Dictionary with single item assertion

gistfile1.cs

// headers is a Dictionary<string, string>
Assert.AreEqual(1, headers.Count);
Assert.AreEqual("bar", headers["foo"]);
// Do I need both of these? Only first? Only second?

// Only 1st: What if the item is different than what I expected?
// Only 2nd: If dictionary is empty, this line will throw an exception before assertion is evaluated, is this OK?
// Both: Isn't this against "one assert per unit test" rule?

Another option is to use CollectionAssert.AreEquivalent

one test per concept is okay but one assert per test is most likely overkill.

I'd do some variation of:

var expected = new Dictionary<string, string> { { "bar", "foo" } };
Assert.True(expected.SequenceEquals(headers));

That states what you expect, and what you verify, in a single assertion, so it addresses all your questions in one go.