Introduction
The error Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContextOptions'1[LibraryData.LibraryContext]' while attempting to activate means your DbContext was not registered in the ASP.NET Core dependency injection container. The DI system cannot provide the DbContextOptions<LibraryContext> that Entity Framework Core needs to construct your context. The fix is to call AddDbContext<LibraryContext>() in your Program.cs (or Startup.cs in older templates).
The Error
System.InvalidOperationException: Unable to resolve service for type
'Microsoft.EntityFrameworkCore.DbContextOptions`1[LibraryData.LibraryContext]'
while attempting to activate 'LibraryData.LibraryContext'.
This occurs when a controller or service requests LibraryContext via constructor injection, but the DI container has no registration for it.
The Fix
.NET 6+ (Minimal API / Program.cs)
1var builder = WebApplication.CreateBuilder(args);
2
3// Register the DbContext — THIS IS THE FIX
4builder.Services.AddDbContext<LibraryContext>(options =>
5 options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection"))
6);
7
8var app = builder.Build();
.NET 5 and Earlier (Startup.cs)
1public class Startup
2{
3 public void ConfigureServices(IServiceCollection services)
4 {
5 // Register the DbContext
6 services.AddDbContext<LibraryContext>(options =>
7 options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"))
8 );
9
10 services.AddControllers();
11 }
12}
The DbContext Class
1// LibraryData/LibraryContext.cs
2using Microsoft.EntityFrameworkCore;
3
4namespace LibraryData
5{
6 public class LibraryContext : DbContext
7 {
8 // This constructor is REQUIRED for DI
9 public LibraryContext(DbContextOptions<LibraryContext> options)
10 : base(options)
11 {
12 }
13
14 public DbSet<Book> Books { get; set; }
15 public DbSet<Author> Authors { get; set; }
16 }
17}
Common Cause 1: Missing AddDbContext Call
The most frequent cause. You created the DbContext class but forgot to register it in the DI container.
1// BROKEN — no DbContext registration
2var builder = WebApplication.CreateBuilder(args);
3builder.Services.AddControllers();
4// Missing: builder.Services.AddDbContext<LibraryContext>(...)
5var app = builder.Build();
6
7// FIXED
8var builder = WebApplication.CreateBuilder(args);
9builder.Services.AddDbContext<LibraryContext>(options =>
10 options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection"))
11);
12builder.Services.AddControllers();
13var app = builder.Build();
Common Cause 2: Wrong DbContextOptions Type
The constructor must accept DbContextOptions<LibraryContext> (generic), not DbContextOptions (non-generic).
1// BROKEN — uses non-generic DbContextOptions
2public class LibraryContext : DbContext
3{
4 public LibraryContext(DbContextOptions options) // Wrong type
5 : base(options) { }
6}
7
8// FIXED — uses generic DbContextOptions<LibraryContext>
9public class LibraryContext : DbContext
10{
11 public LibraryContext(DbContextOptions<LibraryContext> options)
12 : base(options) { }
13}
The DI container registers DbContextOptions<LibraryContext> specifically, so asking for the non-generic DbContextOptions will not match.
Common Cause 3: Missing Constructor
If your DbContext has a parameterless constructor or no constructor at all, EF Core cannot inject the options.
1// BROKEN — no constructor accepting options
2public class LibraryContext : DbContext
3{
4 // EF Core has no way to pass options in
5 public DbSet<Book> Books { get; set; }
6}
7
8// BROKEN — parameterless constructor only
9public class LibraryContext : DbContext
10{
11 public LibraryContext() { }
12 public DbSet<Book> Books { get; set; }
13}
14
15// FIXED — constructor accepts options
16public class LibraryContext : DbContext
17{
18 public LibraryContext(DbContextOptions<LibraryContext> options)
19 : base(options) { }
20
21 public DbSet<Book> Books { get; set; }
22}
Common Cause 4: Connection String Missing
AddDbContext is called, but the connection string is null or missing from appsettings.json.
1// appsettings.json
2{
3 "ConnectionStrings": {
4 "DefaultConnection": "Server=localhost;Database=LibraryDb;Trusted_Connection=True;"
5 }
6}
1// This returns null if "DefaultConnection" is not in appsettings.json
2var connectionString = builder.Configuration.GetConnectionString("DefaultConnection");
3
4// AddDbContext still registers the context, but it will fail at runtime
5// when trying to connect to the database
6builder.Services.AddDbContext<LibraryContext>(options =>
7 options.UseSqlServer(connectionString)
8);
Common Cause 5: Multiple DbContexts
When you have multiple DbContext classes, each must be registered separately.
1public class LibraryContext : DbContext { /* ... */ }
2public class IdentityContext : DbContext { /* ... */ }
3
4// Must register BOTH
5builder.Services.AddDbContext<LibraryContext>(options =>
6 options.UseSqlServer(builder.Configuration.GetConnectionString("LibraryDb"))
7);
8builder.Services.AddDbContext<IdentityContext>(options =>
9 options.UseSqlServer(builder.Configuration.GetConnectionString("IdentityDb"))
10);
Using Different Database Providers
1// SQL Server
2builder.Services.AddDbContext<LibraryContext>(options =>
3 options.UseSqlServer(connectionString)
4);
5
6// PostgreSQL (Npgsql)
7builder.Services.AddDbContext<LibraryContext>(options =>
8 options.UseNpgsql(connectionString)
9);
10
11// SQLite
12builder.Services.AddDbContext<LibraryContext>(options =>
13 options.UseSqlite("Data Source=library.db")
14);
15
16// In-Memory (for testing)
17builder.Services.AddDbContext<LibraryContext>(options =>
18 options.UseInMemoryDatabase("TestDb")
19);
Injecting the Context
Once registered, inject LibraryContext via constructor injection:
1public class BooksController : ControllerBase
2{
3 private readonly LibraryContext _context;
4
5 public BooksController(LibraryContext context)
6 {
7 _context = context;
8 }
9
10 [HttpGet]
11 public async Task<ActionResult<IEnumerable<Book>>> GetBooks()
12 {
13 return await _context.Books.ToListAsync();
14 }
15}
You can also inject into services:
1public class BookService
2{
3 private readonly LibraryContext _context;
4
5 public BookService(LibraryContext context)
6 {
7 _context = context;
8 }
9}
10
11// Register the service
12builder.Services.AddScoped<BookService>();
Common Pitfalls
Registering AddDbContext after Build(): AddDbContext must be called before builder.Build(). Any service registration after Build() has no effect and the context will not be found.
Using new LibraryContext() instead of DI: Manually constructing the context bypasses DI and loses connection string configuration. Always inject the context through constructors.
Wrong project reference: If LibraryContext is in a separate class library project (e.g., LibraryData), ensure the web project references that project and the correct NuGet packages (Microsoft.EntityFrameworkCore.SqlServer) are installed in the right project.
Service lifetime mismatch: AddDbContext registers the context as Scoped by default. Injecting a scoped service into a singleton causes a runtime error. If you need a context in a singleton, use IServiceScopeFactory to create a scope.
Confusing AddDbContext with AddDbContextFactory: AddDbContextFactory<T> (EF Core 5+) registers a factory, not the context directly. Injecting LibraryContext directly will fail — you must inject IDbContextFactory<LibraryContext> and call CreateDbContext().
Summary
Call AddDbContext<LibraryContext>(options => ...) in Program.cs or Startup.ConfigureServices
The DbContext must have a constructor accepting DbContextOptions<LibraryContext> (generic version)
Ensure the connection string exists in appsettings.json under ConnectionStrings
Register each DbContext separately when using multiple contexts
The context is Scoped by default — do not inject directly into singleton services
