Bridge from Beginner: Diagnose a Study Planner State Bug

You add fifteen minutes to a preview. Why does the saved plan change too?

Use the reference-copying model from Beginner: predict the two printed numbers before running this complete Program.cs in a .NET 10 console project. This diagnostic asks you to turn that model into a method guarantee.

using System;

StudyPlan saved = new StudyPlan("C#", 30);
StudyPlan preview = saved;
preview.Minutes += 15;

Console.WriteLine($"Saved: {saved.Minutes}");
Console.WriteLine($"Preview: {preview.Minutes}");

class StudyPlan
{
    public string Topic;
    public int Minutes;

    public StudyPlan(string topic, int minutes)
    {
        Topic = topic;
        Minutes = minutes;
    }
}

Count the objects

The output is Saved: 45, then Preview: 45: two variables reach one object, and += changes its Minutes field.

Topic and Minutes are public fields, stored data callers can assign directly. Beginner used properties; the next lesson restricts these mutation routes.

The new requirement is that creating a preview leaves the saved plan unchanged. That invariant is a condition the operation must preserve; the compiler cannot infer it.

Your attempt: an independent preview

Write CreatePreview for a C# plan holding thirty minutes. It receives a ten-minute bonus and returns a separate plan with forty minutes. It must never assign to a source field. A negative bonus must throw ArgumentOutOfRangeException before producing a result; zero is allowed and must still return a separate object.

Before looking below, check both totals and use ReferenceEquals to check that the objects differ.

Hint: a second variable is insufficient. Decide where a second object must be created.

Compare after trying

Copying this string and int into a new object is sufficient for this model.

using System;

StudyPlan saved = new StudyPlan("C#", 30);
StudyPlan preview = CreatePreview(saved, 10);
Console.WriteLine($"Saved: {saved.Topic}, {saved.Minutes}");
Console.WriteLine($"Preview: {preview.Topic}, {preview.Minutes}");

static StudyPlan CreatePreview(StudyPlan source, int bonus)
{
    if (bonus < 0)
    {
        throw new ArgumentOutOfRangeException("bonus");
    }

    return new StudyPlan(source.Topic, source.Minutes + bonus);
}

class StudyPlan
{
    public string Topic;
    public int Minutes;

    public StudyPlan(string topic, int minutes)
    {
        Topic = topic;
        Minutes = minutes;
    }
}

Assignment copies the reference. In the first run, both variables reach the same plan, so preview.Minutes += 15 changes what both read to 45. In the separate worked program, CreatePreview(saved, 10) constructs another plan, leaving saved at 30 and producing preview at 40.

If StudyPlan gains a mutable List, decide which nested data the preview must own independently before extending the copy.

Reference: Microsoft: classes, objects, and encapsulation.