generated-patch.md

T.Patch

T.Patch is a sealed mutable command type nested in a generated model. It has no retained baseline and does not detect concurrent overwrites by itself.

Construction and member commands

Member	Contract
Patch()	Empty command.
Patch(T.Fragment fragment)	Creates Set commands for supplied contributions.
bool IsEmpty	Whether neither whole-root nor member operations request changes.
Scalar member	Ref-returning FragmentOperation<TValue> supporting assignment, Set, Keep, and Remove.
Nested model member	Child Patch for member-level edits and whole-child operations.
Structural collection member	Shape-specific command supporting keyed or dictionary mutation.
void Set(T value)	Replaces the whole contribution with a captured model.
void SetNull()	Makes the whole contribution present null.
void Remove()	Removes the whole contribution.
Implicit conversion from FragmentOperation<T.Fragment?>	Wraps a whole-root command.

patch.Name = value is a Set command. patch.Name.Remove() removes the member from the source contribution; Keep leaves it untouched. See operation value types.

Application and algebra

Here state means Optional<T.Fragment?>, including whole-root missing and null states.

Member	Result and behavior
state Apply(state current)	Returns the resulting presence-aware sparse state.
T ApplyTo(T current)	Returns a model result without modifying current.
ApplyInPlaceResult ApplyInPlace(T current)	Attempts writes to the existing model and reports unsupported members.
static Patch Between(state before, state after)	Derives commands, including removal, between sparse endpoints.
Patch Compose(Patch next)	Sequential composition: this, then next.
static Patch Compose(Patch first, Patch second)	Equivalent static form.
Patch Invert(state baseline)	Derives an inverse using the supplied baseline.
static RebaseResult<Patch> Rebase(state baseState, Patch local, state currentState, ChangePayloadRebaseOptions? options = null)	Three-way rebase with an explicit baseline.

The notation state above is explanatory, not a C# type alias.

ApplyInPlaceResult exposes bool Succeeded and IReadOnlyList<string> UnsupportedMembers. Immutable members can prevent in-place application even when ApplyTo can construct a new model. Inspect the result before treating the operation as applied. Model-returning APIs cannot represent a missing or null model root; use Apply with optional fragments for those transitions.

Composition does not invent before-state. Invert needs the actual baseline from which the patch was applied. For retained before-values and baseline checks, use T.ChangeSet.

Example

Uses the reference models.

var layer = new ReferenceSettings.Fragment { Theme = "dark", RetryCount = 3 };
var patch = new ReferenceSettings.Patch();
patch.Theme.Remove();
patch.RetryCount.Set(5);

var updated = layer.Apply(patch);
var undo = patch.Invert(Optional<ReferenceSettings.Fragment?>.Present(layer));
var restored = updated.Apply(undo);

updated.Theme is missing and RetryCount is 5. Applying undo restores the original contribution, including Theme "dark".

Collection commands vary by shape; see generated paths and transitions and keyed collection semantics.