var client = new HttpClient();
var response = await client.GetAsync(uri);

string sample;
IEnumerable<string> values;

if (response.Content.Headers.TryGetValues("Sample-Header", out values))
{
   // This never happens!
   sample = values.First();
}

var client = new HttpClient();
var response = await client.GetAsync(uri);

string sample;
IEnumerable<string> values;

if (response.Headers.TryGetValues("Sample-Header", out values))
{
   // This happens!
   sample = values.First();
}