如何使我的所有用户控件依赖值在控制访问其值之前加载?

当我调用此自定义控件时,必须将属性值 正确的秩序 因为第三个依赖属性( iTimeType码 )访问前两个值( 关键字段,值字段 )要在数据库中查找数据,如果它们位于第一个属性之后,则它们的值为空。

<controls:DropDown x:Name="TheItemTypes" 
  KeyField="idCode" ValueField="title" 
  ItemTypeIdCode="itemTypes" 
  Width="150" HorizontalAlignment="Left" Margin="0 0 0 5"/>

如何使用户控件属性的顺序任意? 也就是说,在任何一个值注册为已更改之前,它们都首先加载它们的值?

以下是我的用户控件代码:

using System.Windows;
using System.Collections.Generic;
using TestApp.DataLayer;

namespace TestApp.Controls
{
    public partial class DropDown : DependencyComboBox
    {
        #region DependencyProperty: ItemTypeIdCode
        public string ItemTypeIdCode
        {
            get
            {
                if (GetValue(ItemTypeIdCodeProperty) != null)
                    return GetValue(ItemTypeIdCodeProperty).ToString();
                else
                    return "";
            }
            set { SetValue(ItemTypeIdCodeProperty, value); }
        }

        public static readonly DependencyProperty ItemTypeIdCodeProperty =
            DependencyProperty.Register("ItemTypeIdCode", typeof(string), typeof(DropDown),
                new PropertyMetadata(string.Empty, OnItemTypeIdCodePropertyChanged));

        private static void OnItemTypeIdCodePropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
        {
            DropDown dropDown = dependencyObject as DropDown;
            dropDown.OnPropertyChanged("ItemTypeIdCode");
            dropDown.OnItemTypeIdCodePropertyChanged(e);
        }

        private void OnItemTypeIdCodePropertyChanged(DependencyPropertyChangedEventArgs e)
        {

            Items.Clear();
            foreach (var kvp in Datasource.GetInstance().GetKeyValues(ItemTypeIdCode + "(" + KeyField + "," + ValueField + "); all; orderby displayOrder"))
            {
                Items.Add(new KeyValuePair<string, string>(kvp.Key, kvp.Value));
            }
            this.SelectedIndex = 0;

        }
        #endregion


        #region DependencyProperty: KeyField
        public string KeyField
        {
            get
            {
                if (GetValue(KeyFieldProperty) != null)
                    return GetValue(KeyFieldProperty).ToString();
                else
                    return "";
            }
            set { SetValue(KeyFieldProperty, value); }
        }

        public static readonly DependencyProperty KeyFieldProperty =
            DependencyProperty.Register("KeyField", typeof(string), typeof(DropDown),
                new PropertyMetadata(string.Empty, OnKeyFieldPropertyChanged));

        private static void OnKeyFieldPropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
        {
            DropDown dropDown = dependencyObject as DropDown;
            dropDown.OnPropertyChanged("KeyField");
            dropDown.OnKeyFieldPropertyChanged(e);
        }

        private void OnKeyFieldPropertyChanged(DependencyPropertyChangedEventArgs e)
        {

        }
        #endregion

        #region DependencyProperty: ValueField
        public string ValueField
        {
            get
            {
                if (GetValue(ValueFieldProperty) != null)
                    return GetValue(ValueFieldProperty).ToString();
                else
                    return "";
            }
            set { SetValue(ValueFieldProperty, value); }
        }

        public static readonly DependencyProperty ValueFieldProperty =
            DependencyProperty.Register("ValueField", typeof(string), typeof(DropDown),
                new PropertyMetadata(string.Empty, OnValueFieldPropertyChanged));

        private static void OnValueFieldPropertyChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
        {
            DropDown dropDown = dependencyObject as DropDown;
            dropDown.OnPropertyChanged("ValueField");
            dropDown.OnValueFieldPropertyChanged(e);
        }

        private void OnValueFieldPropertyChanged(DependencyPropertyChangedEventArgs e)
        {

        }
        #endregion

        public DropDown()
        {
            InitializeComponent();
            DataContext = this;
        }

    }
}