Editing a Project#

This guide covers how to change a live Project as the user works: adding, updating and removing tasks, links, resources and assignments, rescheduling after the edit, and recovering when part of an edit is refused.

How editing works#

A Project holds one accepted state: the data from the last Solve(data, …) plus every edit since, with the dates the last solve accepted. You don't send the whole plan again for each change. You describe the change as a SchedulingChanges object and pass it to Project.Apply.

Editing takes two separate calls:

1. Apply(changes) writes the edit into the accepted state and marks the project unsolved. It doesn't schedule anything.
2. Solve(null, options) schedules the edited state, accepts the new dates and returns what moved.

Between the two calls the project has no baseline. Preview, LoadProfile, Evaluate, SuggestAssignments and Roster throw ProjectException with Code == ProjectError.NoBaseline until you solve. You can call Apply several times before one Solve, and the edits accumulate.

Apply accepts what it can. Each operation in the change set is applied or rejected on its own. A rejection doesn't throw: it's listed in the returned ApplyReport and sets NeedsResync. From then on, Solve(null, …), Apply and Level throw ProjectException with ProjectError.NeedsResync. Your model and the project's state may now disagree, and the only way back is to send the full data with Solve(data, …).

When Apply itself throws, the call didn't fit the project's state: NeedsResync from an earlier rejection, or StaleBaseline when the change set names a baseline that isn't current anymore. In both cases nothing was applied. See Error handling.

All calls are synchronous. A Project isn't thread-safe, so let one thread at a time edit a shared instance.

Change sets#

A SchedulingChanges groups the edits by record kind. Each group is a Changes<TAdd, TUpdate> with three lists: Add, Update and Remove. All of them start empty.

Additions use the same record types as ProjectData, except links: LinkAdd has a nullable Id. Leave a link or assignment Id as null and Apply generates one ($generated_link_1, $generated_assignment_1, ...). Generated ids come back in ApplyReport.Effects, so you can store them in your model.

using Plen;

var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");

var data = new ProjectData
{
    Tasks =
    [
        new TaskData(1) { StartDate = monday, Duration = 2.0 },
        new TaskData(2) { StartDate = monday, Duration = 3.0 },
    ],
    Links = [new LinkData { Id = 1, Source = 1, Target = 2 }],
};
var project = new Project();
var options = new SolveOptions();
project.Solve(data, options);

var changes = new SchedulingChanges();
// A new task, linked after task 2. The link has no id: Apply generates one.
changes.Tasks.Add.Add(new TaskData(3) { StartDate = monday, Duration = 1.0 });
changes.Links.Add.Add(new LinkAdd { Source = 2, Target = 3 });
// Task 1 now takes three days.
changes.Tasks.Update.Add(new TaskUpdate(1) { Duration = 3.0 });

var report = project.Apply(changes);
foreach (var op in report.Applied)
    Console.WriteLine($"applied {op.Path}");
Console.WriteLine($"generated links: {string.Join(", ", report.Effects.GeneratedLinks)}");

var result = project.Solve(null, options);
foreach (var update in result.Updates)
    Console.WriteLine($"task {update.Id}: {update.StartDate} .. {update.EndDate}");

The report lists tasks.add[0], tasks.update[0] and links.add[0], and the new link is $generated_link_1. After the solve, task 1 runs until 2026-08-13 08:00, task 2 moves to 2026-08-13 08:00 .. 2026-08-18 08:00, and task 3 starts when task 2 ends.

Operations don't run in the order you added them. Apply runs them in a fixed order, so every reference comes after the adds it may need and every removal comes before the edits it could break:

1. tasks: remove, add, update
2. links: remove, update, add
3. resources: remove, add, update
4. assignments: remove, update, add

An operation is named by its group, list and index, such as tasks.update[0]. The same path shows up in ApplyReport.Applied and ApplyReport.Rejected.

Update records and Patch#

Update records (TaskUpdate, LinkUpdate, ResourceUpdate, AssignmentUpdate) name the record by Id and carry every other field as a Patch<T>. A patch has three states:

• Patch<T>.Missing (the default) leaves the field as it is.
• Patch<T>.Of(value) sets it. A plain value converts implicitly, so Duration = 3.0 works.
• Patch<T>.Null clears an optional field, or restores the default of a field that always has a value: AutoScheduling goes back to true, Unscheduled to false, Progress to 0, Priority to 500, LevelingDelay to 0, Pauses to empty, LinkType to Fs and Lag to 0.

new TaskUpdate(id) and the other update constructors leave every field Missing, so you only touch the fields you set in the initializer.

using Plen;

var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");
var thursday = Moment.FromYmdHm(2026, 8, 13, 8, 0) ?? throw new InvalidOperationException("invalid date");

var data = new ProjectData
{
    Tasks = [new TaskData("review") { StartDate = monday, Duration = 1.0 }],
};
data.Config.ApplyConstraints = true;

var project = new Project();
var options = new SolveOptions();
project.Solve(data, options);

// A value sets a field: "start no earlier than Thursday".
var pin = new SchedulingChanges();
pin.Tasks.Update.Add(new TaskUpdate("review")
{
    ConstraintType = ConstraintType.Snet,
    ConstraintDate = thursday,
});
project.Apply(pin);
foreach (var update in project.Solve(null, options).Updates)
    Console.WriteLine($"pinned: {update.Id} starts {update.StartDate}");

// Null clears the constraint; StartDate moves the task back.
// Every field left Missing keeps its value.
var unpin = new SchedulingChanges();
unpin.Tasks.Update.Add(new TaskUpdate("review")
{
    ConstraintType = Patch<ConstraintType>.Null,
    ConstraintDate = Patch<Moment>.Null,
    StartDate = monday,
});
project.Apply(unpin);
foreach (var update in project.Solve(null, options).Updates)
    Console.WriteLine($"unpinned: {update.Id} starts {update.StartDate}");

The first solve moves the task to 2026-08-13 08:00. The second puts it back on Monday. Clearing the constraint alone wouldn't do that: the accepted state already holds the Thursday dates, so the update also sets StartDate.

An Id patch takes one implicit conversion only, so Parent = "phase" doesn't compile. Write Parent = Id.Str("phase") or Patch<Id>.Of("phase").

Which dates a task update keeps#

A task has three linked date fields: StartDate, EndDate and Duration. When an update sets some of them, Apply keeps what you sent and derives the rest:

• StartDate alone keeps the duration and recomputes the end (a drag).
• EndDate alone keeps the start and recomputes the duration (a resize from the right edge).
• Duration alone keeps the start and recomputes the end.
• StartDate and EndDate together recompute the duration.
• A new CalendarId, or a new Parent while calendars are inherited (CalendarMode.TaskInherited), keeps the start and duration and recomputes the end on the new calendar. Under an inherited calendar, the descendants that inherit it re-derive their ends too.

An explicit StartDate that falls on non-working time is kept as sent when the task has no predecessor. The end is computed forward from it on the working calendar, so only the start sits outside working time. Snap the date in your UI if you don't want that.

Feeding results back#

A Mutation from a result uses the same field names as TaskUpdate, so a result entry maps field by field onto an update. That's useful when you want to commit dates you got from Preview or a leveled clone:

using Plen;

var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");
var wednesday = Moment.FromYmdHm(2026, 8, 12, 8, 0) ?? throw new InvalidOperationException("invalid date");

var project = new Project();
project.Solve(
    new ProjectData
    {
        Tasks =
        [
            new TaskData(1) { StartDate = monday, Duration = 2.0 },
            new TaskData(2) { StartDate = monday, Duration = 3.0 },
        ],
        Links = [new LinkData { Id = 1, Source = 1, Target = 2 }],
    },
    new SolveOptions());

// The user drags task 1 to Wednesday and sees the preview.
var drag = new SchedulingChanges();
drag.Tasks.Update.Add(new TaskUpdate(1) { StartDate = wednesday });
var preview = project.Preview(drag, new PreviewOptions());

// They confirm: commit the previewed dates as they are.
var commit = new SchedulingChanges { BaselineId = preview.BaselineId };
commit.Tasks.Update.AddRange(preview.Updates.Select(ToUpdate));
var report = project.Apply(commit);
Console.WriteLine($"applied {report.Applied.Count}, rejected {report.Rejected.Count}");

static Patch<T> Set<T>(T? value) where T : struct =>
    value is T v ? v : Patch<T>.Missing;

static TaskUpdate ToUpdate(Mutation mutation) => new(mutation.Id)
{
    StartDate = Set(mutation.StartDate),
    EndDate = Set(mutation.EndDate),
    Duration = Set(mutation.Duration),
    LevelingDelay = Set(mutation.LevelingDelay),
    // The result is shared with the project: give the update its own copy.
    Pauses = mutation.Pauses is { } pauses
        ? pauses.ConvertAll(p => p.DeepClone())
        : Patch<List<Pause>>.Missing,
};

The preview moves task 1 to Wednesday and task 2 after it, so the commit applies two updates. Mutation sets only the fields that changed, and a null field becomes Patch<T>.Missing. Results are shared with the project, which is why the pauses are deep-cloned before they go into the update.

Pauses replaces the whole list. Set it to an empty list or Patch<List<Pause>>.Null to remove every pause. See Clear leveling.

Applying#

Apply returns an ApplyReport:

• Applied lists the path of every accepted operation.
• Rejected lists the path and message of every refused one.
• Effects lists what the edit changed beyond the records you named (below).
• NeedsResync is true when anything was rejected.
• Revision is the project's revision number after this edit.

Removals cascade, and Effects tells you what went with them. Removing a task removes its descendants, every link that touches any of them and every assignment on them. Removing a resource removes its assignments.

using Plen;

var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");

var data = new ProjectData
{
    Tasks =
    [
        new TaskData("phase") { TaskType = TaskType.Summary },
        new TaskData("spec") { Parent = "phase", StartDate = monday, Duration = 2.0 },
        new TaskData("release") { StartDate = monday, Duration = 1.0 },
    ],
    Links = [new LinkData { Id = "spec-release", Source = "spec", Target = "release" }],
    Resources = [new ResourceData { Id = "ann" }],
    Assignments = [new AssignmentData { Id = "ann-spec", Task = "spec", Resource = "ann", Value = 1.0 }],
};
var project = new Project();
project.Solve(data, new SolveOptions());

var changes = new SchedulingChanges();
changes.Tasks.Remove.Add("phase");
var report = project.Apply(changes);

var effects = report.Effects;
Console.WriteLine($"tasks: {string.Join(", ", effects.RemovedTasks)}");
Console.WriteLine($"links: {string.Join(", ", effects.RemovedLinks)}");
Console.WriteLine($"assignments: {string.Join(", ", effects.RemovedAssignments)}");

Removing the summary phase also removes spec, the link spec-release and the assignment ann-spec. Delete the same records from your own model.

The other Effects lists:

• GeneratedLinks and GeneratedAssignments hold the ids Apply made up for additions without one.
• DroppedLinks and DroppedAssignments hold records that were accepted but are now inactive, for example a link to an unscheduled task. They stay in the project and come back into play when the task does.

Apply edits the project in place. Clones made earlier don't see the edit, so you can Clone() a project, apply a multi-step edit to the clone and drop it if the user cancels. For a single edit, Preview does the same job without a clone. See Previewing changes.

Field-only task edits and assignment updates take a fast path that patches the affected records without rebuilding the whole model. Frequent small edits, like drags, stay cheap.

Rescheduling#

After Apply, call Solve(null, options). It schedules the edited state, accepts the dates as the new baseline and returns the updates relative to the previous one.

Tasks that a successful operation added or updated are always reported, even when the planner leaves them exactly where the edit put them. The same goes for tasks whose working calendars changed because of a resource or assignment edit. Your model gets the derived end or duration for every task you edited, not only for the ones that moved. These reported tasks accumulate across several Apply calls until the next acyclic Solve(null, …) or Level. A cyclic solve keeps them pending.

Guarding against stale edits#

A UI usually edits the dates it showed the user. If the project changed in between (an edit from another tab, a leveling run), applying the edit can do the wrong thing. Set SchedulingChanges.BaselineId to the token of the result the user saw, and Apply refuses the whole change set with ProjectError.StaleBaseline when that token isn't current.

using Plen;

var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");

var project = new Project();
var options = new SolveOptions();
var loaded = project.Solve(
    new ProjectData { Tasks = [new TaskData(1) { StartDate = monday, Duration = 2.0 }] },
    options);
var seen = loaded.BaselineId;

// The edit was made against the dates the user saw.
var changes = new SchedulingChanges { BaselineId = seen };
changes.Tasks.Update.Add(new TaskUpdate(1) { Duration = 3.0 });
project.Apply(changes);

// A second edit still carrying the old token is refused as a whole.
try
{
    project.Apply(changes);
}
catch (ProjectException e) when (e.Code == ProjectError.StaleBaseline)
{
    Console.WriteLine("stale: reload first");
}

var solved = project.Solve(null, options);
Console.WriteLine($"new baseline differs: {solved.BaselineId != seen}");

A token names a baseline only while the project is solved and clean. After an Apply, any BaselineId is stale until the next solve, so take a fresh token from each result. Tokens are opaque and unique to the process: don't store them or compare them across runs. null (the default) skips the check.

Rejections and resync#

An operation is rejected when it can't apply: an unknown id in Update or Remove, a duplicate id in Add, Patch<T>.Null on a required field (a link's Source or Target, an assignment's Task, Resource or Value), or a record that fails validation after the edit, such as an invalid pause or assignment slice. Each operation is tried on its own copy, so a rejected operation leaves no partial edit behind. The operations around it still apply.

using Plen;

var project = new Project();
var options = new SolveOptions();
project.Solve(Load(), options);

var changes = new SchedulingChanges();
changes.Tasks.Remove.Add("deleted-elsewhere");
changes.Tasks.Update.Add(new TaskUpdate("design") { Duration = 4.0 });

var report = project.Apply(changes);
foreach (var op in report.Applied)
    Console.WriteLine($"applied {op.Path}");
foreach (var op in report.Rejected)
    Console.WriteLine($"rejected {op.Path}: {op.Message}");

if (report.NeedsResync)
{
    // Solve(null, …), Apply and Level now throw until the full data is sent again.
    try
    {
        project.Solve(null, options);
    }
    catch (ProjectException e) when (e.Code == ProjectError.NeedsResync)
    {
        project.Solve(Load(), options);
    }
}

static ProjectData Load()
{
    var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");
    return new ProjectData
    {
        Tasks = [new TaskData("design") { StartDate = monday, Duration = 2.0 }],
    };
}

The update applies and the removal is rejected with rejected tasks.remove[0]: unknown task id. The project now holds the update but not the removal, which is probably not what your model holds. That's why any rejection blocks further edits: Solve(null, …), Apply and Level throw ProjectException with ProjectError.NeedsResync until a successful Solve(data, …) replaces the whole state with data you know is right.

The resync data must pass validation. If it doesn't, Solve throws InputException, and the project still needs a resync. See Recover from rejected changes.

Evaluate and SuggestAssignments run the same apply on disposable copies, so a rejected candidate shows up as its CandidateResult.Error (a RejectedOperation) instead of blocking the project. See Evaluating alternatives.

What change sets can't do#

Some edits have no operation. For these, rebuild the data in your model and send it with Solve(data, …):

• Calendars can't be added, changed or removed. Tasks and resources can switch to another existing calendar through CalendarId.
• SchedulingConfig can't change: DurationUnit, UseCalendars, CalendarMode, ApplyConstraints and the other settings are fixed until the next full solve.
• A task's TaskType and DurationUnit aren't in TaskUpdate. To turn a task into a summary or a milestone, remove it and add it again (its links and assignments go with the removal), or resync.
• An id can't be renamed. An update finds the record by its Id and never changes it.
• ResourceUpdate changes only Capacity and CalendarId.

Calls that don't need a change set don't take one. Level writes its own LevelingDelay and Pauses edits and accepts the result. See Leveling resources.