[转] EF Configuring a DbContext

本文转自：https://docs.microsoft.com/en-us/ef/core/miscellaneous/configuring-dbcontext

Note

This documentation is for EF Core. For EF6.x, see Entity Framework 6.+

This article shows patterns for configuring a DbContext with DbContextOptions. Options are primarily used to select and configure the data store.+

Configuring DbContextOptions

DbContext must have an instance of DbContextOptions in order to execute. This can be configured by overriding OnConfiguring, or supplied externally via a constructor argument.+

If both are used, OnConfiguring is executed on the supplied options, meaning it is additive and can overwrite  options supplied to the constructor argument.+

Constructor argument

Context code with constructor+

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public class BloggingContext : DbContext
{
    public BloggingContext(DbContextOptions<BloggingContext> options)
        : base(options)
    { }

    public DbSet<Blog> Blogs { get; set; }
}

Tip

The base constructor of DbContext also accepts the non-generic version of DbContextOptions. Using the non-generic version is not recommended for applications with multiple context types.+

Application code to initialize from constructor argument+

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var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
optionsBuilder.UseSqlite("Filename=./blog.db");

using (var context = new BloggingContext(optionsBuilder.Options))
{
    // do stuff
}

OnConfiguring

Warning

OnConfiguring occurs last and can overwrite options obtained from DI or the constructor. This approach does not lend itself to testing (unless you target the full database).+

Context code with OnConfiguring+

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public class BloggingContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlite("Filename=./blog.db");
    }
}

Application code to initialize with "OnConfiguring"+

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using (var context = new BloggingContext())
{
    // do stuff
}

Using DbContext with dependency injection

EF supports using DbContext with a dependency injection container. Your DbContext type can be added to the service container by using AddDbContext<TContext>.+

AddDbContext will add make both your DbContext type, TContext, and DbContextOptions<TContext> to the available for injection from the service container.+

See more reading below for information on dependency injection.+

Adding dbcontext to dependency injection+

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public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<BloggingContext>(options => options.UseSqlite("Filename=./blog.db"));
}

This requires adding a constructor argument to you DbContext type that accepts DbContextOptions.+

Context code+

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public class BloggingContext : DbContext
{
    public BloggingContext(DbContextOptions<BloggingContext> options)
      :base(options)
    { }

    public DbSet<Blog> Blogs { get; set; }
}

Application code (in ASP.NET Core)+

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public MyController(BloggingContext context)

Application code (using ServiceProvider directly, less common)+

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using (var context = serviceProvider.GetService<BloggingContext>())
{
  // do stuff
}

var options = serviceProvider.GetService<DbContextOptions<BloggingContext>>();

+

Using IDbContextFactory<TContext>

As an alternative to the options above, you may also provide an implementation of IDbContextFactory<TContext>. EF command line tools and dependency injection can use this factory to create an instance of your DbContext. This may be required in order to enable specific design-time experiences such as migrations.+

Implement this interface to enable design-time services for context types that do not have a public default constructor. Design-time services will automatically discover implementations of this interface that are in the same assembly as the derived context.+

Example:+

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using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Infrastructure;

namespace MyProject
{
    public class BloggingContextFactory : IDbContextFactory<BloggingContext>
    {
        public BloggingContext Create()
        {
            var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
            optionsBuilder.UseSqlite("Filename=./blog.db");

            return new BloggingContext(optionsBuilder.Options);
        }
    }
}

More reading

• Read Getting Started on ASP.NET Core for more information on using EF with ASP.NET Core.
• Read Dependency Injection to learn more about using DI.
• Read Testing with InMemory for more information.
• Read Understanding EF Services for more details on how EF uses dependency injection internally.