Reading results#

Every scheduling call returns a ScheduleResult. This guide shows how to write the new dates back into your own model and explain why tasks moved. It also covers constraint conflicts, dependency cycles, dropped records, the baseline token, and why you should treat a result as read-only.

How a result is organized#

ScheduleResult is what Engine.Schedule, Project.Solve, Project.Preview and Project.Analyze return. Project.Level wraps one in LevelingResult.Schedule. Its properties fall into four groups:

Group	Properties	What it tells you
Dates	`Updates`	The tasks that changed, with their new values
Problems	`Conflicts`, `Cycles`, `Notes`, `Converged`	Rules the plan breaks, loops in the links, records left out, whether the planner settled
Extras	`Analytics`, `Overallocations`, `Profile`	Critical path and slack, resource overload, load profile. Each one is `null` unless you requested it or it applies
Identity	`BaselineId`	Which accepted baseline the result belongs to

These rules hold for every result:

Updates are sparse. Updates lists only the tasks whose dates changed. Each Mutation sets only the properties that changed. A task that didn't move isn't in the list. Set SolveOptions.IncludeUnchanged when you need every task's dates, for example on the first load.
Problems are data, not exceptions. A plan with conflicts, cycles or dropped links still returns normally. Calls throw InputException for invalid input and ProjectException for a call that doesn't fit the project's state. See Error handling.
Order is stable. The same input and options always give the same result, in the same order. BaselineId is the one exception, because it's an opaque token.
Updates use the input names. A Mutation has the same property names as TaskData and TaskUpdate: StartDate, EndDate, Duration, LevelingDelay, Pauses. You can copy a mutation onto a TaskUpdate property by property.

Updates#

Read Updates after each call and copy the changed values into your model. Every date and number property of a Mutation is nullable, and null means "unchanged", not "cleared".

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 = 1.0 },
    ],
    Links = [new LinkData { Id = 1, Source = 1, Target = 2 }],
};

// The caller's own copy of the dates.
var model = new Dictionary<Id, (Moment Start, double Duration)>
{
    [1] = (monday, 2.0),
    [2] = (monday, 1.0),
};

var result = Engine.Schedule(data, new SolveOptions());
foreach (var update in result.Updates)
{
    Console.WriteLine(
        $"{update.Id} ({update.Kind}): start {update.StartDate}, end {update.EndDate}, " +
        $"duration {update.Duration?.ToString() ?? "unchanged"}");

    if (model.TryGetValue(update.Id, out var row))
    {
        if (update.StartDate is { } start)
            row.Start = start;
        if (update.Duration is { } duration)
            row.Duration = duration;
        model[update.Id] = row;
    }
}

Output:

2 (Task): start 2026-08-12 08:00, end 2026-08-13 08:00, duration unchanged

Task 1 isn't listed because it didn't move. Task 2 moved, so it reports a new start and end. Its Duration is null because its working time stayed the same.

Kind is a MutationKind: Task, Summary or Milestone. Summaries show up in Updates too, because their spans roll up from their children. LevelingDelay and Pauses appear only in results from Project.Level, and only when leveling changed them. Leveling resources shows how to write them back.

To get every task's dates instead of only the changed ones, see Load all task dates.

Reasons#

Set SolveOptions.Reasons to learn why each task ended up where it did. Each update then carries a Reason. Reason is a closed set of nested records, so read it with a switch on the nested types:

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 = 1.0 },
        new TaskData(2) { StartDate = monday, Duration = 1.0 },
        new TaskData(3) { StartDate = monday, Duration = 1.0 },
    ],
    Links =
    [
        new LinkData { Id = 10, Source = 1, Target = 2 },
        new LinkData { Id = 11, Source = 2, Target = 3 },
    ],
};

var result = Engine.Schedule(data, new SolveOptions { Reasons = true });
foreach (var update in result.Updates)
{
    var why = update.Reason switch
    {
        Reason.Asap r => $"pushed by task {r.SourceId} through link {r.LinkId}",
        Reason.Alap r => $"pulled by task {r.TargetId} through link {r.LinkId}",
        Reason.SummaryTranslation r => $"moved with summary {r.SummaryId}",
        Reason.Constraint => "placed by its constraint",
        Reason.Preferred => "kept at its preferred date",
        _ => "no reason recorded",
    };
    Console.WriteLine($"task {update.Id}: {why}");
}

Output:

task 2: pushed by task 1 through link 10
task 3: pushed by task 2 through link 11

The variants:

Reason.Asap(LinkId, SourceId): a predecessor link pushed the task later.
Reason.Alap(LinkId, TargetId): a successor link pulled the task earlier (backward planning).
Reason.SummaryTranslation(SummaryId): the task moved as part of its summary block.
Reason.Constraint: the task's own constraint placed it.
Reason.Preferred: the task stayed at, or returned to, its own date.

PreviewOptions has the same Reasons flag, so a drag preview can explain itself too. A cyclic solve reports no reasons, because the planner doesn't run.

Conflicts#

A conflict is a rule the scheduled plan breaks. It doesn't stop scheduling. The dates are still valid output, and Conflicts lists what doesn't fit. Conflict has two variants:

Conflict.ConstraintViolation(Task, Constraint, Violation, Required, Actual): a task's constraint and its links disagree. Violation names the bound that broke: EarliestStart, LatestStart, EarliestEnd or LatestEnd.
Conflict.CalendarNonWorking(Task, Proposed, Snapped): with Config.StrictCalendar, a task that moved with its summary block landed on non-working time.

Constraints act only when Config.ApplyConstraints is true. In this example, task 2 must start no later than Tuesday, but its predecessor runs until Wednesday:

using Plen;

var monday = Moment.FromYmdHm(2026, 8, 10, 8, 0) ?? throw new InvalidOperationException("invalid date");
var tuesday = Moment.FromYmdHm(2026, 8, 11, 8, 0) ?? throw new InvalidOperationException("invalid date");

var data = new ProjectData
{
    Tasks =
    [
        new TaskData(1) { StartDate = monday, Duration = 2.0 },
        // Build must start no later than Tuesday, but design runs until Wednesday.
        new TaskData(2)
        {
            StartDate = monday,
            Duration = 1.0,
            ConstraintType = ConstraintType.Snlt,
            ConstraintDate = tuesday,
        },
    ],
    Links = [new LinkData { Id = 1, Source = 1, Target = 2 }],
};
data.Config.ApplyConstraints = true;

var result = Engine.Schedule(data, new SolveOptions());
foreach (var conflict in result.Conflicts)
{
    switch (conflict)
    {
        case Conflict.ConstraintViolation c:
            Console.WriteLine(
                $"task {c.Task}: {c.Constraint} violated ({c.Violation}), required {c.Required}, actual {c.Actual}");
            break;
        case Conflict.CalendarNonWorking c:
            Console.WriteLine($"task {c.Task}: lands on non-working time at {c.Proposed} (snapped {c.Snapped})");
            break;
    }
}
foreach (var update in result.Updates)
    Console.WriteLine($"task {update.Id}: start {update.StartDate}");

Output:

task 2: Snlt violated (LatestStart), required 2026-08-11 08:00, actual 2026-08-12 08:00
task 2: start 2026-08-11 08:00

The upper-bound constraint wins: task 2 starts on Tuesday, at Required. Actual is where the link wanted it to start. Show both dates to the user, so they can relax the constraint or move the predecessor. Links, constraints and summaries explains how each constraint type resolves.

Cycles and notes#

A dependency cycle (task 1 waits for task 2, which waits for task 1) has no valid schedule. Planning Engine doesn't throw on it. It returns the dates unchanged, leaves Updates empty and lists every unique simple cycle in Cycles. Each Cycle holds the task ids (Tasks) and link ids (Links) that form the loop.

Notes lists records the scheduler left out or changed. A Note is one of:

Note.DroppedLink(Link, Reason): a link that can't apply, for example one that points to an unscheduled task.
Note.DroppedTask(Task, Reason): an unscheduled task.
Note.DroppedAssignment(Assignment, Reason): an assignment that doesn't take part.
Note.Repaired(Path, Message): a fix applied under Validation.Repair.

This example has a cycle between tasks 1 and 2, plus a parked task 3:

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 = 1.0 },
        new TaskData(2) { StartDate = monday, Duration = 1.0 },
        // Task 3 is parked: it isn't scheduled, and neither is the link to it.
        new TaskData(3) { StartDate = monday, Duration = 1.0, Unscheduled = true },
    ],
    Links =
    [
        // 1 -> 2 -> 1 is a cycle.
        new LinkData { Id = 10, Source = 1, Target = 2 },
        new LinkData { Id = 11, Source = 2, Target = 1 },
        new LinkData { Id = 12, Source = 2, Target = 3 },
    ],
};

var result = Engine.Schedule(data, new SolveOptions());
foreach (var cycle in result.Cycles)
    Console.WriteLine($"cycle: tasks [{string.Join(", ", cycle.Tasks)}], links [{string.Join(", ", cycle.Links)}]");
Console.WriteLine($"updates: {result.Updates.Count}");

foreach (var note in result.Notes)
{
    var line = note switch
    {
        Note.DroppedLink n => $"link {n.Link} dropped: {n.Reason}",
        Note.DroppedTask n => $"task {n.Task} dropped: {n.Reason}",
        Note.DroppedAssignment n => $"assignment {n.Assignment} dropped: {n.Reason}",
        Note.Repaired n => $"repaired {n.Path}: {n.Message}",
        _ => note.ToString(),
    };
    Console.WriteLine(line);
}
if (!result.Converged)
    Console.WriteLine("the planner stopped before a fixed point");

Output:

cycle: tasks [1, 2], links [10, 11]
updates: 0
task 3 dropped: unscheduled
link 12 dropped: target task "3" is unscheduled

Highlight the links in cycle.Links so the user can break the loop. A cyclic solve still installs a baseline. If you requested analytics, they come back with Analytics.Status == AnalyticsStatus.Invalid.

Converged is false when the planner didn't reach a stable state within Config.MaxIterations passes (default 8). The dates are still a valid result, so treat it as a warning, not a failure.

To clean up messy input instead of rejecting it, see Accept messy input.

Baseline token#

BaselineId names the accepted baseline of a Project: the dates the last Solve or Level accepted. These calls fill it in:

Solve and Level: the baseline they just accepted.
Preview: the baseline it read.
Analyze(null, …) on a solved project with no pending edits: the current baseline. In every other case it's null.
Engine.Schedule: sets it too, but the project behind the call is gone afterwards, so the token has no use.

The token changes whenever the data changes: new data, an Apply, a solve that moves dates, or leveling edits. Keep it next to the dates you display. Pass it back in PreviewOptions.BaselineId or SchedulingChanges.BaselineId. If the project moved on in the meantime, the call throws ProjectException with Code == ProjectError.StaleBaseline:

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 = 1.0 },
    ],
    Links = [new LinkData { Id = 1, Source = 1, Target = 2 }],
};

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

// Keep the token next to the dates you show.
var shown = loaded.BaselineId;

var changes = new SchedulingChanges();
changes.Tasks.Update.Add(new TaskUpdate(1) { Duration = 3.0 });

var guarded = new PreviewOptions { BaselineId = shown };
var preview = project.Preview(changes, guarded);
Console.WriteLine($"preview read the shown baseline: {preview.BaselineId == shown}");

// Someone else commits an edit; the accepted baseline changes.
project.Apply(changes);
var committed = project.Solve(null, new SolveOptions());
Console.WriteLine($"token changed: {committed.BaselineId != shown}");

try
{
    project.Preview(changes, guarded);
    Console.WriteLine("unexpected: preview accepted a stale token");
}
catch (ProjectException e) when (e.Code == ProjectError.StaleBaseline)
{
    Console.WriteLine("stale: reload the dates before previewing again");
}

Output:

preview read the shown baseline: True
token changed: True
stale: reload the dates before previewing again

The token is opaque. Don't parse it, don't store it as a persistent id, and don't compare tokens across runs: the same data solved twice may get two different tokens. Compare it only with the current one, and always take it from the latest result. Editing a project shows the full edit cycle.

Results are shared#

ScheduleResult is a mutable class, but treat every result you get from a Project as read-only. The project keeps the result of Solve and Level as its accepted baseline. It returns that same object from a later Solve(null, …) with equal options and nothing changed. Its clones share it, and Evaluate and SuggestAssignments read it as the baseline. If you change it, you change what the project sees.

Call DeepClone() to get a copy you own:

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 = 1.0 },
    ],
    Links = [new LinkData { Id = 1, Source = 1, Target = 2 }],
};

var project = new Project();
var options = new SolveOptions();
var first = project.Solve(data, options);
var again = project.Solve(null, options);
var fork = project.Clone();

Console.WriteLine($"same object: {ReferenceEquals(first, again)}");
Console.WriteLine($"clone shares it: {ReferenceEquals(first, fork.Solve(null, options))}");

// Copy before you change anything.
var mine = first.DeepClone();
mine.Updates.Clear();
Console.WriteLine($"project result untouched: {first.Updates.Count}");

Output:

same object: True
clone shares it: True
project result untouched: 1

The copy shares no list or nested record with the original. LevelingResult.DeepClone() is the exception: it doesn't copy Schedule, so call Schedule.DeepClone() before you change the leveled schedule.

ScheduleResult compares by value with Equals, list by list. Two results of the same data can still differ in BaselineId, so set it to null on both copies before comparing them in tests.

A result from Engine.Schedule belongs to you alone, so you can change it freely.

The core package has no JSON support. To send results to a client as JSON, see Exchange JSON with a client.