System.Text.Json code generation fails on recursive IReadOnlyDictionary

[ejball]

Description

System.Text.Json code generation fails with error CS1031: Type expected if the serializable class has an IReadOnlyDictionary whose value type is that same serializable class.

Reproduction Steps

Include these two classes in a C# library or console app that references System.Text.Json 6.0.6:

public class Dto
{
    public IReadOnlyDictionary<string, Dto>? Data { get; set; }
}

[JsonSerializable(typeof(Dto))]
public partial class JsonContext : JsonSerializerContext
{
}

Expected behavior

Compilation fails with: ...\System.Text.Json.SourceGeneration\System.Text.Json.SourceGeneration.JsonSourceGenerator\JsonContext.IReadOnlyDictionaryStringDto.g.cs(27,128,27,129): error CS1031: Type expected

That badly generated line of code looks like this: ObjectCreator = () => new global::System.Collections.Generic.Dictionary<global::System.String, >(),

Actual behavior

It should succeed without error, as it does if Dictionary is used instead of IReadOnlyDictionary.

Regression?

No response

Known Workarounds

Use Dictionary instead of IReadOnlyDictionary.

Configuration

.NET 6.0.401 on Windows x64.

Other information

No response

[ghost]

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Issue Details

---

Description

System.Text.Json code generation fails with error CS1031: Type expected if the serializable class has an IReadOnlyDictionary whose values is that same serializable class.

Reproduction Steps

Include these two classes in a C# library or console app that references System.Text.Json 6.0.6:

public class Dto
{
    public IReadOnlyDictionary<string, Dto>? Data { get; set; }
}

[JsonSerializable(typeof(Dto))]
public partial class JsonContext : JsonSerializerContext
{
}

Expected behavior

Compilation fails with: ...\System.Text.Json.SourceGeneration\System.Text.Json.SourceGeneration.JsonSourceGenerator\JsonContext.IReadOnlyDictionaryStringDto.g.cs(27,128,27,129): error CS1031: Type expected

That badly generated line of code looks like this: ObjectCreator = () => new global::System.Collections.Generic.Dictionary<global::System.String, >(),

Actual behavior

It should succeed without error, as it does if Dictionary is used instead of IReadOnlyDictionary.

Regression?

No response

Known Workarounds

Use Dictionary instead of IReadOnlyDictionary.

Configuration

.NET 6.0.401 on Windows x64.

Other information

No response

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

[eiriktsarpalis]

I can repro the issue -- fundamentally the issue stems from the fact that the source generator's "parser" component eagerly evaluates properties of the recursive type, even though it hasn't been fully populated yet:

runtime/src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs

Lines 957 to 960 in 957f76f

	if (needsRuntimeType)
	{
	runtimeTypeRef = GetDictionaryTypeRef(collectionKeyTypeSpec, collectionValueTypeSpec);
	}

I suspect that this issue has the potential to surface in other cases as well, so in fixing this we should carefully audit the parser code for any other instances where it could be reading from uninitialized metadata, perhaps by making such scenaria impossible by construction.