Postgresql 无法创建类型为'；AppContext'；。Npgsql+；埃夫科尔

如何解决我的“HelloEFCore”.NET核心控制台应用程序异常？我使用Npgsql实体框架核心提供程序（）

无法创建“AppContext”类型的对象。有关设计时支持的不同模式，请参见

它发生在我尝试进行初始迁移时

dotnet ef migrations add CreateDatabase

我正在使用的代码：

Program.cs

class Program
{
    static void Main(string[] args)
    {
        AppContext a = new AppContext("Server=127.0.0.1; port=5432; user_id=postgres; password=root; database=db; pooling=true");
    }
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Description { get; set; }
    public int Quantity { get; set; }
    public int BrandId { get; set; }
    public Brand Brand { get; set; }
}

public class Brand
{
    public int BrandId { get; set; }
    public string Name { get; set; }
    public List<Product> Products { get; set; } = new List<Product>();
}

public class AppContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<Brand> Brands { get; set; }
    private readonly string _connectionString;
    public AppContext(string connectionString)
    {
        _connectionString = connectionString ?? 
            throw new ArgumentException("connectionString is empty.");
    }
    protected override void OnConfiguring(DbContextOptionsBuilder builder)
    {
        builder.UseNpgsql(_connectionString);
    }
}

Product.cs

class Program
{
    static void Main(string[] args)
    {
        AppContext a = new AppContext("Server=127.0.0.1; port=5432; user_id=postgres; password=root; database=db; pooling=true");
    }
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Description { get; set; }
    public int Quantity { get; set; }
    public int BrandId { get; set; }
    public Brand Brand { get; set; }
}

public class Brand
{
    public int BrandId { get; set; }
    public string Name { get; set; }
    public List<Product> Products { get; set; } = new List<Product>();
}

public class AppContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<Brand> Brands { get; set; }
    private readonly string _connectionString;
    public AppContext(string connectionString)
    {
        _connectionString = connectionString ?? 
            throw new ArgumentException("connectionString is empty.");
    }
    protected override void OnConfiguring(DbContextOptionsBuilder builder)
    {
        builder.UseNpgsql(_connectionString);
    }
}

Brand.cs

class Program
{
    static void Main(string[] args)
    {
        AppContext a = new AppContext("Server=127.0.0.1; port=5432; user_id=postgres; password=root; database=db; pooling=true");
    }
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Description { get; set; }
    public int Quantity { get; set; }
    public int BrandId { get; set; }
    public Brand Brand { get; set; }
}

public class Brand
{
    public int BrandId { get; set; }
    public string Name { get; set; }
    public List<Product> Products { get; set; } = new List<Product>();
}

public class AppContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<Brand> Brands { get; set; }
    private readonly string _connectionString;
    public AppContext(string connectionString)
    {
        _connectionString = connectionString ?? 
            throw new ArgumentException("connectionString is empty.");
    }
    protected override void OnConfiguring(DbContextOptionsBuilder builder)
    {
        builder.UseNpgsql(_connectionString);
    }
}

公共级品牌
{
public int BrandId{get；set；}
公共字符串名称{get；set；}
公共列表产品{get；set；}=new List（）；
}

AppContext.cs

class Program
{
    static void Main(string[] args)
    {
        AppContext a = new AppContext("Server=127.0.0.1; port=5432; user_id=postgres; password=root; database=db; pooling=true");
    }
}

public class Product
{
    public int ProductId { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Description { get; set; }
    public int Quantity { get; set; }
    public int BrandId { get; set; }
    public Brand Brand { get; set; }
}

public class Brand
{
    public int BrandId { get; set; }
    public string Name { get; set; }
    public List<Product> Products { get; set; } = new List<Product>();
}

public class AppContext : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<Brand> Brands { get; set; }
    private readonly string _connectionString;
    public AppContext(string connectionString)
    {
        _connectionString = connectionString ?? 
            throw new ArgumentException("connectionString is empty.");
    }
    protected override void OnConfiguring(DbContextOptionsBuilder builder)
    {
        builder.UseNpgsql(_connectionString);
    }
}

公共类AppContext:DbContext
{
公共数据库集产品{get；set；}
公共DbSet品牌{get；set；}
私有只读字符串_connectionString；
公共AppContext（字符串连接字符串）
{
_connectionString=connectionString？？
抛出新ArgumentException（“connectionString为空”）；
}
配置时受保护的覆盖无效（DbContextOptionsBuilder）
{
builder.UseNpgsql（_connectionString）；
}
}

efstart.csproj

<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
  <OutputType>Exe</OutputType>
  <TargetFramework>netcoreapp3.1</TargetFramework>
</PropertyGroup>
<ItemGroup>
  <PackageReference Include="Microsoft.EntityFrameworkCore" Version="3.1.8" />
  <PackageReference Include="Npgsql" Version="4.1.3"/>
  <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="3.1.3"/>
  <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="3.1.4"/>
</ItemGroup>
</Project>

Exe
netcoreapp3.1

---

应用程序编译良好。我在Ubuntu 20.04上使用pgAdmin。

您需要实现IDesignTimeDbContextFactory接口，使您的解决方案能够正常工作

public class AppContext Factory : IDesignTimeDbContextFactory< AppContext>
{
    public AppContext CreateDbContext(string[] args)
    {
        var builder = new DbContextOptionsBuilder<AppContext>();

        builder.UseNpgsql(connectionString);

        return new DataContext(builder.Options);
    }
}

公共类AppContext工厂：IDesignTimeDbContextFactory
{
公共AppContext CreateDbContext（字符串[]args）
{
var builder=new DbContextOptionsBuilder（）；
builder.UseNpgsql（connectionString）；
返回新的DataContext（builder.Options）；
}
}

请检查我对以下问题的回答：解决方案也应该在您的上下文中工作，只需根据您的需要调整上下文名称、数据库和路径/文件名。我删除了一个构造函数：an可以正常工作，但我希望它能与DI一起工作。您能否显示实现IDesignTimeDbContextFactory的完整代码，因为它看起来很痛苦。我特别想知道如何注入connectionstring。我想这就是你要找的