C#•2y 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>()!)
);

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

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>()!)
);

And the

provider.GetService<AppDb>()!

provider.GetService<AppDb>()!

returns 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 is a class that inherrits from DbContext Thanks in advance

8 Replies

undisputed world champions•2y ago

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>() instead 😉 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_kun•2y ago

putting ! behind variables, does not change the behavior

Ah yeah sorry i think i mistaken that for another language how do i know that the AppDb is registered as scoped service? 🤔

undisputed world champions•2y ago

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_kun•2y ago

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•2y ago

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_kun•2y ago

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

undisputed world champions•2y ago

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.

kunio_kun•2y ago

I see i could get IServiceProvider in the service thanks a lot that answers it 😄 that resolves my misunderstanding too 😄

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