C# - transform an async task from one type to another

The most direct translation with existing methods would be:

Task<int> statusCode = response.ContinueWith(t => t.Result.StatusCode)

However in practice you almost always await the task to get the result. Maybe you should look into async/await.

I'm slightly surprised there isn't anything in the framework for this, to be honest. (More likely, there is an I haven't seen it.) You can build it fairly easily though:

public static async Task<TResult> Map<TSource, TResult>
    (Task<TSource> task, Func<TSource, TResult> selector)
    => selector(await task.ConfigureAwait(false));

Note: using ConfigureAwait here isn't always clear-cut. You may want to include a continueOnCapturedContext parameter:

public static async Task<TResult> Map<TSource, TResult>(
    Task<TSource> task,
    Func<TSource, TResult> selector,
    bool continueOnCapturedContext = false) =>
    selector(await task.ConfigureAwait(continueOnCapturedContext));