I came across something that I found counter-intuitive and I was just wondering if this was a bug or is the desired functionality...

[TestMethod]
public void TestMethod1()
{
	var myList = new List<string>();
	myList.Should().OnlyContain(x => x == "bar"); //This passes
	
	myList.Should().Contain(x => x == "bar"); //This fails

	myList.Add("foo");
	
	myList.Should().OnlyContain(x => x == "bar"); //This fails

}

To me it seems like OnlyContain should verify that "bar" is in the collection and if the collection is empty it should through an error.

Can someone describe a scenario where it would be acceptable for OnlyContain to not throw an error when the collection is empty?

You're right. When the collection is empty, it should throw. I've fixed this in this changeset.

Thanks!