如何在介面中抽象EntityFrameworkCoreDBSet<T>？

我正在使用 EF Core 6，我的 DbContext 看起來像這樣：

public class SpaceBattlesDatabase: DbContext, ISpaceBattlesDataBase
{
     public DbSet<Building> Buildings => Set<Building>();
     //   other irrelevant code
}

我想用一個介面抽象它，在一個干凈的架構中的中心層使用..

public interface ISpaceBattlesDataBase
{
    public IQueryable<Building> Buildings { get; }
}

但是編譯器警告我：

錯誤 CS0738“SpaceBattlesDatabase”未實作介面成員“ISpaceBattlesDataBase.Buildings”。“SpaceBattlesDatabase.Buildings”無法實作“ISpaceBattlesDataBase.Buildings”，因為它沒有匹配的回傳型別“IQueryable”。

但根據檔案和元資料，DbSet<TEntity>應該實作IQueryable<TEntity>..

IEnumerable<T>也不起作用..我在這里錯過了什么嗎？

我怎樣才能抽象出DbSet在界面中使用？

謝謝

uj5u.com熱心網友回復：

我在這里錯過了什么嗎？

是的，正如 Eric Lippert在這里解釋的那樣， C# 不支持虛擬方法回傳型別協方差。在這種特殊情況下，您可以使用顯式介面實作來解決：

public class SpaceBattlesDatabase: DbContext, ISpaceBattlesDataBase
{
    public DbSet<Building> Buildings => Set<Building>();
    IQueryable<Building> ISpaceBattlesDataBase.Buildings => Buildings;
}

public interface ISpaceBattlesDataBase
{
    IQueryable<Building> Buildings { get; }
}

uj5u.com熱心網友回復：

在這些情況下，我使用不同的方法。我首先創建一個ContextFactory：

public class ContextFactory : IDesignTimeDbContextFactory<ApplicationDbContext>
{
    public ApplicationDbContext CreateDbContext(string[]? args = null)
    {
        return new ApplicationDbContext();
    }
}

然后我創建一個IDataService界面：

public interface IDataService<T>
{
    Task<IEnumerable<T>> GetAll();
    Task<T> Get(int id);
    Task<T> Create(T entity);
    Task<bool> CreateMultiple(List<T> entity);
    Task<bool> Delete(T entity);
    Task<T> Update(T entity);
}

并像這樣實作它：

public class DataService<T> : IDataService<T> where T : class
{
    private readonly ContextFactory contextFactory;

    public DataService(ContextFactory contextFactory)
    {
        this.contextFactory = contextFactory;
    }

    public async Task<T> Create(T entity)
    {
        using ApplicationDbContext context = contextFactory.CreateDbContext();
        EntityEntry<T> createdResult = await context.Set<T>().AddAsync(entity);
        await context.SaveChangesAsync();
        return createdResult.Entity;
    }

    public async Task<bool> CreateMultiple(List<T> entity)
    {
        using ApplicationDbContext context = contextFactory.CreateDbContext();
        await context.Set<T>().AddRangeAsync(entity);
        await context.SaveChangesAsync();
        return true;
    }

    public async Task<bool> Delete(T entity)
    {
        try
        {
            using ApplicationDbContext context = contextFactory.CreateDbContext();
            context.Set<T>().Remove(entity);
            await context.SaveChangesAsync();
            return true;
        }
        catch (Exception)
        {
            return false;
        }
    }

    public async Task<T> Get(int id)
    {
        using ApplicationDbContext context = contextFactory.CreateDbContext();
        return await context.Set<T>().FindAsync(id) ?? await new ValueTask<T>();
    }

    public async Task<IEnumerable<T>> GetAll()
    {
        using ApplicationDbContext context = contextFactory.CreateDbContext();
        return await context.Set<T>().ToListAsync();
    }

    public async Task<T> Update(T entity)
    {
        using ApplicationDbContext context = contextFactory.CreateDbContext();
        context.Set<T>().Update(entity);
        await context.SaveChangesAsync();
        return entity;
    }
}

您可以根據需要對其進行修改。最后，對于Building模型，我會做類似的事情：

public class BuildingCrudService
{
    private readonly IDataService<Building> dataService;

    public BuildingCrudService()
    {
        dataService = new DataService<Building>(new ContextFactory());
    }

    /// <summary>
    /// Adds a building to the database.
    /// </summary>
    /// <param name="building"></param>
    /// <returns>List of buildings.</returns>
    public async Task<List<Building>> AddBuildingAsync(Building building)
    {
        await dataService.Create(building);
        return await ListBuildingsAsync();
    }

    /// <summary>
    /// Deletes the building from the database.
    /// </summary>
    /// <param name="id"></param>
    /// <returns>True if deleted, else false.</returns>
    public async Task<bool> DeleteBuildingAsync(int id)
    {
        Building b = await GetBuildingByIDAsync(id);
        return await dataService.Delete(b);
    }

    /// <summary>
    /// Fetches the list of all the buildings from the database.
    /// </summary>
    /// <returns>List of buildings.</returns>
    public async Task<List<Building>> ListBuildingsAsync()
    {
        return (List<Building>)await dataService.GetAll();
    }

    /// <summary>
    /// Searches for a building that matches the id.
    /// </summary>
    /// <param name="id"></param>
    /// <returns>The building that matches the id.</returns>
    public async Task<Building> GetBuildingByIDAsync(int id)
    {
        return await dataService.Get(id);
    }

    /// <summary>
    /// Updates an existing building to the database.
    /// </summary>
    /// <param name="building"></param>
    /// <returns>The updated building.</returns>
    public async Task<Building> UpdateBuildingAsync(Building building)
    {
        Building b = await GetBuildingByIDAsync(building.BuildingId);
        b.Name = building.Name;
        // Also update other properties
        return await dataService.Update(b);
    }
}

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