What does ConfigureAwait(false) do, and when is it appropriate to use?

ConfigureAwait(false) tells the awaited Task NOT to resume its continuation on the original captured SynchronizationContext — appropriate in library code that doesn't need to run back on a specific UI/request context, improving performance and avoiding deadlock risk; generally unnecessary (and often omitted) in modern ASP.NET Core, which has no SynchronizationContext by default.

public async Task<string> FetchDataAsync() {
  var result = await httpClient.GetStringAsync(url).ConfigureAwait(false);
  return result.ToUpper(); // this line doesn't need the original context
}