[TemplatePart(Name = "PART_SearchTextBox", Type = typeof(TextBox))]

     [TemplatePart(Name = "PART_ClearButton", Type = typeof(Button))]

      public class SearchBox : TextBox

2.创建2个子控件的Style， 注意ControlTemplate中的子控件命名和上述属性声明一致

<Style TargetType="{x:Type local:SearchBox}">
      <Setter Property="Template">   
         <Setter.Value>                
               <ControlTemplate TargetType="{x:Type local:SearchBox}">
                    <!-------------子控件PART_SearchTextBox,PART_ClearButton及其它设置--------------->
               </ControlTemplate>
          </Setter.Value>
      </Setter>
</Style>
3.如果在当前程序集里，必须放在.\Themes文件夹下; 必须在程序集的AssemblyInfo.cs里指定ThemeInfo（用来指定控件默认Style）
[assembly: ThemeInfo(

    ResourceDictionaryLocation.None, //where theme specific resource dictionaries are located

    //(used if a resource is not found in the page,

    // or application resource dictionaries)

    ResourceDictionaryLocation.SourceAssembly //where the generic resource dictionary is located

    //(used if a resource is not found in the page,

    // app, or any theme specific resource dictionaries)

)]

4.在自定义控件的实现中注册路由事件等等其它逻辑处理
比如：对子元素Clear按钮进行处理

       public override void OnApplyTemplate()
        {

            base.OnApplyTemplate();

            Button btnClear = base.GetTemplateChild("PART_ClearButton") as Button;

            if (btnClear != null)
            {

               btnClear.Click += new RoutedEventHandler(OnClearButton_Click);
            }

        }

        private void OnClearButton_Click(Object sender, RoutedEventArgs e)
        {

            TextBox txtSearch = base.GetTemplateChild("PART_SearchTextBox") as TextBox;

            if (txtSearch!= null)
            {

               txtSearch.Text = String.Empty;
               txtSearch.Focus();
            }

        }