如何在 iqueryable 中强制转换派生 BaseModel 具有 isdelete 标志，在实体框架核心中制作软删除存储库

Question

why this does not work and how I can fix it, I want to make soft delete/hard delete using one repository

public async Task<IEnumerable<TEntity>> GetAsync(Expression<Func<TEntity, bool>> predicate)
{
    var query = Context.Set<TEntity>().Where(predicate);

    if (query is IQueryable<BaseModel>)
    {
        var baseQuery = query as IQueryable<BaseModel>;
        query = baseQuery.Where(x => !x.isDelete) as IQueryable<TEntity>;
    }

    return await query.ToListAsync();
}

this base model class

public class BaseModel
{
    public bool isDelete { get; set; }
}

this the model

public class Jobs : BaseModel
{
    public int Id { get; set; }
    public string Description { get; set; }
    public string ProjectName { get; set; }
    public string Project_Status { get; set; }
}

i have this exception

{"Expression of type 'System.Linq.IQueryable`1[Models.BaseModel]' cannot be used for parameter of type 'System.Linq.IQueryable`1[Models.Jobs]' of method 'System.Linq.IQueryable`1[Models.Jobs] Where[Jobs](System.Linq.IQueryable`1[Models.Jobs], System.Linq.Expressions.Expression`1[System.Func`2[Models.Jobs,System.Boolean]])' (Parameter 'arg0')"}

Answer 1

You cannot cast the IQueryable<BaseModel> to IQueryable<TEntity> because the IQueryable<out T> is covariant . So you can only cast from IQueryable of more derived types (eg: TEntity ) to IQueryable of less derived types (eg: BaseModel ).

So here is the solution, you can use .Cast<T> to cast the element type instead of the whole IQueryable<TEntity> :

public async Task<IEnumerable<TEntity>> GetAsync(Expression<Func<TEntity, bool>> predicate)
{
    var query = Context.Set<TEntity>().Where(predicate);

    if (typeof(BaseModel).IsAssignableFrom(typeof(TEntity)))
    {
        query = query.Cast<BaseModel>().Where(e => !e.isDelete).Cast<TEntity>();
    }        

    return await query.ToListAsync();
}