c# - How to successfully implement WPF textbox validation? -

i'm trying implement should simple textbox validation wpf application, i'm having problems.

i used guide: http://www.codeproject.com/tips/690130/simple-validation-in-wpf

my textbox in mainwindow.xaml:

     <textbox x:name="textbox1" horizontalalignment="left" height="23"               margin="93,111,0,0" textwrapping="wrap" verticalalignment="top"               width="120" style="{staticresource textboxinerror}"              validation.errortemplate="{staticresource validationerrortemplate}">         <textbox.text>             <binding path="name" updatesourcetrigger="propertychanged">                 <binding.validationrules>                     <local:namevalidator/>                 </binding.validationrules>             </binding>         </textbox.text>     </textbox> 

my namevalidator class in mainwindow.xaml.cs:

    public class namevalidator : validationrule      {       public override validationresult validate(object value, system.globalization.cultureinfo cultureinfo)       {         if (value == null)             return new validationresult(false, "value cannot empty.");         else         {             if (value.tostring().length > 3)                 return new validationresult(false, "name cannot more 3 characters long.");         }         return validationresult.validresult;       }   } 

my static resources in app.xaml:

        <controltemplate x:key="validationerrortemplate">             <dockpanel>                 <textblock foreground="red" dockpanel.dock="top">!</textblock>                 <adornedelementplaceholder x:name="erroradorner"></adornedelementplaceholder>             </dockpanel>         </controltemplate>         <style x:key="textboxinerror" targettype="{x:type textbox}">             <style.triggers>                 <trigger property="validation.haserror" value="true">                     <setter property="tooltip"                             value="{binding relativesource={x:static relativesource.self},                             path=(validation.errors)[0].errorcontent}"/>                 </trigger>             </style.triggers>         </style> 

i can run application without errors, validation never triggered.

using posted, works fine me, produces red "!" above textbox. however, did remember set datacontext, ie.

public mainwindow()     {         initializecomponent();         this.datacontext = this;     } 

without this, won't work.