Add Roslyn expression support for ChildSelector.Child

[Copilot]

• Confirm current branch contents and identify .csproj edits
• Revert .csproj changes while preserving non-.csproj build fixes
• Re-run restore/build checks to verify resulting behavior
• Summarize what remains fixed vs environment-limited

Original prompt

Add support for using C# Roslyn expressions for ChildSelector.Child values in microsoft/Forge.

Context gathered from the repository:

• ChildSelector.Child is currently declared as string and documented as a string key pointer to a child TreeNode.
• TreeWalkerSession.SelectChild(TreeNode treeNode) currently evaluates only ShouldSelect, then returns cs.Child directly.
• Other dynamic schema properties in Forge already support Roslyn evaluation via EvaluateDynamicProperty, using the C#| prefix (TreeNode.Properties, action Input, Timeout, etc.).
• README/examples already emphasize Roslyn evaluation in schema properties and child selection conditions.

Requested change:

• Add C# Roslyn expression support to ChildSelector.Child, so schema authors can provide a dynamic expression like "C#|..." for the target child node key.

Implementation guidance:

1. Update the ChildSelector contract/documentation in Forge.TreeWalker/contracts/ForgeTree.cs to reflect that Child may be either a literal child node key or a Roslyn/evaluated expression that resolves to a child node key.
2. Update any schema validation rules if needed so string values for Child remain valid while clarifying dynamic-expression support.
3. Modify TreeWalkerSession.SelectChild in Forge.TreeWalker/src/TreeWalkerSession.cs so that when a selector matches, it resolves cs.Child through the same dynamic evaluation pipeline used elsewhere (likely EvaluateDynamicProperty(..., typeof(string))) before returning the selected child key.
   • Preserve existing behavior for plain literal strings.
   • Preserve behavior for empty/null ShouldSelect defaulting to true.
   • Ensure failures surface consistently with existing EvaluateDynamicPropertyException handling.
4. Add or update unit tests in Forge.TreeWalker.UnitTests to cover:
   • literal Child values still working,
   • Roslyn expression in Child resolving to a valid node key,
   • interaction with ShouldSelect, including the default-select path when ShouldSelect is empty,
   • any relevant failure case if expression evaluation yields an invalid type or invalid child.
5. Update user-facing docs/examples (README and/or sample schemas) to show a ChildSelector with a dynamic Child expression.

Relevant files identified:

• Forge.TreeWalker/contracts/ForgeTree.cs
• Forge.TreeWalker/src/TreeWalkerSession.cs
• Forge.TreeWalker/contracts/ForgeSchemaValidationRules.json
• Forge.TreeWalker.UnitTests/test/ForgeSchemaHelper.cs
• Forge.TreeWalker.UnitTests/test/TreeWalkerUnitTests.cs
• README.md

Acceptance criteria:

• A schema can set Child to a Roslyn expression prefixed with C#| and the tree walker navigates to the evaluated node key.
• Existing schemas using literal child names continue to work unchanged.
• Unit tests cover the new behavior.
• Documentation/examples mention the new capability.

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: add C# roslyn expression support to Child

Assistant: I’m waiting for your response to the confirmation dialog so I can continue with the change.

This pull request was created from Copilot chat.

[kijujjav]

looks good

[kijujjav]

looks good

[Copilot]

Pull request overview

This PR extends Forge TreeWalker’s ChildSelector.Child to support dynamic resolution via the existing Roslyn expression pipeline (e.g., C#|...), enabling schemas to compute the next child node key at runtime.

Changes:

• Updated TreeWalkerSession.SelectChild to evaluate ChildSelector.Child using EvaluateDynamicProperty(..., typeof(string)).
• Documented dynamic ChildSelector.Child support in the contract (ForgeTree.cs), schema validation rules, and README.md.
• Added unit tests covering literal child selection, dynamic child selection, and invalid dynamic child type failures.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Updates example and documentation to mention dynamic ChildSelector.Child support.
Forge.TreeWalker/src/TreeWalkerSession.cs	Evaluates ChildSelector.Child via dynamic property evaluation when selecting a child.
Forge.TreeWalker/Forge.TreeWalker.csproj	No functional change (formatting/whitespace only).
Forge.TreeWalker/contracts/ForgeTree.cs	Updates contract documentation to describe dynamic Child capability.
Forge.TreeWalker/contracts/ForgeSchemaValidationRules.json	Adds schema description clarifying that Child may be a dynamic expression.
Forge.TreeWalker.UnitTests/test/TreeWalkerUnitTests.cs	Adds tests validating literal/dynamic child selection and invalid-type failure.
Forge.TreeWalker.UnitTests/test/ForgeSchemaHelper.cs	Adds helper schemas used by the new unit tests.

---

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