/// <summary>
        /// Initializes a new instance of the <see cref="Xamarin.Auth.FormAuthenticatorField"/> class.
        /// </summary>
        /// <param name='key'>
        /// A key used to identify the field.
        /// </param>
        /// <param name='title'>
        /// The title of the field when presented in a UI.
        /// </param>
        /// <param name='fieldType'>
        /// The type of the field.
        /// </param>
        /// <param name='placeholder'>
        /// Placeholder text shown when there is no input value for the field.
        /// </param>
        /// <param name='defaultValue'>
        /// The value of the field.
        /// </param>
        public FormAuthenticatorField(string key, string title, FormAuthenticatorFieldType fieldType, string placeholder = "", string defaultValue = "")
        {
            if (string.IsNullOrWhiteSpace(key))
            {
                throw new ArgumentException("key must not be blank", "key");
            }
            Key = key;

            if (string.IsNullOrWhiteSpace(title))
            {
                throw new ArgumentException("title must not be blank", "title");
            }
            Title = title;

            Placeholder = placeholder ?? "";

            Value = defaultValue ?? "";

            FieldType = fieldType;
        }
		/// <summary>
		/// Initializes a new instance of the <see cref="Xamarin.Auth.FormAuthenticatorField"/> class.
		/// </summary>
		/// <param name='key'>
		/// A key used to identify the field.
		/// </param>
		/// <param name='title'>
		/// The title of the field when presented in a UI.
		/// </param>
		/// <param name='fieldType'>
		/// The type of the field.
		/// </param>
		/// <param name='placeholder'>
		/// Placeholder text shown when there is no input value for the field.
		/// </param>
		/// <param name='defaultValue'>
		/// The value of the field.
		/// </param>
		public FormAuthenticatorField (string key, string title, FormAuthenticatorFieldType fieldType, string placeholder = "", string defaultValue = "")
		{
			if (string.IsNullOrWhiteSpace (key)) {
				throw new ArgumentException ("key must not be blank", "key");
			}
			Key = key;

			if (string.IsNullOrWhiteSpace (title)) {
				throw new ArgumentException ("title must not be blank", "title");
			}
			Title = title;

			Placeholder = placeholder ?? "";

			Value = defaultValue ?? "";

			FieldType = fieldType;
		}