Question about JSON (De)Serialization using System.Text.Json.

Question

This is a Q about JSON (De)Serialization using System.Text.Json.

There are three classes involved, abbreviated here:

    internal class FolderState
    {
        /// <summary>
        /// Stack elemens
        /// </summary>
        /// 
        [JsonInclude] public int LastFileNum { get; set; }
        [JsonInclude] public int LastFolderNum { get; set; }
        [JsonInclude] public String Path { get; set; }
		.
		.
		.
	}
	
	internal class FolderStack<T> : Stack<T>
    {
        public FolderStack()
        {
        }
    }
	
	// For non-reflection JSON (De)Serialization
    [JsonSourceGenerationOptions(WriteIndented = true)]
    [JsonSerializable(typeof(FolderStack<FolderState>))]
    [JsonSerializable(typeof(FolderState))]
    internal partial class SourceGenerationContext : JsonSerializerContext { }
	
	internal class FolderStateStack
    {
        public FolderStack<FolderState> FolderStack { get; set; }
		.
		.
		.
	}

The FolderStateStack class has a member FolderStack which holds a stack of FolderState objects. Also added are the markers, described in How to use source generation in System.Text.Json - .NET | Microsoft Learn, to enable source-code generation mode (instead of reflection mode).

The FolderStack is serialized to JSON like this

String jsonStr = JsonSerializer.Serialize(FolderStack, SourceGenerationContext.Default.FolderStackFolderState);

Which results in this JSON

[
  {
    "LastFileNum": 0,
    "LastFolderNum": -1,
    "Path": "C:\\Users\\William\\Pictures"
  }
]

The attempt to deserialize the JSON codes like this

FolderStack = JsonSerializer.Deserialize(fileStream, SourceGenerationContext.Default.FolderStackFolderState);

This results in an exception, it seems because the fields do not match, the JSON does not fully define the FolderState elements.

In order to Deserialize the JSON it should look more like this, with the FolderState elements labeled?

[
	"FolderState":
	[
  		{
    		"LastFileNum": 0,
    		"LastFolderNum": -1,
    		"Path": "C:\\Users\\Robert\\Pictures"
  		}
	]
]

Any thoughts on how to correct the Serialization so that the FolderState is labeled??

TY!!

Answer 1

System.Text.Json does not require or support “labeling” the element type inside a JSON array. The JSON produced by

[
  {
    "LastFileNum": 0,
    "LastFolderNum": -1,
    "Path": "C:\\Users\\William\\Pictures"
  }
]

is already the correct representation for FolderStack<FolderState> (a stack of FolderState objects). The deserialization error is not because FolderState is unlabeled.

Key points and fixes:

1. Array of objects is the correct shape
   JsonSerializer.Deserialize<FolderStack<FolderState>>(...) expects a JSON array of objects whose properties match FolderState. Extra properties in JSON are ignored by default; missing properties are allowed unless marked as required. There is no need for:

      [
        FolderState:
        [
          { ... }
        ]
      ]
   

   That is not valid JSON and will not be understood by System.Text.Json.
2. Use the generated type info correctly
   With source generation, the correct call for a FolderStack<FolderState> is:

      // Serialize
      string jsonStr = JsonSerializer.Serialize(
          folderStateStack.FolderStack,
          SourceGenerationContext.Default.FolderStackFolderState);
      
      // Deserialize from string
      folderStateStack.FolderStack = JsonSerializer.Deserialize(
          jsonStr,
          SourceGenerationContext.Default.FolderStackFolderState);
   

   When deserializing from a stream:

      using var fileStream = File.OpenRead(path);
      folderStateStack.FolderStack = JsonSerializer.Deserialize(
          fileStream,
          SourceGenerationContext.Default.FolderStackFolderState);
   

   The second parameter (FolderStackFolderState) is the generated JsonTypeInfo<FolderStack<FolderState>>, which matches the JSON array of FolderState objects.
3. Ensure the generic type is included in the context
   The context already includes:

      [JsonSerializable(typeof(FolderStack<FolderState>))]
      [JsonSerializable(typeof(FolderState))]
      internal partial class SourceGenerationContext : JsonSerializerContext { }
   

   This is correct. No additional labeling is required in JSON.
4. Check for other causes of the exception
   Since System.Text.Json ignores unknown JSON properties and allows missing ones by default, the usual causes of exceptions here are:
   • The JSON is not valid (trailing commas, comments, or the “FolderState:” text).
   • The stream is not positioned at the beginning (e.g., reused stream without Position = 0).
   • A different type is being deserialized than the one registered in the context.
   Ensure the JSON is exactly the array shown above and that the deserialization call uses the same type info that was used for serialization.

In summary, the JSON shape is already correct; remove any extra labels like FolderState: and ensure the deserialization uses SourceGenerationContext.Default.FolderStackFolderState against a valid JSON array.

---

References:

• How to read JSON as .NET objects (deserialize)
• JSON serialization and deserialization in .NET - overview
• Migrate from Newtonsoft.Json to System.Text.Json (dotnet-8-0)
• Migrate from Newtonsoft.Json to System.Text.Json (dotnet-10-0)