Getting return value from Task.Run

Remove the Result from the end. When you await you will get the Result back from the await-able method.

var val = await Task.Run(() => RunLongTask(i.ToString(CultureInfo.InvariantCulture)));

This is not a direct answer to old question, but for others searching:

"Normally" you shouldn't do this, but sometimes you need to match a library API so you can use a wrapper function like below:

private async Task<string> WrapSomeMethod(string someParam)
{
    //adding .ConfigureAwait(false) may NOT be what you want but google it.
    return await Task.Run(() => SomeObj.SomeMethodAsync(someParam)).ConfigureAwait(false);
}

And then call that instead with .Result like below:

string blah = WrapSomeMethod(someParam).Result;