Fix double-encoding of non-string JSON values in nested blocks and add SingleBlock mapper

[koty10]

Fix: Double-encoding of non-string JSON values inside block properties during import + missing SingleBlock mapper

Problem

When importing content that contains nested block structures (e.g. a BlockGrid with an area containing items that have SingleBlock properties with DropDown.Flexible fields), non-string JSON values get corrupted during import.

For example, a dropdown value ["Orange"] (a JSON array) is correctly stored in the serialized XML, but after import it becomes the JSON string "[\"Orange\"]" (double-encoded). This causes JsonReaderException errors when rendering the page and shows invalid values in the backoffice.

Root cause

SyncBlockMapperBase<T>.GetImportProperty() converts each block property value to a string via GetStringValue(), sends it through the mapper collection, and assigns the result back to BlockPropertyValue.Value. For editors without a dedicated mapper (e.g. Umbraco.DropDown.Flexible), the raw string is returned and assigned back. When the parent block is re-serialized via SerializeJsonString(), values that were originally JSON arrays/objects/numbers (but are now C# strings) get double-encoded.

The export side (GetExportProperty) already handles this correctly by converting the result back to a JsonNode:

return result.ConvertToJsonNode()?.ExpandAllJsonInToken() ?? result;

But the import side was missing this conversion.

Fix

1. SyncBlockMapperBase.GetImportProperty() — When the original BlockPropertyValue.Value was a non-string JSON type (JsonElement with ValueKind of Array, Object, Number, etc.), the string result is now converted back to a JsonNode before being assigned back. This preserves the correct JSON type and prevents double-encoding. String-type values are left unchanged to avoid altering their type.

2. New SingleBlockMapper — Added a mapper for Umbraco.SingleBlock (analogous to existing BlockListMapper and BlockGridMapper). Without this, properties inside SingleBlock editors were never recursively processed during import — UDIs in MultiUrlPickers, media references, etc. inside SingleBlock were not mapped, and the entire SingleBlock value was subject to the double-encoding issue.

Affected scenarios

Any property editor without a dedicated ISyncMapper that stores non-string JSON values (arrays, objects, numbers) inside any block type (BlockGrid, BlockList, SingleBlock). The most visible case is Umbraco.DropDown.Flexible which stores values as ["SelectedValue"].

How to reproduce

1. Create a page with a BlockGrid containing blocks that have a SingleBlock field
2. Inside the SingleBlock, add a block with a DropDown.Flexible property, select a value (e.g. "Orange")
3. Export via uSync
4. On a different instance (clean DB), import via uSync
5. The dropdown value is corrupted — page rendering fails with JsonReaderException

[Copilot]

Pull request overview

This PR fixes a double-encoding issue where non-string JSON values (e.g. ["Orange"] from Umbraco.DropDown.Flexible) inside nested block properties were being re-serialized as JSON strings on import, corrupting the value. It also introduces a missing SingleBlockMapper so that properties inside Umbraco.SingleBlock editors are recursively processed in the same way as BlockList and BlockGrid.

Changes:

• In SyncBlockMapperBase.GetImportProperty, when the original value was a non-string JSON type, convert the mapped string result back to a JsonNode to preserve the original JSON shape.
• Add a IsNonStringJsonValue helper that detects JsonElement (any ValueKind except String/Undefined) as well as JsonArray/JsonObject.
• Add a new SingleBlockMapper registered via ISyncMapper discovery, mirroring BlockListMapper/BlockGridMapper but typed as SyncBlockMapperBase<SingleBlockValue>.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
uSync.Core/Mapping/SyncBlockMapperBase.cs	Adds JSON-type preservation on the import path to avoid double-encoding non-string values.
uSync.Core/Mapping/Mappers/SingleBlockMapper.cs	New mapper enabling recursive import/export processing of Umbraco.SingleBlock properties.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[KevinJump]

Hi Thanks for this, done some tests and it looks good 👍

• i've just moved the static IsNonStringJsonValue into out JsonExtensions class, in case we want to use it somewhere else.

[KevinJump]

In release v17.3.2 - https://www.nuget.org/packages/uSync/17.3.2