Coroutine chaining in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of chained coroutines with WaitForSeconds

What will be the output sequence and timing when running this Unity coroutine chaining code?

Unity

using System.Collections;
using UnityEngine;

public class TestChain : MonoBehaviour
{
    void Start()
    {
        StartCoroutine(First());
    }

    IEnumerator First()
    {
        Debug.Log("Start First");
        yield return StartCoroutine(Second());
        Debug.Log("End First");
    }

    IEnumerator Second()
    {
        Debug.Log("Start Second");
        yield return new WaitForSeconds(1);
        Debug.Log("End Second");
    }
}

A Start First, Start Second, End First, End Second (after 1 second)

B Start First, End First, Start Second, End Second (after 1 second)

C Start First, Start Second, End Second (after 1 second), End First immediately after

D Start Second, Start First, End Second (after 1 second), End First

💡 Hint

Remember that yield return StartCoroutine waits for the inner coroutine to finish before continuing.

The First coroutine starts and logs 'Start First'. It then yields return StartCoroutine(Second()), so it waits until Second finishes. Second logs 'Start Second', waits 1 second, then logs 'End Second'. After Second finishes, First resumes and logs 'End First'.

🧠 Conceptual

intermediate

Understanding yield return null in coroutine chaining

In Unity coroutine chaining, what does yield return null do inside a coroutine?

A Immediately continues without waiting

B Stops the coroutine permanently

C Waits for 1 second before continuing

D Pauses the coroutine until the next frame before continuing

💡 Hint

Think about what happens when you want to wait just one frame.

yield return null pauses the coroutine and resumes it in the next frame, allowing other code to run in between.

🔧 Debug

advanced

Identify the error in coroutine chaining code

What error will this Unity coroutine chaining code produce when run?

Unity

IEnumerator First()
{
    Debug.Log("First start");
    yield return Second();
    Debug.Log("First end");
}

IEnumerator Second()
{
    Debug.Log("Second start");
    yield return new WaitForSeconds(1);
    Debug.Log("Second end");
}

A Second coroutine runs but First does not wait for it to finish

B No error, runs correctly with expected output

C Compile-time error due to missing StartCoroutine call

D Runtime error: NullReferenceException

💡 Hint

Check how coroutines are started and chained properly.

yield return Second() returns the IEnumerator but does not start it as a coroutine. So First does not wait for Second to finish. To wait properly, use yield return StartCoroutine(Second()).

📝 Syntax

advanced

Correct syntax for chaining multiple coroutines

Which option correctly chains two coroutines so that Second runs after First finishes?

Unity

IEnumerator First() { Debug.Log("First"); yield return new WaitForSeconds(1); }
IEnumerator Second() { Debug.Log("Second"); yield return new WaitForSeconds(1); }

A StartCoroutine(First()); StartCoroutine(Second());

B yield return StartCoroutine(First()); yield return StartCoroutine(Second());

C yield return First(); yield return Second();

D yield return new WaitForSeconds(1); yield return new WaitForSeconds(1);

💡 Hint

Remember how to wait for one coroutine to finish before starting another.

Using yield return StartCoroutine(...) waits for the coroutine to finish before continuing. Option B just returns the IEnumerator without starting it properly. Option B starts both coroutines simultaneously. Option B just waits fixed times without chaining coroutines.

🚀 Application

expert

Chaining coroutines with conditional wait and result passing

Given these two coroutines, what will be the value of 'result' after running the Chain coroutine?

Unity

IEnumerator WaitAndReturn(int waitSeconds, System.Action<int> callback)
{
    yield return new WaitForSeconds(waitSeconds);
    callback(waitSeconds * 2);
}

IEnumerator Chain()
{
    int result = 0;
    yield return StartCoroutine(WaitAndReturn(2, x => result = x));
    Debug.Log($"Result is {result}");
}

A Result is 4

B Result is 2

C Result is 0

D No output, coroutine never finishes

💡 Hint

Look at how the callback sets the result after waiting.

WaitAndReturn waits 2 seconds, then calls callback with 2*2=4. Chain waits for WaitAndReturn to finish, so result is set to 4 before logging.