Disable the default constructor on a Database-First workflow

Question

How can I prevent the use of the parameterless constructor of the generated DbContext?

var dcx = new DataEntities();

The default constructor is generated by the T4 template, and I thus cannot override it in a partial class. I would prefer it not compile, but a runtime error would also be good.

Answer 1

You can modify the template to provide the constructor you want.

• Open the *.Context.tt file
• Go to line ~59

• Change this code.

  public <#=code.Escape(container)#>()
      : base("name=<#=container.Name#>")
  

• Into the default constructor you want, for example.

  public <#=code.Escape(container)#>(string nameOrConnectionString)
      : base(nameOrConnectionString)
  

• Save

Answer 2

You can inherit the DbContext created by the template, define your own constructor, and use the inherited DbContext instead of the one generated by the template.

public class MyModifiedDbContext : TheTemplateGeneratedDbContext
{
   public MyModifiedDbContext() 
   {
       // define your own constructor
   }
}

Or make it private to avoid its use, so you get the error at compile time

public class MyModifiedDbContext : TheTemplateGeneratedDbContext
{
   private MyModifiedDbContext() 
   // ...
}

Use MyModifiedDbContext instead of TheTemplateGeneratedDbContext

Answer 3

I gave up waiting for EF Core team to add this as an option. I don't want to make and maintain my own T4 templates for this - that's nuts!

Solution for me was just to run some regex on the generated code as part of a powershell script.

fix-dbcontext.ps1

$filename=$args[0]

# load context file
$content = (Get-Content -Raw $filename) 

[regex] $commentOutConstructorRegex = '(?ms)(?<=: DbContext\s*.*?)(public.*?)(?=\s*public)'
$content = $commentOutConstructorRegex.Replace($content, '// Default constructor removed', 1)

[regex] $removeOnConfiguringRegex = '(?ms)(protected override void OnConfiguring).*?(?=\s*protected)'
$content = $removeOnConfiguringRegex.Replace($content, '// Generated OnConfiguring removed', 1)

[regex] $dateCommentRegex = '(?ms)(?=\s*public partial class)'
$content = $dateCommentRegex.Replace($content, "`r`n`t// Generated " + (Get-Date).ToString() + "`r`n", 1)

$content | Out-File -Encoding UTF8 $filename

This will:

1. Remove the default constructor
2. Remove the OnConfiguring method including hardcoded connection string
3. Add the date in a comment

Just run it with .\fix-dbcontext.ps1 .\MyDBContext.cs.

You probably want to change that last line to context.txt instead of $filename until you're sure it does what you want.

IMPORTANT: This was tested only on EFCore templates, but if you understand my Regexes you should be able to modify it for EntityFramework if it doesn't already work.