType)
{
your code ..
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
//explicitly specify the concrete type we want to create
return serializer.Deserialize<T>(reader);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
your code ...
}
}
I have this structure :
List<dynamic> lst = new List<dynamic>();
lst.Add(new{objId = 1,myOtherColumn = 5});
lst.Add(new{objId = 2,myOtherColumn = 6});
lst.Add(new{lala = "asd" ,lala2 = 7});
I'm serializing it via :
string st= JsonConvert.SerializeObject(lst);
Question:
How can I cause the serializer to change only values of the "objId" property , to something else ?
I know I should use class Myconverter : JsonConverter , but I didn't find any example which keeps the default behavior and in addition - allow me to add condition logic of serialization.
Here's a converter that will handle it, at least for simple objects as per your example. It looks for objects containing objId properties and then serialises all properties it finds on them. You may need to expand it to deal with other member types/more complex properties as required:
class MyConverter : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteStartObject();
foreach (var prop in value.GetType().GetProperties()) {
writer.WritePropertyName(prop.Name);
if (prop.Name == "objId") {
//modify objId values for example
writer.WriteValue(Convert.ToInt32(prop.GetValue(value, null)) + 10);
} else {
writer.WriteValue(prop.GetValue(value, null));
}
}
writer.WriteEndObject();
}
public override bool CanConvert(Type objectType)
{
//only attempt to handle types that have an objId property
return (objectType.GetProperties().Count(p => p.Name == "objId") == 1);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Alternatively, you can use a converter that specifies it will only convert int types, then query where in the JSON path you are before doing any conversions. This has the benefit of not needing to deal with all the other members of the anonymous type.
class MyConverter : JsonConverter
{
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
if (writer.Path.EndsWith(".objId")) {
writer.WriteValue(Convert.ToInt32(value) + 10);
}
else {
writer.WriteValue(value);
}
}
public override bool CanConvert(Type objectType)
{
return objectType == typeof (int);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}