Question

I need to register an Event Handler for a class which is generated by a Template – the T4 Template within the EntityFramework.

Currently, we have edited the generated code to register the handler within the Constructor of the generated class (the Model Context).

Current code:

    public MyAppContext(string connectionString)
        : base(connectionString, ContainerName)
    {
        this.ContextOptions.LazyLoadingEnabled = true;
        // Register the event handler
        this.Connection.StateChange += Connection_StateChange;
    }

The problem is, if the code is ever re-generated in the future, then the above code will be clobbered and the Event Handler will no longer get hooked up…

Code re-generation happens automatically from the smallest thing such as opening the EF Designer and moving a Table around on the canvas! So its imperative we DO NOT rely on leaving the custom code in the generated class.

Is there anyway we can put the registration in a partial class and leave the generated code untouched???

EG is there some sort of event that will always be fired once a constructor is called ?

Answer 1

The answer to this problem is to edit the T4 Template to put in a method call at the end of the Constructor.

This method, in the context of partial classes generated by a template, needs to be a partial method.

The template needs to contain a definition for the partial method.

Then your custom partial class can implement that method and it will be called by the Constructors defined in the generated partial class – now you can regenerate that partial class as many times as you like and be guranteed that the partial method will always be called – assuming no one edits the template.

If someone does edit the template and removes the definition of the partial method, then you will get a compiler error – easy to fix.

If someone edits the template and removes the call to the partial method from the Constructor, then unfortunately, the compiler can’t help you – something to be aware of!

Here is my solution in tid-bits:

A snippet of the constructor and partial method definition in the T4 Template code ‘MyApp.Context.tt’ (see here for a great explanation of T4 syntax and its use within the EntityFramework):

public <#=code.Escape(container)#>(string connectionString)
    : base(connectionString, ContainerName)
{
<#
    WriteLazyLoadingEnabled(container);
#>
    // Call the OnContextCreated() method to perform any necessary 'post creation' setup
    OnContextCreated();
}

// Define the OnContextCreated partial method so that the accompanying partial Context 
// class can implement this method.
partial void OnContextCreated();

The custom partial class which implements the partial method and wires up the event handler:

public partial class MyAppContext 
{
    /// <summary>
    /// Performs all 'post creation' operations for the MyAppContext 
    /// 
    /// *********************************
    /// NOTE: If you get a compiler error:
    /// 'No defining declaration found for implementing declaration of partial method 'OnContextCreated()'  
    /// then it is likely that the partial class MyApp.Context.cs does not contain a corresponding
    /// definition for the partial method OnContextCreated().
    /// This can occur if the MyApp.Context.tt template no longer generates the definition.
    /// SOLUTION: Edit the MyApp.Context.tt T4 template to ensure that that partial method is defined AND
    /// that it is called from EACH MyAppContext() constructor.
    /// *********************************
    /// 
    /// </summary>
    partial void OnContextCreated()
    {
        // Register the event handler
        this.Connection.StateChange += Connection_StateChange;
    }
}