JsonSerializer throws InvalidCastException

[folkcoder]

Using .NET Core 3.0 or 3.1.100-preview3-014645, an ICollection<string> property with a private backing field assigned a custom implementation fails to serialize.

    public class Client
    {
        private ICollection<string> _allowedGrantTypes = new GrantTypeValidatingHashSet();

        // this works 
        // public GrantTypeValidatingHashSet AllowedGrantTypes { get; set; }

        // this works 
        // public ICollection<string> AllowedGrantTypes { get; set; } = new GrantTypeValidatingHashSet();

        // this errors
        public ICollection<string> AllowedGrantTypes
        {
            get { return _allowedGrantTypes; }
            set
            {
                _allowedGrantTypes = new GrantTypeValidatingHashSet();
            }
        }
    }

Error message

System.InvalidCastException: 'Unable to cast object of type 'SerializationSample.GrantTypeValidatingHashSet' to type 'System.Collections.Generic.IList`1[System.String]'.'

Stack trace

at System.Text.Json.JsonSerializer.ApplyValueToEnumerable[TProperty](TProperty& value, ReadStack& state)
   at System.Text.Json.JsonPropertyInfoNotNullable`4.OnReadEnumerable(ReadStack& state, Utf8JsonReader& reader)
   at System.Text.Json.JsonPropertyInfo.ReadEnumerable(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
   at System.Text.Json.JsonPropertyInfo.Read(JsonTokenType tokenType, ReadStack& state, Utf8JsonReader& reader)
   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)
   at SerializationSample.Program.Main() in C:\SerializationSample\SerializationSample\Program.cs:line 13

This was discovered while attempting to serialize a Client model from IdentityServer4. I've created a bare-bones project duplicating the error. Serialization occurs correctly with Newtonsoft.Json.

[Fr1z2r]

Here is another simpler example. 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)

[gingters]

This should also be fixed in 3.0 (still in Maintenance) and 3.1 (LTS). I reproduced that issue also in 3.1.101 and in 3.1.200-preview-014883.

[GioviQ]

After this upgrade https://devblogs.microsoft.com/aspnet/blazor-webassembly-3-2-0-preview-3-release-now-available/
I still have the issue of @folkcoder

[benedict1986]

I am using 3.1.0 and also having this issue.

[GioviQ]

In fact System.Text.Json.JsonSerializer does not support HashSet<>

[am11]

Seems like it has been fixed in the master branch for .NET 5 - https://github.com/dotnet/installer/blob/master/README.md#installers-and-binaries:

#!/usr/bin/env bash

# linux

mkdir /tmp/dotnet5
curl -sSL https://aka.ms/dotnet/net5/dev/Sdk/dotnet-sdk-linux-x64.tar.gz | tar -xvz -C /tmp/dotnet5 > /dev/null

/tmp/dotnet5/dotnet new console -n testhashset

cat <<'EOF' > testhashset/Program.cs
using System;
using System.Collections.Generic;
using System.Text.Json;

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

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

        Console.WriteLine("Count of {0} is: {1}", nameof(myObject.X), myObject.X.Count);
    }
}
EOF

/tmp/dotnet5/dotnet publish testhashset/ -o my-publish-dir/ /p:PublishSingleFile=true -r linux-x64

./my-publish-dir/testhashset

# prints
#> Count of X is: 2

[benedict1986]

Does it mean I won't be able to use it in my Asp.Net Core 3.1 project? That sounds terrible since .net core 5.0 is in preview and some other dependencies may have some issue.

[GioviQ]

Until then

public static class HttpClientJsonExtensions
    {
        //System.Text.Json does not like HashSet. So I have to use Newtonsoft
        //See https://github.com/dotnet/runtime/issues/31553
        //TODO remove and use GetFromJsonAsync in the future
        public static async Task<T> GetNewtonsoftJsonAsync<T>(this HttpClient httpClient, string requestUri)
        {
            var stringContent = await httpClient.GetStringAsync(requestUri);
            return Newtonsoft.Json.JsonConvert.DeserializeObject<T>(stringContent);
        }
    }

[benedict1986]

Thank you @GioviQ for your tip. I eventually make it working by installing Microsoft.AspNetCore.Mvc.NewtonsoftJson and then use .AddNewtonsoftJson() in the following location:

services
.AddControllersWithViews(option => option.EnableEndpointRouting = false) 
.SetCompatibilityVersion(CompatibilityVersion.Version_3_0)
.AddNewtonsoftJson();