JsonSerializer throws InvalidCastException

[Fr1z2r]

Using .NET Core App 3.1.101
This code throws an exception when IgnoreNullValues=true, and doesn't throw when IgnoreNullValues=false:

class MyClass
{
    public HashSet<int> X { get; set; } = new HashSet<int>();
}

public static void Main()
{
    JsonSerializer.Deserialize<MyClass>(@"{""X"":[2,1]}", 
        new JsonSerializerOptions {IgnoreNullValues = true});
}

System.InvalidCastException : Unable to cast object of type 'System.Collections.Generic.HashSet`1[System.Int32]' to type 'System.Collections.IList'.
   at System.Text.Json.JsonSerializer.ApplyObjectToEnumerable(Object value, ReadStack& state, Boolean setPropertyDirectly)
   at System.Text.Json.JsonSerializer.HandleEndArray(JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
   at System.Text.Json.JsonSerializer.ReadCore(Type returnType, JsonSerializerOptions options, Utf8JsonReader& reader)
   at System.Text.Json.JsonSerializer.Deserialize(String json, Type returnType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options)

[jozkee]

This seems to be broken in 3.1 and 3.0. It was fixed for 5.0 under dotnet/corefx#42420

Repro steps:

• Deserialize into properties that contain a default value, e.g. MyClass implemented above.
• use IgnoreNullValues = true option.
• Property needs to be a collection that uses an internal converter.
• A few type affected by this
  • HashSet<T>
  • Stack<T>
  • Queue<T>
  • Stack
  • Queue
  • T[]

cc @ahsonkhan @layomia

[layomia]

This is fixed in 5.0, and the workaround for 3.0/3.1 is to not initialize the collection. Does this work for you, or can you switch to 5.0?

Will close - this can be reopened if this is blocking on 3.1.