Entityframework.docs: IDesignTimeDbContextFactory with dependency injection?

[weitzhandler]

When trying to create a DI-dependent implementation of IDesignTimeDbContextFactory, I get the following error upong trying to add migrations:

PM> add-migration InitialCreate -context AppDbContext
System.MissingMethodException: No parameterless constructor defined for this object.
at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean wrapExceptions, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor)
at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean wrapExceptions, Boolean skipCheckThis, Boolean fillCache)
at System.Activator.CreateInstance(Type type, Boolean nonPublic, Boolean wrapExceptions)
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContextFromFactory(Type factory)
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass12_1.b__9()
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func1 factory) at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType) at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_1.<.ctor>b__0() at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_01.b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
No parameterless constructor defined for this object.
PM>

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Document Details

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• ID: 2fa64390-1232-8dd3-f170-7d5c68e29ad9
• Version Independent ID: 245bbba8-3102-4a66-15eb-650e05eb9e62
• Content: Design-time DbContext Creation - EF Core
• Content Source: entity-framework/core/miscellaneous/cli/dbcontext-creation.md
• Product: entity-framework
• Technology: entity-framework-core
• GitHub Login: @bricelam
• Microsoft Alias: bricelam

[bricelam]

Think of IDesignTimeDbContextFactory.CreateDbContext() like Program.Main() but for tools. It's the entry point. You'll need to build your service container before you can use DI.

[bricelam]

Your IDesignTimeDbContextFactory implementation needs a parameterless constructor. There are no services to inject yet.

[weitzhandler]

Thank you @bricelam. Here's my implementation, for future reference. I believe once deployed I'm gonna need to work on it again, but it works now. Would have been nice if there was a better way to access the configuration off DI.

```c#
public class AppDbContextFactory : IDesignTimeDbContextFactory
{
public AppDbContext CreateDbContext(params string[] args)
{
var options = new DbContextOptionsBuilder();
var config = GetAppConfiguration();
options.UseSqlServer(config.GetConnectionString("DesignTimeAppDbConnection"));

return new AppDbContext(options.Options);

}

IConfiguration GetAppConfiguration()
{
var environmentName =
Environment.GetEnvironmentVariable(
"ASPNETCORE_ENVIRONMENT");

var dir = Directory.GetParent(AppContext.BaseDirectory);    
do
  dir = dir.Parent;
while (dir.Name != "bin");
dir = dir.Parent;
var path = dir.FullName;

var builder = new ConfigurationBuilder()
        .SetBasePath(path)
        .AddJsonFile("appsettings.json")
        .AddJsonFile($"appsettings.{environmentName}.json", true)
        .AddEnvironmentVariables();

return builder.Build();

}
}
```

[bricelam]

Would using the Program. CreateHostBuilder() pattern work better for you?

[weitzhandler]

I'll check out your suggestion, but I realize that my real problem is this, which is happening at runtime.

Thank you @bricelam.