Android : Xarmin.Android AsyncTask strange behaviour

I have to implement my Asynctask to implement some work. This is my code:

class LoaderInfo : AsyncTask<Java.Lang.Void, Java.Lang.Void, Dictionary<String, String>>
    protected override void OnPreExecute()
    {
        base.OnPreExecute();
        //Set the dialog for the user.
        pDialog = new ProgressDialog(this.context);
        pDialog.SetMessage(message);
        //It means the "loading amount" is not measured.
        pDialog.Indeterminate = true;
        //Sets whether this dialog is cancelable with the BACK key.
        pDialog.SetCancelable(false);
        pDialog.Show();
    }
protected override Dictionary<String, String> RunInBackground(paramsJava.Lang.Void[] @params)
{
    Dictionary<String, String> d = new Dictionary<String, String>();
    return d;
}
protected override void OnPostExecute(Dictionary<String, String> result)
{
    base.OnPostExecute(result);
    this.pDialog.Dismiss();
}

The problem is that OnPostExecute is never called! I have tried with the debug, but seems that the "RunInBackground" (in the official documentation I didn't see this method, but visual studio say me that I must implement this override method and not DoInBackground, Why?) doesn't return nothing.

I have tried with this code and everything works:

class LoaderInfo : AsyncTask<Java.Lang.Void, Java.Lang.Void, Java.Lang.Void>
{

ProgressDialog pDialog;
Context context;

public LoaderInfo(Context context)
{
    this.context = context;
}
protected override void OnPreExecute()
{
    base.OnPreExecute();
    //Set the dialog for the user.
    pDialog = new ProgressDialog(this.context);
    String message = this.context.GetString(Resource.String.load);
    pDialog.SetMessage(message);
    //It means the "loading amount" is not measured.
    pDialog.Indeterminate = true;
    //Sets whether this dialog is cancelable with the BACK key.
    pDialog.SetCancelable(false);
    pDialog.Show();
}
protected override void OnPostExecute(Java.Lang.Void result)
{
    System.Console.WriteLine("OnPostExecute");
    base.OnPostExecute(result);
    this.pDialog.Dismiss();
}

protected override Java.Lang.Void RunInBackground(params Java.Lang.Void[] @params)
{
    return null; 
}

} }

Why I have this problem? I am I'm doing something wrong?

Thanks in advance for the help.