System.Text.Json cannot deserialize to IReadOnlySet

[StasJS]

Description

Reading through https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/supported-collection-types JsonSerializer.Deserialize will support deserialization to a ton of different collection interfaces.

Strangely, IReadOnlySet is not mentioned at all in the page I linked. Not even to say it's not supported, as some collection types have e.g. LinkedListNode<T>.

When trying to deserialize to an IReadOnlySet, JsonSerializer throws a NotSupportedException

Reproduction Steps

• Run dotnet new console
• Write Program.cs

using System.Collections.Immutable;
using System.Text.Json;

// Both of these work as expected
var setResult = JsonSerializer.Deserialize<ISet<int>>("[1]");
var immutableSetResult = JsonSerializer.Deserialize<IImmutableSet<int>>("[1]");

// throws NotSupportedException
JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]");

Expected behavior

JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]"); ought to work, like it does for ISet or other readonly collections e.g. IReadOnlyList.

I would also expect IReadOnlySet to be featured in this page https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/supported-collection-types one way or another.

Actual behavior

JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]"); throws NotSupportedException and completely absent from https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/supported-collection-types

Regression?

Don't know.

Known Workarounds

Use ISet or similar.

Configuration

> dotnet sdk check
Version      Status
------------------------
7.0.400      Up to date.

Microsoft.NETCore.App             7.0.10       Up to date.
Microsoft.WindowsDesktop.App      7.0.10       Up to date.

Windows 11 Pro x64

> [System.Environment]::OSVersion
Platform ServicePack Version      VersionString
-------- ----------- -------      -------------
 Win32NT             10.0.22623.0 Microsoft Windows NT 10.0.22623.0

Other information

No response

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

Reading through https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/supported-collection-types JsonSerializer.Deserialize will support deserialization to a ton of different collection interfaces.

Strangely, IReadOnlySet is not mentioned at all in the page I linked. Not even to say it's not supported, as some collection types have e.g. LinkedListNode<T>.

When trying to deserialize to an IReadOnlySet, JsonSerializer throws a NotSupportedException

Reproduction Steps

• Run dotnet new console
• Write Program.cs

using System.Collections.Immutable;
using System.Text.Json;

// Both of these work as expected
var setResult = JsonSerializer.Deserialize<ISet<int>>("[1]");
var immutableSetResult = JsonSerializer.Deserialize<IImmutableSet<int>>("[1]");

// throws NotSupportedException
JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]");

Expected behavior

JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]"); ought to work, like it does for ISet or other readonly collections e.g. IReadOnlyList.

I would also expect IReadOnlySet to be featured in this page https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/supported-collection-types one way or another.

Actual behavior

JsonSerializer.Deserialize<IReadOnlySet<int>>("[1]"); throws NotSupportedException and completely absent from https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/supported-collection-types

Regression?

Don't know.

Known Workarounds

Use ISet or similar.

Configuration

> dotnet sdk check
Version      Status
------------------------
7.0.400      Up to date.

Microsoft.NETCore.App             7.0.10       Up to date.
Microsoft.WindowsDesktop.App      7.0.10       Up to date.

Windows 11 Pro x64

> [System.Environment]::OSVersion
Platform ServicePack Version      VersionString
-------- ----------- -------      -------------
 Win32NT             10.0.22623.0 Microsoft Windows NT 10.0.22623.0

Other information

No response

Author:	StasJS
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

Because contract customization has not been fully implemented for collection types, it is not possible to work around this issue by customizing the JsonTypeInfo.CreateObject delegate. Instead, implementing a custom converter for the interface is necessary. Until this is addressed, my recommendation is to deserialize on models that use concrete collection types.

We should audit all built-in collection interfaces and check whether they are supported by the serializer. One possibility is to honor the newly added CollectionBuilderAttribute and use the delegate as specified by the interface declaration.

[huoyaoyuan]

Since HashSet<T> and ISet<T> are supported, it's reasonable to also include IReadOnlySet<T>.

[StasJS]

Thanks for the update. @eiriktsarpalis do you see there being value in reconciling the documentation with current behaviour?
It sounds like any fix won't ship for quite some time.

[eiriktsarpalis]

Yes, I think we should be documenting what types are supported.

[JohnathanBarclay]

Since HashSet<T> and ISet<T> are supported, it's reasonable to also include IReadOnlySet<T>.

Other read-only collection types e.g. IReadOnlyCollection<T> & IReadOnlyList<T> are also supported, so this seems like an oversight.

[KrzysztofBranicki]

@eiriktsarpalis

Until this is addressed, my recommendation is to deserialize on models that use concrete collection types.

There is no concrete type ReadOnlySet<T> like there is e.g. ReadOnlyCollection<T>, also in this documentation documentation you say that ReadOnlyCollection<T> does not support deserialization anyway (which is a separate question on it's own why that is).

[eiriktsarpalis]

There is no concrete type ReadOnlySet like there is e.g. ReadOnlyCollection

Don't ImmutableHashSet<T> or ImmutableSortedSet<T> work?

[KrzysztofBranicki]

Don't ImmutableHashSet<T> or ImmutableSortedSet<T> work?

They do if you are willing to accept worst performance of those collections in scenarios where they are just used in DTOs (no expected mutations). And if someone doesn't care about performance he may just stick with Newtonsoft.Json.

[eiriktsarpalis]

If that is a concern, then HashSet<T> and SortedSet<T> are alternatives, the trade-off being that they expose mutable APIs.