EF Core: 'Unable to resolve service for type AppDbContext while attempting to activate'

System.InvalidOperationException: Unable to resolve service for type 'FixLab.AppDbContext' while attempting to activate 'FixLab.CustomerService'. means a class asks for your DbContext in its constructor, but the context was never registered with dependency injection. Register it with AddDbContext in Program.cs and the error goes away.

One line does it: builder.Services.AddDbContext<AppDbContext>(...) with your provider and connection string. Registering the service that uses the context is not enough; the container has to know how to build the context too.

The error

Unhandled exception. System.InvalidOperationException: Unable to resolve service for type 'FixLab.AppDbContext' while attempting to activate 'FixLab.CustomerService'.
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateArgumentCallSites(ServiceIdentifier serviceIdentifier, Type implementationType, CallSiteChain callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)

The rest of the stack trace ends in GetRequiredService, called from Program.cs. FixLab and CustomerService are the test project's names; yours will show your own context and the class that needs it.

Why it happens

The message names two types. The second, CustomerService, is registered and the container is trying to build it. The first, AppDbContext, is a constructor parameter of that service, and the container has no registration for it. The program registered only the service:

builder.Services.AddScoped<CustomerService>();

A DbContext is not registered by referencing the EF Core packages, and creating it yourself elsewhere with new does not help either: the container only hands out what was added to builder.Services. The error appears the first time something resolves the service, which here was GetRequiredService<CustomerService>(), not the line that built the host.

The fix

builder.Services.AddDbContext<AppDbContext>(o => o.UseSqlServer(@"Server=(localdb)\MSSQLLocalDB;Database=Fix4Ef;Trusted_Connection=True;TrustServerCertificate=True"));

dotnet run -- fixed
CustomerService resolved, provider: Microsoft.EntityFrameworkCore.SqlServer

AddDbContext registers the context as a scoped service, so each request or scope gets its own instance. The context also needs a constructor that accepts the options the container builds and passes them on:

public AppDbContext(DbContextOptions<AppDbContext> options) : base(options) { }

Without that constructor the options never reach the context, and EF Core answers with "No database provider has been configured for this DbContext", whose own text points at this constructor. In a real app read the connection string from configuration rather than writing it in code.

How it was reproduced

A console project on .NET SDK 10.0.401 using the generic host (Host.CreateApplicationBuilder) with Microsoft.Extensions.Hosting 10.0.12, Microsoft.EntityFrameworkCore.SqlServer 10.0.12 and Microsoft.EntityFrameworkCore.Design 10.0.12 on Windows 11. CustomerService takes AppDbContext in its constructor; without the AddDbContext line, resolving it threw the error above, and with it the service resolved and reported the SQL Server provider.

Frequently asked

How do I fix 'Unable to resolve service for type DbContext while attempting to activate'?

Register the context in Program.cs with builder.Services.AddDbContext, passing your provider and connection string, and give the context a constructor that takes DbContextOptions of that context and passes it to base.

Which type in the message is missing?

The first one. The message reads: unable to resolve the first type while activating the second, so the first type is the missing registration and the second is the class that asked for it.

Why does the error appear when the service is used and not at startup?

The container builds a service the first time something asks for it. In the reproduction the exception came from GetRequiredService, two lines after the host was built, because nothing resolved CustomerService before that.

More decoded errors in the Fixes category. The same message for services other than a DbContext is covered in Unable to resolve service for type ... while attempting to activate.