var mySecondEditText = FindViewById<EditText>(Resource.Id.MySecondEditText);
mySecondEditText.AttachBehavior(new SelectAllOnFocusBehavior());

/// <summary>
/// Behavior used on an EditText object and used to hide the software keyboard when one of the following key is pressed: "Done", "Search" or "Go"
/// </summary>
public class HideKeyboardOnEnterKeyBehavior : Behavior<EditText>
{
    /// <summary>
    /// Gets or sets the application context.
    /// </summary>
    /// <value>
    /// The application context.
    /// </value>
    public Context ApplicationContext { get; set; }

    private EventHandler<TextView.EditorActionEventArgs> _editorActionEventHandler;

    /// <summary>
    /// Method to override when the behavior is attached to the view.
    /// </summary>
    /// <exception cref="System.InvalidOperationException">ApplicationContext property needs to be set in order to use this behavior.</exception>
    protected override void OnAttached()
    {
        if (this.ApplicationContext == null)
        {
            throw new InvalidOperationException("ApplicationContext property needs to be set in order to use this behavior.");
        }

        _editorActionEventHandler = (sender, args) =>
        {
            if (args.ActionId == ImeAction.Done || args.ActionId == ImeAction.Search || args.ActionId == ImeAction.Go || args.ActionId == ImeAction.Next)
            {
                var inputManager = (InputMethodManager)this.ApplicationContext.GetSystemService(Context.InputMethodService);
                inputManager.HideSoftInputFromWindow(this.AssociatedObject.WindowToken, HideSoftInputFlags.None);

                args.Handled = true;
            }
        };

        this.AssociatedObject.EditorAction += _editorActionEventHandler;
    }

    /// <summary>
    /// Method to override when the behavior is removed from the view.
    /// </summary>
    protected override void OnDetaching()
    {
        if (_editorActionEventHandler != null)
        {
            this.AssociatedObject.EditorAction -= _editorActionEventHandler;
        }
    }
}

/// <summary>
/// Behavior used on an EditText object and used to select all the text within it when it has focus
/// </summary>
public class SelectAllOnFocusBehavior : Behavior<UITextField>
{
    private EventHandler _editingDidBeginEventHandler;

    /// <summary>
    /// Method to override when the behavior is attached to the view.
    /// </summary>
    protected override void OnAttached()
    {
        _editingDidBeginEventHandler = (sender, e) =>
        {
            this.AssociatedObject.PerformSelector(new Selector("selectAll"), null, 0.0f);
        };

        this.AssociatedObject.EditingDidBegin += _editingDidBeginEventHandler;
    }

    /// <summary>
    /// Method to override when the behavior is removed from the associatedObject.
    /// </summary>
    protected override void OnDetaching()
    {
        if (_editingDidBeginEventHandler != null)
        {
            this.AssociatedObject.EditingDidBegin -= _editingDidBeginEventHandler;
        }
    }
}