How do I use DbContext in a separate class library .NET core?

When you want to use `DbContext` in a separate class library in .NET Core, you typically follow these steps. Let's assume you have a class library called `DataAccessLibrary` and a separate project (e.g., a web application) that references and uses this class library.

1. **Create a Class Library:**
   - Create a new class library project in your solution, for example, named `DataAccessLibrary`.

2. **Install Entity Framework Core:**
   - Install the Entity Framework Core NuGet package in your `DataAccessLibrary` project. You can do this using the Package Manager Console or the NuGet Package Manager in Visual Studio:
     ```
     Install-Package Microsoft.EntityFrameworkCore
     ```

3. **Define Your DbContext:**
   - Create a class that derives from `DbContext` in your `DataAccessLibrary`. Define your database entities and the database context itself. For example:
     ```csharp
     // YourDbContext.cs
     using Microsoft.EntityFrameworkCore;

     namespace DataAccessLibrary
     {
         public class YourDbContext : DbContext
         {
             public YourDbContext(DbContextOptions<YourDbContext> options)
                 : base(options)
             {
             }

             public DbSet<YourEntity> YourEntities { get; set; }
         }
     }
     ```

4. **Configure DbContext in Startup (if using ASP.NET Core):**
   - If you're working with an ASP.NET Core application, configure your `DbContext` in the `Startup.cs` file by adding it to the services collection:
     ```csharp
     // Startup.cs
     using DataAccessLibrary;
     using Microsoft.EntityFrameworkCore;

     public void ConfigureServices(IServiceCollection services)
     {
         // Other services...

         // Configure DbContext
         services.AddDbContext<YourDbContext>(options =>
             options.UseSqlServer(Configuration.GetConnectionString("YourConnectionString")));
     }
     ```

5. **Use Dependency Injection (if needed):**
   - If you want to use your `DbContext` in other classes or controllers, you can inject it using dependency injection. For example, in an ASP.NET Core controller:
     ```csharp
     // YourController.cs
     using DataAccessLibrary;

     public class YourController : Controller
     {
         private readonly YourDbContext _context;

         public YourController(YourDbContext context)
         {
             _context = context;
         }

         // Your actions using _context...
     }
     ```

6. **Configure Connection String:**
   - Make sure to configure your database connection string in your application's configuration. This is important, especially if you are using ASP.NET Core. For example, in `appsettings.json`:
     ```json
     {
       "ConnectionStrings": {
         "YourConnectionString": "your_actual_connection_string"
       },
       // other settings...
     }
     ```

Remember to adjust class and variable names according to your application's requirements. This example assumes you're using Entity Framework Core with SQL Server, but you can replace the database provider and connection string as needed.

By following these steps, you can create a class library that encapsulates your data access logic and use it in other projects like ASP.NET Core applications.