C#•4y ago

Dependency Injection in ASP.NET Core and issue with null suppresion

Hi, I'm having some issues with null suppression and dependency injection in ASP.NET Core

I have this block of code

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

And it wasn't able to resolve

Cannot resolve scoped service 'RemoteGatewayManager.Types.Classes.AppDb' from root provider

Cannot resolve scoped service 'RemoteGatewayManager.Types.Classes.AppDb' from root provider

Cannot resolve scoped service 'RemoteGatewayManager.Types.Classes.AppDb' from root provider

Cannot resolve scoped service 'RemoteGatewayManager.Types.Classes.AppDb' from root provider

I also tried with DbContext

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

builder.Services.AddDbContext<AppDb>();
builder.Services.AddHostedService(provider =>
    new DbMigrator(provider.GetService<ILogger<DbMigrator>>()!, provider.GetService<AppDb>()!)
);

And the

provider.GetService<AppDb>()!

provider.GetService<AppDb>()!

provider.GetService<AppDb>()!

provider.GetService<AppDb>()!

returns

null

null

, but doesn't throw. Isn't it supposed to throw for being null?
Also why is it returning null instead of telling that it is not able to resolve service?

Edit:

AppDb

AppDb

AppDb

AppDb is a class that inherrits from

DbContext

DbContext

DbContext

DbContext

Thanks in advance

undisputed world champions 🍉•10/12/22, 6:25 AM

putting !! behind variables, does not change the behavior
it just tells the compiler "i know it looks like this could be null, i (the developer) made sure it is not null", so the compiler does not keep telling you this variable could be null and that you should check it
if you wanna throw if the type can not be resolved you can use provider.GetRequiredService<T>()provider.GetRequiredService<T>() instead

undisputed world champions 🍉•10/12/22, 6:27 AM

the "Cannot resolve scoped service 'RemoteGatewayManager.Types.Classes.AppDb' from root provider" error just says you did not create a scope before trying to resolve
in asp.net core there should be a scope created for each client-request, so you dont have to worry about it there
but in backgroundservices and similar you will have to create a scope before resolving scoped-services

kunio_kunOP•10/12/22, 6:27 AM

putting ! behind variables, does not change the behavior

Ah yeah sorry i think i mistaken that for another language

kunio_kunOP•10/12/22, 6:28 AM

how do i know that the

AppDb

AppDb

is registered as scoped service?

undisputed world champions 🍉•10/12/22, 6:31 AM

probably says so somewhere in the documentation ^^
but if you think about it, it makes sense, since you dont want database accesses from different requests to influence each other
registering the db-context as scoped service, makes sure you get separate db-contexts for separate requests

kunio_kunOP•10/12/22, 6:31 AM

does that mean have to use this https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.dependencyinjection.serviceproviderserviceextensions.createscope?view=dotnet-plat-ext-6.0 to create a scope first. and then get the dbcontext?

ServiceProviderServiceExtensions.CreateScope(IServiceProvider) Meth...

Creates a new IServiceScope that can be used to resolve scoped services.

undisputed world champions 🍉•10/12/22, 6:33 AM

if you want to resolve it in a background service or similar, yeah
in controllers and stuff called from controllers you dont need to worry about it, since asp.net core will create scopes for each request for you already

kunio_kunOP•10/12/22, 6:33 AM

yeah i'm using it in hosted services and background services

undisputed world champions 🍉•10/12/22, 6:34 AM

here is a article about how to use scopes in background services

https://learn.microsoft.com/en-us/dotnet/core/extensions/scoped-service

Use scoped services within a BackgroundService - .NET

Learn how to use scoped services within a BackgroundService in .NET.