JsonSerializer.Serialize an anonymous type, with duplicate nested names throws an unexpected exception

[dbaghdanov]

Description

Upgrading my project from .netcore 3.1 to .net6 I came across one of my tests failing due to a change in how JsonSerializer.Serialize will serialize an anonymous type.

Reproduction Steps

var obj = new 
{
  input = new {
    foo = "foo"
  },
  INPUT = new {
    foo = "foo"
  }
};

var json = JsonSerializer.Serialize(obj); // throws an exception

Expected behavior

In .netcore 3.1 the above would have returned:

{"input":{"foo":"bar"},"INPUT":{"foo":"bar"}}

Actual behavior

In .net5+ this exception is thrown instead:

Unhandled exception. System.InvalidOperationException: Members 'input' and 'INPUT' on type '<>f__AnonymousType2`2[<>f__AnonymousType1`1[System.String],<>f__AnonymousType1`1[System.String]]' cannot both bind with parameter 'input' in the deserialization constructor.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_MultiplePropertiesBindToConstructorParameters(Type parentType, String parameterName, String firstMatchName, String secondMatchName)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.InitializeConstructorParameters(JsonParameterInfoValues[] jsonParameters, Boolean sourceGenMode)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo..ctor(Type type, JsonConverter converter, Type runtimeType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.<InitializeForReflectionSerializer>g__CreateJsonTypeInfo|112_0(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.GetClassFromContextOrCreate(Type type)
   at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type type)
   at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type runtimeType)
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)

Unhandled exception. System.InvalidOperationException: Members 'input' and 'INPUT' on type '<>f__AnonymousType22[<>f__AnonymousType11[System.String],<>f__AnonymousType1`1[System.String]]' cannot both bind with parameter 'input' in the deserialization constructor.

Regression?

Worked as I expected in .netcore 3.1 but not in .net5+

Known Workarounds

If I were to create a class to represent my anonymous type instead, then I do receive the expected JSON string.

public sealed class Obj
{
    public Input input {get; set;}
    public Input INPUT {get; set;}
}

public sealed class Input
{
    public string foo {get;set;}	
}

And instantiate similar to my anonymous type:

var obj = new Obj()
{
    input = new Input()
    {
        foo="bar"
    }, 
    INPUT = new Input()
    {
        foo="bar"
    }
};

var json = JsonSerializer.Serialize(obj);

I do get my expected JSON string:

{"input":{"foo":"bar"},"INPUT":{"foo":"bar"}}

Configuration

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.405
 Commit:    27ab36058b

Runtime Environment:
 OS Name:     debian
 OS Version:  11
 OS Platform: Linux
 RID:         debian.11-x64
 Base Path:   /usr/share/dotnet/sdk/6.0.405/

global.json file:
  /workspaces/bipsli/global.json

Host:
  Version:      6.0.13
  Architecture: x64
  Commit:       1af80ba017

.NET SDKs installed:
  6.0.405 [/usr/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.13 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.13 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

Other information

I posted this same question to StackOverflow where it was suggested that I open an issue here.
https://stackoverflow.com/questions/75317065/exception-serializing-anonymous-type-to-json-when-nested-property-names-are-the

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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

Upgrading my project from .netcore 3.1 to .net6 I came across one of my tests failing due to a change in how JsonSerializer.Serialize will serialize an anonymous type.

Reproduction Steps

var obj = new 
{
  input = new {
    foo = "foo"
  },
  INPUT = new {
    foo = "foo"
  }
};

var json = JsonSerializer.Serialize(obj); // throws an exception

Expected behavior

In .netcore 3.1 the above would have returned:

{"input":{"foo":"bar"},"INPUT":{"foo":"bar"}}

Actual behavior

In .net5+ this exception is thrown instead:

Unhandled exception. System.InvalidOperationException: Members 'input' and 'INPUT' on type '<>f__AnonymousType2`2[<>f__AnonymousType1`1[System.String],<>f__AnonymousType1`1[System.String]]' cannot both bind with parameter 'input' in the deserialization constructor.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_MultiplePropertiesBindToConstructorParameters(Type parentType, String parameterName, String firstMatchName, String secondMatchName)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo.InitializeConstructorParameters(JsonParameterInfoValues[] jsonParameters, Boolean sourceGenMode)
   at System.Text.Json.Serialization.Metadata.JsonTypeInfo..ctor(Type type, JsonConverter converter, Type runtimeType, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.<InitializeForReflectionSerializer>g__CreateJsonTypeInfo|112_0(Type type, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializerOptions.GetClassFromContextOrCreate(Type type)
   at System.Text.Json.JsonSerializerOptions.GetOrAddClass(Type type)
   at System.Text.Json.JsonSerializer.GetTypeInfo(JsonSerializerOptions options, Type runtimeType)
   at System.Text.Json.JsonSerializer.Serialize[TValue](TValue value, JsonSerializerOptions options)

Unhandled exception. System.InvalidOperationException: Members 'input' and 'INPUT' on type '<>f__AnonymousType22[<>f__AnonymousType11[System.String],<>f__AnonymousType1`1[System.String]]' cannot both bind with parameter 'input' in the deserialization constructor.

Regression?

Worked as I expected in .netcore 3.1 but not in .net5+

Known Workarounds

If I were to create a class to represent my anonymous type instead, then I do receive the expected JSON string.

public sealed class Obj
{
    public Input input {get; set;}
    public Input INPUT {get; set;}
}

public sealed class Input
{
    public string foo {get;set;}	
}

And instantiate similar to my anonymous type:

var obj = new Obj()
{
    input = new Input()
    {
        foo="bar"
    }, 
    INPUT = new Input()
    {
        foo="bar"
    }
};

var json = JsonSerializer.Serialize(obj);

I do get my expected JSON string:

{"input":{"foo":"bar"},"INPUT":{"foo":"bar"}}

Configuration

$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.405
 Commit:    27ab36058b

Runtime Environment:
 OS Name:     debian
 OS Version:  11
 OS Platform: Linux
 RID:         debian.11-x64
 Base Path:   /usr/share/dotnet/sdk/6.0.405/

global.json file:
  /workspaces/bipsli/global.json

Host:
  Version:      6.0.13
  Architecture: x64
  Commit:       1af80ba017

.NET SDKs installed:
  6.0.405 [/usr/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.13 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.13 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

Other information

I posted this same question to StackOverflow where it was suggested that I open an issue here.
https://stackoverflow.com/questions/75317065/exception-serializing-anonymous-type-to-json-when-nested-property-names-are-the

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

[eiriktsarpalis]

I can reproduce. This appears to have been caused by the introduction of constructor deserialization support. Constructor deserialization will attempt to bind constructor parameters to properties using case insensitive comparison, so if the type uses two properties with different casing it will fail at that binding phase. We should consider relaxing this restriction in line with work in #71944.

In the meantime, it should be possible to work around your issue using classes instead of anonymous types when you serialize.