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?

The answer depends entirely on what you are testing. I also believe that "one assert per unit test" rule is a myth.

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.