String interpolation issues : <p>I'm trying to figure out why my unit test fails (The third assert below):</p>

<pre><code>var date = new DateTime(2017, 1, 1, 1, 0, 0);

var formatted = "{countdown|" + date.ToString("o") + "}";

//Works
Assert.AreEqual(date.ToString("o"), $"{date:o}");
//Works
Assert.AreEqual(formatted, $"{{countdown|{date.ToString("o")}}}");
//This one fails
Assert.AreEqual(formatted, $"{{countdown|{date:o}}}");
</code></pre>

<p>AFAIK, this should work correctly, but it appears that it doesn't pass the formatting parameter in correctly, it it appears as just <code>{countdown|o}</code> to the code. Any idea why this is failing?</p>
