Is it possible to set code behind a resource dictionary in WPF for event handling?

I think what you're asking is you want a code-behind file for a ResourceDictionary. You can totally do this! In fact, you do it the same way as for a Window:

Say you have a ResourceDictionary called MyResourceDictionary. In your MyResourceDictionary.xaml file, put the x:Class attribute in the root element, like so:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:Class="MyCompany.MyProject.MyResourceDictionary"
                    x:ClassModifier="public">

Then, create a code behind file called MyResourceDictionary.xaml.cs with the following declaration:

namespace MyCompany.MyProject
{
    partial class MyResourceDictionary : ResourceDictionary
    { 
       public MyResourceDictionary()
       {
          InitializeComponent();
       }     
       ... // event handlers ahead..
    }
}

And you're done. You can put whatever you wish in the code behind: methods, properties and event handlers.

== Update for Windows 10 apps ==

And just in case you are playing with UWP there is one more thing to be aware of:

<Application x:Class="SampleProject.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:rd="using:MyCompany.MyProject">
<!-- no need in x:ClassModifier="public" in the header above -->

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>

                <!-- This will NOT work -->
                <!-- <ResourceDictionary Source="/MyResourceDictionary.xaml" />-->

                <!-- Create instance of your custom dictionary instead of the above source reference -->
                <rd:MyResourceDictionary />

            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>

I disagree with "ageektrapped"... using the method of a partial class is not a good practice. What would be the purpose of separating the Dictionary from the page then?

From a code-behind, you can access a x:Name element by using:

Button myButton = this.GetTemplateChild("ButtonName") as Button;
if(myButton != null){
   ...
}

You can do this in the OnApplyTemplate method if you want to hookup to controls when your custom control loads. OnApplyTemplate needs to be overridden to do this. This is a common practice and allows your style to stay disconnected from the control. (The style should not depend on the control, but the control should depend on having a style).

Gishu - whilst this might seem to be a "generally not to be encouraged practice" Here is one reason you might want to do it:

The standard behaviour for text boxes when they get focus is for the caret to be placed at the same position that it was when the control lost focus. If you would prefer throughout your application that when the user tabs to any textbox that the whole content of the textbox was highlighted then adding a simple handler in the resource dictionary would do the trick.

Any other reason where you want the default user interaction behaviour to be different from the out of the box behaviour seems like good candidates for a code behind in a resource dictionary.

Totally agree that anything which is application functionality specific ought not be in a code behind of a resource dictionary.