A BPMN interpreter for .NET. Not a workflow engine.

using Bpmn.Interchange;

// Analyze and Read share one code path, so a dry run cannot drift from the real one.
var result = new BpmnXmlReader().Read(File.ReadAllText("order-intake.bpmn"));

foreach (var issue in result.Analysis.Issues)
    Console.WriteLine($"{issue.Severity,-8} {issue.ElementId ?? "-",-24} {issue.Message}");

var definitions = result.Definitions;
Console.WriteLine($"{definitions.Processes.Count} process(es), {result.Analysis.Issues.Count} finding(s)");

// Vendor annotations other readers discard are still here.
foreach (var element in definitions.Processes.SelectMany(p => p.Elements))
    if (!element.Extensions.IsEmpty)
        Console.WriteLine($"{element.ElementId}: retained {string.Join(", ", element.Extensions.RetainedNamespaces())}");

using Bpmn.Interchange;
using Bpmn.Model;

var definitions = new BpmnDefinitionsBuilder()
    .TargetNamespace("http://valence.works/orders")
    .Process("order-intake", process => process
        .StartEvent("start")
        .ExclusiveGateway("large-order")
        .UserTask("manual-review", "Manual review")
        .EndEvent("accepted")
        .Connect("start", "large-order")
        .Connect("large-order", "manual-review", condition: "large")
        .Connect("large-order", "accepted", isDefault: true)
        .Connect("manual-review", "accepted"))
    .Build();

// Layout is synthesized where the model carries none, and every edge gets at least
// two waypoints, so the output opens in a BPMN modeler without complaint.
File.WriteAllText("order-intake.bpmn", new BpmnXmlWriter().Write(definitions));

using Bpmn.Interchange;
using Bpmn.Runtime.InMemory;

// A reference host: virtual clock, single process, nothing durable.
var definitions = new BpmnXmlReader().Read(File.ReadAllText("order-intake.bpmn")).Definitions;

var host = new InMemoryBpmnHost();
var instance = host.Start(definitions.Processes[0]);

instance.CompleteWork("node-manual-review");
instance.Clock.Advance(TimeSpan.FromDays(7)); // a seven-day timer resolves in microseconds

foreach (var work in instance.PendingWork)
    Console.WriteLine($"waiting on {work.ElementId}");

Console.WriteLine(instance.IsCompleted ? $"completed: {instance.Outcome}" : "still running");

// Every evaluation is recorded, so you can see exactly what the interpreter decided.
Console.WriteLine(instance.Transcript);