Partial class constructors and event handler registration

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 ?

JTech · Accepted Answer · 2012-02-22T22:20:53.507

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;
    }
}

score 2 · Answer 2 · answered Feb 21 '12 at 02:29

2

Are you talking about EF? There is a special extension mechanism for this : OnContextCreated() partial method.

You can use it this way:

partial class MyAppContext
{
        partial void OnContextCreated()
        {
            // Register the event handler
            this.Connection.StateChange += Connection_StateChange;
        }

        void Connection_StateChange(object sender, StateChangeEventArgs e) {

        }
}

answered Feb 21 '12 at 02:29

Dmitry Nogin

3,670
1
19
35

Hiya, thanks for the answer. Yes, I am talking about EF. OnContextCreated() sounds perfect! However, when putting OnContextCreated() within the partial class i get compiler error: `No defining declaration found for implementing declaration of partial method '...OnContextCreated()'`. For some reason, the Context created by EF is not putting the partial method in the generated code. I'm not sure how to get it to do that...aside from editing the *.tt Template file myself. Any suggestions? – JTech Feb 21 '12 at 06:51
Yes, I would say that *.tt has to be modified a little bit. (None *.tt EF4 model always has it.) – Dmitry Nogin Feb 21 '12 at 07:02

score 0 · Answer 3 · answered Feb 21 '12 at 02:18

0

How about an intermediate base class?

public class Intermediate : WhateverYourBaseWas {
    public Intermediate(string connectionString, string containerName) : base(connectionString, containerName) {
        this.Connection.StateChange += Connection_StateChange;
    }
}

Then your generated class can inherit from that.

answered Feb 21 '12 at 02:18

Chris Shain

50,833
6
93
125

Unfortunately, this still requires that we edit the generated code to inherit from the intermediate - really no different from setting up the handler in the generated classe's constructor. – JTech Feb 21 '12 at 04:06

Partial class constructors and event handler registration

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