I am writing a Cmdlet and need to pass object structures into an API client that may contain PSObjects. Currently, these serialise as a JSON string containing CLIXML. Instead, I need it to be treated like an object (including the NoteProperties in PSObject.Properties as properties, and recursively serialising their values).
I tried writing my own JsonConverter but for some reason it only gets called for the top level object, not for nested PSObjects:
public class PSObjectJsonConverter : JsonConverter {
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
if (value is PSObject) {
JObject obj = new JObject();
foreach (var prop in ((PSObject)value).Properties) {
obj.Add(new JProperty(prop.Name, value));
}
obj.WriteTo(writer);
} else {
JToken token = JToken.FromObject(value);
token.WriteTo(writer);
}
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {
throw new NotImplementedException();
}
public override bool CanRead {
get { return false; }
}
public override bool CanConvert(Type objectType) {
return true;
}
}
Additionally, I am using serializing to camel case using CamelCasePropertyNamesContractResolver. Is there a way to make the converter respect that?
The following converter should correctly serialize recursively nested objects of type PSObject:
public class PSObjectJsonConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(PSObject).IsAssignableFrom(objectType);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var psObj = (PSObject)value;
writer.WriteStartObject();
foreach (var prop in psObj.Properties)
{
//Probably we shouldn't try to serialize a property that can't be read.
//https://learn.microsoft.com/en-us/dotnet/api/system.management.automation.pspropertyinfo.isgettable?view=powershellsdk-1.1.0#System_Management_Automation_PSPropertyInfo_IsGettable
if (!prop.IsGettable)
continue;
writer.WritePropertyName(prop.Name);
serializer.Serialize(writer, prop.Value);
}
writer.WriteEndObject();
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override bool CanRead { get { return false; } }
}
Notes:
In WriteJson you serialize the incoming object value as the value of each property. Surely you meant prop.Value.
By only returning true from CanConvert() when the incoming object type is of type PSObject, you avoid the need to implement default serialization for non-PSObject types in WriteJson().
When you call JToken.FromObject(value) you are not using the incoming JsonSerializer serializer. Thus, any JsonSerializerSettings (including converters) will be lost. In theory you could use JToken.FromObject(Object, JsonSerializer) instead, which would preserve settings, but if you did, you would encounter the bug described in JSON.Net throws StackOverflowException when using [JsonConvert()]. Luckily, since we now return false from CanConvert when default serialization is required, this is no longer necessary.
There is no need to construct an intermediate JObject. You can write directly to the JsonWriter, which will be somewhat more performant.
Update: Additionally, I am using serializing to camel case using CamelCasePropertyNamesContractResolver. Is there a way to make the converter respect that?
Once you introduce a custom JsonConverter for your type, you need to do everything manually, including remapping of property names. Here's a version of WriteJson() that handles this by using DefaultContractResolver.NamingStrategy:
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var psObj = (PSObject)value;
writer.WriteStartObject();
var resolver = serializer.ContractResolver as DefaultContractResolver;
var strategy = (resolver == null ? null : resolver.NamingStrategy) ?? new DefaultNamingStrategy();
foreach (var prop in psObj.Properties)
{
//Probably we shouldn't try to serialize a property that can't be read.
//https://learn.microsoft.com/en-us/dotnet/api/system.management.automation.pspropertyinfo.isgettable?view=powershellsdk-1.1.0#System_Management_Automation_PSPropertyInfo_IsGettable
if (!prop.IsGettable)
continue;
writer.WritePropertyName(strategy.GetPropertyName(prop.Name, false));
serializer.Serialize(writer, prop.Value);
}
writer.WriteEndObject();
}
Note that naming strategies were introduced in Json.NET 9.0.1 so if you are using an earlier version you will need to create your own camel case name mapper such as the one shown in this answer.
Related
I am trying to deserialize the following JSON (which validates on https://jsonlint.com/):
{"pandoc-api-version":[1,22],"meta":{"title":{"t":"MetaBlocks","c":[{"t":"Para","c":[{"t":"Str","c":"Dynamic"},{"t":"Space"},{"t":"Str","c":"Language"},{"t":"Space"},{"t":"Str","c":"Runtime"}]},{"t":"Para","c":[]}]}},"blocks":[{"t":"Para","c":[{"t":"Strong","c":[{"t":"Str","c":"Bill"},{"t":"Space"},{"t":"Str","c":"Chiles"},{"t":"Space"},{"t":"Str","c":"and"},{"t":"Space"},{"t":"Str","c":"Alex"},{"t":"Space"},{"t":"Str","c":"Turner"}]}]},{"t":"Para","c":[{"t":"Emph","c":[{"t":"Strong","c":[{"t":"Str","c":"Reading"},{"t":"Space"},{"t":"Str","c":"this"},{"t":"Space"},{"t":"Str","c":"Document:"}]}]}]}]}
into the following classes:
internal record TagContent(string T, OneOf<TagContent[], string>? C);
internal class RawPandoc {
[JsonProperty] public int[] PandocApiVersion = default!;
[JsonProperty] public Dictionary<string, TagContent> Meta = default!;
[JsonProperty] public TagContent[] Blocks = default!;
}
using the following code:
var settings = new JsonSerializerSettings {
ContractResolver = new DefaultContractResolver { NamingStrategy = new KebabCaseNamingStrategy() },
Converters = new JsonConverter[] { new OneOfJsonConverter() }
};
var pandoc = JsonConvert.DeserializeObject<RawPandoc>(s, settings);
and I get the following error:
Unexpected token when deserializing object: StartObject. Path 'meta.title.c[0]', line 1, position 69.
How can I resolve this?
For completeness, here is the current and incomplete code for OneOfJsonConverter. OneOf is a library for union types in C#:
using OneOf;
namespace PandocFilters {
public class OneOfJsonConverter : JsonConverter {
public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer) {
if (value is IOneOf of) {
value = of.Value;
}
serializer.Serialize(writer, value);
}
public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer) {
if (reader.Value is null) { return null; }
// TODO not implemented yet
return reader.Value;
}
public override bool CanConvert(Type objectType) => objectType.UnderlyingIfNullable().GetInterfaces().Contains(typeof(IOneOf));
}
}
Problem is you are not advancing the reader in your ReadJson implementation. You declared your converter can handle IOneOf objects, and so JSON.NET expects your converter to actually read and handle it, however it does nothing as of now. So ReadJson is called (at the start of first array in json which should be deserialized to OneOf), and then after it returns - reader position is still where it was before (at start of array), which is not what JSON.NET expects. Then it fails trying to continue reading next object, because its assumptions are violated. So, just implement ReadJson, and meanwhile you can advance a reader for example like that:
public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer) {
// advance reader as expected
var eitherStringOrArray = JObject.ReadFrom(reader);
return reader.Value;
}
With an attribute [JsonProperty(Required = Required.Always)] annotating my model I am able to check, that the property value is not null.
For strings I would like to check for empty values as well. Is there a way to check if string property value is empty while using attributes in Json.NET? Or is there any other way?
This JSON should cause a problem
{
"Property1": "",
...
}
By using a JSON schema you can define minimum lengths for specific properties, among other things. This is reasonably simple using the additional Json.NET Schema package, after slight modifications to the deserialization code to specify the schema to be used for validation. However, the Json.NET Schema library is not MIT licensed and uses restrictive/commercial licensing unlike Json.NET itself. There may be other implementations, though.
In a similar case I ended up (ab)using a JsonConverter to do validation instead. This is somewhat hacky as it's not really a converter, but the convenient thing is that it works purely by applying a Json.NET attribute to the model without any other code changes.
public class NonEmptyStringConverter : JsonConverter
{
public override bool CanConvert(Type objectType) => objectType == typeof(string);
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)
{
if (reader.TokenType != JsonToken.String)
throw CreateException($"Expected string value, but found {reader.TokenType}.", reader);
var value = (string)reader.Value;
if (String.IsNullOrEmpty(value))
throw CreateException("Non-empty string required.", reader);
return value;
}
private static Exception CreateException(string message, JsonReader reader)
{
var info = (IJsonLineInfo)reader;
return new JsonSerializationException(
$"{message} Path '{reader.Path}', line {info.LineNumber}, position {info.LinePosition}.",
reader.Path, info.LineNumber, info.LinePosition, null);
}
}
Usage:
[JsonProperty(Required = Required.Always)]
[JsonConverter(typeof(NonEmptyStringConverter))]
public string Property1 { get; set; }
The Required.Always should still be applied to handle cases where the property is missing completely, in which case the converter won't be called by Json.NET.
Using json.net this test fails by default:
JsonConvert.DeserializeObject(
JsonConvert.SerializeObject(new object()),
typeof(object)
).ShouldBeOfType<object>(); // actual type is JObject
Is there a way to change this behavior, so it deserializes to the actual requested type?
You have a degenerate test case there. If you instruct Json.Net to deserialize into type object, you are telling it that the JSON could represent any possible object. So it will choose to use a JObject in that case, since you were not specific and a JObject can handle any JSON object. It is not expecting that you want to deserialize into a literal empty object instance, because that is not a very useful thing to do. If the JSON contained any data at all, you would not be able to access that data after the deserialization: object has no properties!
You can fix your test by creating an empty class Foo and using that in place of object:
JsonConvert.DeserializeObject(
JsonConvert.SerializeObject(new Foo()), typeof(Foo)
).ShouldBeOfType<Foo>();
If you really do need to force Json.Net to deserialize into an empty object instance whenever object is specified as the type, you can do it using a custom JsonConverter like this:
public class EmptyObjectConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(object);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JToken token = JToken.Load(reader); // consume the JSON object from the reader
return token.Type == JTokenType.Null ? null : new object();
}
public override bool CanWrite
{
get { return false; }
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Then pass an instance of the converter to JsonConvert.DeserializeObject():
JsonConvert.DeserializeObject(
JsonConvert.SerializeObject(new object()),
typeof(object),
new EmptyObjectConverter()
).ShouldBeOfType<object>();
Fiddle: https://dotnetfiddle.net/7xZ7tm
I’m using Json.net to deserialize json data received by Web API call. Some fields often have html-encoded characters like " or & How can I have this characters automatically decoded during deserialization?
I came to 2 possible solutions:
Calling System.Web.HttpUtility.HtmlDecode() in property setter like:
public string Title
{
set
{
title = System.Web.HttpUtility.HtmlDecode(value);
}
}
Writing custom JsonConverter that calls System.Web.HttpUtility.HtmlDecode() in ReadJson() method:
public class HtmlEncodingConverter : Newtonsoft.Json.JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(String);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
return System.Web.HttpUtility.HtmlDecode((string)reader.Value);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
writer.WriteRawValue(System.Web.HttpUtility.HtmlEncode((string)value));
}
}
But is there any built-in solution that allows to perform html-decoding during json deserialization without additional code?
System.Net.WebUtility.HtmlDecode()
or
HttpUtility.HtmlDecode()
is the way to go, nothing built in regarding the JsonSerializer.
How can I customize the serialized output of ASP.NET Web API?
Let's say, I want all values in uppercase.
You have access to the JSON.NET serializer settings. With JSON.NET you can overide conversions using converters e.g. this datetime one.
You can also implement your own from inheritting from the abstract JsonConverter. See here for details.
For your example create the converter:
public class UpperCaseStringConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(string);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
return reader.Value.ToString();
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var outputValue = value as string;
writer.WriteValue(outputValue == null ? null : outputValue.ToUpper());
}
}
And then to register this globally add this config:
Registration example from here
JsonMediaTypeFormatter jsonFormatter = GlobalConfiguration.Configuration.Formatters.JsonFormatter;
JsonSerializerSettings jSettings = new Newtonsoft.Json.JsonSerializerSettings();
jSettings.Converters.Add(new UpperCaseStringConverter());
jsonFormatter.SerializerSettings = jSettings;
To add to a single property on a model just add the annotation:
[JsonConverter(typeof(UpperCaseStringConverter))]