Retry

RetryAttribute is used on a test method to specify that it should be rerun on failure up to a maximum number of times.

Constructor

RetryAttribute(int tryCount)

Parameter	Type	Description
tryCount	int	The maximum number of times the test should be run if it fails. This is the total number of attempts, not the number of retries after an initial failure. Therefore, [Retry(1)] does nothing and should not be used.

Properties

Property	Type	Description	Default
RetryExceptions	Type[]	An array of exception types that trigger a retry when thrown. These are in addition to the normal behavior of retrying only on an assertion failure.	[] (empty array)

Note

The RetryExceptions property was added in NUnit 4.5.0.

Applies To

Test Methods	Test Fixtures (Classes)	Assembly
✅	❌	❌

Example

[TestFixture]
public sealed class RetryTests
{
    private readonly Random _random = new(42);

    [Test]
    [Retry(5)]
    public async Task OperationShouldPassIn1s()
    {
        var sw = Stopwatch.StartNew();
        string result = await ExpensiveOperation();
        sw.Stop();
        Assert.That(sw.ElapsedMilliseconds, Is.LessThan(1000), "Operation did not complete in time");
        Assert.That(result, Is.Not.Null);
    }

    private async Task<string> ExpensiveOperation()
    {
        // Simulate an expensive operation
        int duration = _random.Next(500, 1500);
        await Task.Delay(duration); // Simulate work
        return "Actual Result"; // Simulate a response
    }
}

Using RetryExceptions

When you need to retry on specific exception types (such as TimeoutException or OperationCanceledException), you can set the RetryExceptions property:

[TestFixture]
public sealed class Retry
{
    private int _delayInMilliseconds;

    [OneTimeSetUp]
    public void Setup()
    {
        _delayInMilliseconds = 2500;
    }

    [Test]
    [Retry(5, RetryExceptions = [typeof(OperationCanceledException)])]
    [CancelAfter(2000)]
    public async Task QueryServiceAsync(CancellationToken cancellationToken)
    {
        string result = await CallExternalServiceAsync(cancellationToken);
        Assert.That(result, Is.Not.Null);
    }

    private async Task<string> CallExternalServiceAsync(CancellationToken cancellationToken)
    {
        // Call an external service that may time out
        int delayInMilliseconds = _delayInMilliseconds;
        if (_delayInMilliseconds > 1000)
            _delayInMilliseconds -= 1000; // Decrease delay for next attempt

        await Task.Delay(delayInMilliseconds, cancellationToken); // Simulate a delay that may exceed
        return "Actual Result"; // Simulate a response
    }
}

Notes

1. It is not currently possible to use RetryAttribute on a TestFixture or any other type of test suite. Only single test methods may be retried.
2. If a test has an unexpected exception, an error result is returned and it is not retried — unless that exception type is specified in the RetryExceptions property.

See Also

• Repeat Attribute