How to write an extension method in DbContext class?

问题

I want to create a helper method to check an item exists or not in datatabase.

I've tried this:

public static class DbContextExtensions
{
    public static bool Exist<TModel>(this DbSet<TModel> model, string id) where TModel : class
    {
        return !string.IsNullOrEmpty(id) && model.SingleOrDefault(x => x.Id == id) != null;
    }
}

I'm getting this error message:

'TModel' does not contain a definition for 'Id' and no extension method 'Id' accepting a first argument of type 'TModel' could be found (are you missing a using directive or an assembly reference?)

What I want to achieve:

public class PostManager
{
    private readonly _dbContext;

    public PostManager(MyDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    public async Task<IdentityResult> UpdateAsync(EditPostViewModel model)
    {
        if (_dbContext.Users.Exist(model?.UserId) && _dbContext.Posts.Exist(model?.Id))
        {
            // the user with "UserId" and the post with "Id" are existing...
        }
    }
}

I don't want to create a helper method inside the PostManager class, like this:

public class PostManager
{
    // contructor...

    // actions...

    private bool UserExist(string id)
    {
        return !string.IsNullOrEmpty(id) && _dbContext.Users.SingleOrDefault(x => x.Id == id) != null;
    }

    private bool PostExist(string id)
    {
        return !string.IsNullOrEmpty(id) && _dbContext.Posts.SingleOrDefault(x => x.Id == id) != null;
    }
}

Because the method UserExist and PostExist may be used in many another classes. So, I don't want to re-declare them in every class before using.

How can I do that? Thank you so much!

回答1:

Your have TModel : class - since TModel is of type class which does not have any property named Id, hence the error. So you need to point it to a base entity class (preferably an interface - composition is always better than inheritance) but this is to give you a general idea - such as where TModel:Entity.

e.g.

 class BaseEntity {
   string Id {get; set;}
 }

 class Person : BaseEntity {
   string Name {get; set;}
 } 

 public static class DbContextExtensions
 {
    public static bool Exist<TModel>(this DbSet<TModel> model, string id) where TModel : BaseEntity
   {
      return !string.IsNullOrEmpty(id) && model.SingleOrDefault(x => x.Id == id) != null;
   }
 }

来源：https://stackoverflow.com/questions/48860276/how-to-write-an-extension-method-in-dbcontext-class

标签

entity-framework