async and await in C#

1.  The async keyword turns a method into an async method, which allows you to use the await keyword in its body. 
2. When the await keyword is applied, it suspends the calling method and yields control back to its caller until the awaited task is complete.
3. await can only be used inside an async method.

private readonly HttpClient _httpClient = new HttpClient();

[HttpGet, Route("DotNetCount")]

public async Task<int> GetDotNetCount()

{

    // Suspends GetDotNetCount() to allow the caller (the web server)

    // to accept another request, rather than blocking on this one.

    var html = await _httpClient.GetStringAsync("https://dotnetfoundation.org");

    return Regex.Matches(html, @"\.NET").Count;

}

Reference: https://docs.microsoft.com