As inspired by this answer by Brian Rogers and other answers to How to handle both a single item and an array for the same property using JSON.net, you can create a generic JsonConverter<List<T>> that checks whether the incoming JSON value is an array, and if not, deserializes an item of type T and returns the item wrapped in an appropriate list. Even better, you can create a JsonConverterFactory that manufactures such a converter for all list types List<T> encountered in your serialization graph.
First, define the following converter and converter factory:
public class SingleOrArrayConverter<TItem> : SingleOrArrayConverter<List<TItem>, TItem>
{
public SingleOrArrayConverter() : this(true) { }
public SingleOrArrayConverter(bool canWrite) : base(canWrite) { }
}
public class SingleOrArrayConverterFactory : JsonConverterFactory
{
public bool CanWrite { get; }
public SingleOrArrayConverterFactory() : this(true) { }
public SingleOrArrayConverterFactory(bool canWrite) => CanWrite = canWrite;
public override bool CanConvert(Type typeToConvert)
{
var itemType = GetItemType(typeToConvert);
if (itemType == null)
return false;
if (itemType != typeof(string) && typeof(IEnumerable).IsAssignableFrom(itemType))
return false;
if (typeToConvert.GetConstructor(Type.EmptyTypes) == null || typeToConvert.IsValueType)
return false;
return true;
}
public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
{
var itemType = GetItemType(typeToConvert);
var converterType = typeof(SingleOrArrayConverter<,>).MakeGenericType(typeToConvert, itemType);
return (JsonConverter)Activator.CreateInstance(converterType, new object [] { CanWrite });
}
static Type GetItemType(Type type)
{
// Quick reject for performance
if (type.IsPrimitive || type.IsArray || type == typeof(string))
return null;
while (type != null)
{
if (type.IsGenericType)
{
var genType = type.GetGenericTypeDefinition();
if (genType == typeof(List<>))
return type.GetGenericArguments()[0];
// Add here other generic collection types as required, e.g. HashSet<> or ObservableCollection<> or etc.
}
type = type.BaseType;
}
return null;
}
}
public class SingleOrArrayConverter<TCollection, TItem> : JsonConverter<TCollection> where TCollection : class, ICollection<TItem>, new()
{
public SingleOrArrayConverter() : this(true) { }
public SingleOrArrayConverter(bool canWrite) => CanWrite = canWrite;
public bool CanWrite { get; }
public override TCollection Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
switch (reader.TokenType)
{
case JsonTokenType.Null:
return null;
case JsonTokenType.StartArray:
var list = new TCollection();
while (reader.Read())
{
if (reader.TokenType == JsonTokenType.EndArray)
break;
list.Add(JsonSerializer.Deserialize<TItem>(ref reader, options));
}
return list;
default:
return new TCollection { JsonSerializer.Deserialize<TItem>(ref reader, options) };
}
}
public override void Write(Utf8JsonWriter writer, TCollection value, JsonSerializerOptions options)
{
if (CanWrite && value.Count == 1)
{
JsonSerializer.Serialize(writer, value.First(), options);
}
else
{
writer.WriteStartArray();
foreach (var item in value)
JsonSerializer.Serialize(writer, item, options);
writer.WriteEndArray();
}
}
}
Then add the the converter factory to JsonSerializerOptions.Converters before deserialization:
var options = new JsonSerializerOptions
{
Converters = { new SingleOrArrayConverterFactory() },
PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
};
var list = JsonSerializer.Deserialize<List<Item>>(json, options);
Or add a specific converter either to options or to your data model directly using JsonConverterAttribute:
class Item
{
public string Email { get; set; }
public int Timestamp { get; set; }
public string Event { get; set; }
[JsonConverter(typeof(SingleOrArrayConverter<string>))]
public List<string> Category { get; set; }
}
If your data model uses some other type of collection, say ObservableCollection<string>, you can apply a lower level converter SingleOrArrayConverter<TCollection, TItem> as follows:
[JsonConverter(typeof(SingleOrArrayConverter<ObservableCollection<string>, string>))]
public ObservableCollection<string> Category { get; set; }
Notes:
If you want the converter(s) to apply only during deserialization, pass canWrite: false to the parameterized constructor:
Converters = { new SingleOrArrayConverterFactory(canWrite: false) }
The converter will still get used, but will unconditionally generate a default serialization.
The converter is not implemented for jagged 2d or nD collections such as List<List<string>>. It is also not implemented for arrays and read-only collections.
According to Serializer support for easier object and collection converters #1562, because JsonConverter<T> lacks an async Read() method,
A limitation of the existing [JsonConverter] model is that it must "read-ahead" during deserialization to fully populate the buffer up to the end up the current JSON level. This read-ahead only occurs when the async+stream JsonSerializer deserialize methods are called and only when the current JSON for that converter starts with a StartArray or StartObject token.
Thus using this converter to deserialize potentially very large arrays may have a negative performance impact.
As discussed in the same thread, the converter API may get redesigned in System.Text.Json - 5.0 to fully support async deserialization by converters for arrays and object, implying that this converter may benefit from being rewritten when .NET 5 (no longer labeled with "Core") is eventually released.
Demo fiddle here.
