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Fluent Assertions Documentation
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    Execution Time
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    Members and Actions

    Fluent Assertions also provides a method to assert that the execution time of particular method or action does not exceed a predefined value. To verify the execution time of a method, use the following syntax:

    C#
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    public class SomePotentiallyVerySlowClass
{
    public void ExpensiveMethod()
    {
        for (short i = 0; i < short.MaxValue; i++)
        {
            string tmp = " ";
            if (!string.IsNullOrEmpty(tmp))
            {
                tmp += " ";
            }
        }
    }
}
var subject = new SomePotentiallyVerySlowClass();
subject.ExecutionTimeOf(s => s.ExpensiveMethod()).Should().BeLessThanOrEqualTo(500.Milliseconds());

    Alternatively, to verify the execution time of an arbitrary action, use this syntax:

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    var someAction = () => Thread.Sleep(100);
someAction.ExecutionTime().Should().BeLessThanOrEqualTo(200.Milliseconds());

    The supported assertions on ExecutionTime() are a subset of those found for TimeSpans, namely:

    C#
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    someAction.ExecutionTime().Should().BeLessThanOrEqualTo(200.Milliseconds());
someAction.ExecutionTime().Should().BeLessThan(200.Milliseconds());
someAction.ExecutionTime().Should().BeGreaterThan(100.Milliseconds());
someAction.ExecutionTime().Should().BeGreaterThanOrEqualTo(100.Milliseconds());
someAction.ExecutionTime().Should().BeCloseTo(150.Milliseconds(), 50.Milliseconds());

    Tasks

    If you're dealing with a Task, you can also assert that it completed within a specified period of time or not completed:

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    var someAsyncWork = () => SomethingReturningATask();
await someAsyncWork.Should().CompleteWithinAsync(100.Milliseconds());
await someAsyncWork.Should().NotCompleteWithinAsync(100.Milliseconds());
await someAsyncWork.Should().ThrowWithinAsync<InvalidOperationException>(100.Milliseconds());

    If the Task is generic and returns a value, you can use that to write a continuing assertion:

    C#
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    Func<Task<int>> someAsyncFunc;

await someAsyncFunc.Should().CompleteWithinAsync(100.Milliseconds()).WithResult(42);

    A fully async version is available as well.