Unit testing using two functions

I've neglected unit testing for some time. I wrote unit tests, but they were fairly poor. I'm now reading through "The art of unit Testing" to bring myself up to scratch.

If I have an interface such as:

public interface INotificationService{ void AddError(string _error); void AddIssue(string _issue); IEnumerable FetchErrors(); IEnumerable FetchIssues();}

A concrete implementation of this interface contains:

private readonly ICollection messages;

Adding an error or issue creates a new message with an enum denoting it's type and adds it to the collection. Calling FetchErrors() / FetchIssues() returns messages of that type from the collection.

Would the following test be valid?:

[Test] public void FetchErrors_LoggingEnabledAddErrorFetchErrors_ReturnsError() { notificationService = new NotificationService(); notificationService.AddError("A new error"); Assert.AreEqual(new []{"A new error"}, notificationService.FetchErrors()); }

My concern is that I'm first calling AddError(), then testing the result of FetchErrors(). So I'm calling two functions. Is this incorrect?

Should I make the collection public and directly assert that it contains a message of the appropriate type containing the logged error message?

What would be the best practice in this scenario?

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