UnitTest.java

public class LogAnalyzer {
	private IExtensionManager manager;
	public LogAnalyzer(IExtensionManager mgr) {
		manager = mgr;
	}

	public bool isValidLogFileName(string fileName) {
		return manager.isValid(fileName);
	}
}

public interface IExtensionManager {
	bool isValid(string fileName);
}

[TestFixture]
public class LogAnalyzerTests {

	[Test]
	public void isValidFileName_NameSupportedExtension_ReturnsTrue() {
		FakeExtensionManager fakeManager = new FakeExtensionManager();
		fakeManager.willBeValid = true;
		LogAnalyzer log = new LogAnalyzer(fakeManager);
		bool result = log.isValidLogFileName("short.txt");
		Assert.True(result);
	}
}

internal class FakeExtensionManager : IExtensionManager {
	public bool willBeValid = false;
	public bool isValid(string fileName) {
		return willBeValid;
	}
}