dotnet ef "Unable to create a DbContext" in CI

This means EF tools could not instantiate your DbContext at design time. It needs either a parameterless constructor, DI registration the tools can resolve, or an IDesignTimeDbContextFactory. In CI it often fails because a connection string or environment variable present locally is absent.

What this error means

The EF command fails with "Unable to create an object of type DbContext" plus a hint about adding a design-time factory. It reproduces every run for the same configuration.

dotnet

Unable to create an object of type 'AppDbContext'. For the different patterns supported
at design time, see https://go.microsoft.com/fwlink/?linkid=851728

Common causes

Design-time configuration is missing

The DbContext relies on DI/config (e.g. a connection string from configuration) that is not available at design time in CI, so the tools cannot construct it.

No design-time factory and no usable constructor

The context has no parameterless constructor and no IDesignTimeDbContextFactory, so EF cannot create it without running the app.

How to fix it

Add a design-time factory

Implement IDesignTimeDbContextFactory<AppDbContext> that builds options from a CI-available connection string.
Supply the connection string via a CI env var or a non-secret design-time default.
Re-run the EF command.

public sealed class DesignFactory : IDesignTimeDbContextFactory<AppDbContext>
{
  public AppDbContext CreateDbContext(string[] args)
  {
    var cs = Environment.GetEnvironmentVariable("EF_CONN") ?? "Host=localhost;Database=app";
    var opts = new DbContextOptionsBuilder<AppDbContext>().UseNpgsql(cs).Options;
    return new AppDbContext(opts);
  }
}

How to prevent it

Provide a design-time factory so EF tooling never depends on app startup.
Set a CI env var for the design-time connection string.