EF Core Generic Repository get Id from new inserted generic entity async

Question

Goal: get id of the inserted generic entity by using async with ASP.NET Core 3

Problem: what part am I missing in order to achieve the goal?

Code:

    public async Task<int> AddAsync(T entity)  
    {  
        if (entity == null)  
        {  
            throw new ArgumentNullException($"{nameof(AddAsync)} entity must not be null");  
        }  
      
        try  
        {  
            await _context.AddAsync(entity);  
            await _context.SaveChangesAsync();  
      
            return <Id Here>     
        }  
        catch (Exception)  
        {  
            throw new Exception($"{nameof(entity)} could not be saved");  
        }  
    }  

Thank you!

------------------------

It doesn't work to use id.

Answer 1

Hi JJTT-0327,
First, you can try to use reflection to obtain the Id property.

var IdProperty = entity.GetType().GetProperty("Id").GetValue(entity,null);  
return (int)IdProperty;  

In general, the table in database has auto-generated integer identity Id as primary key.
If your Id is primary key, kuncevic.dev has provided a better solution in this thread you can refer to.
Best Regards,
Daniel Zhang

---

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Answer 2

You could use an Interface e.g. quick-n-dirty. Customer and Manager will work with AddAsync as per the constraint while SalesRepresentative will not as it fails to meet the where constraint.

public interface TEntity
{
    int Identifier { get;} 
}

public class Customer : TEntity
{
    public int Id { get; set; }
    public int Identifier => Id;
}
public class Manager : TEntity
{
    public int Id { get; set; }
    public int Identifier => Id;
}
public class SalesRepresentative 
{
    public int Id { get; set; }
    public int Identifier => Id;
}

public class GenericRepository<T> where T : TEntity
{
    public async Task<int> AddAsync(T entity)
    {
        return ((TEntity) entity).Identifier;
    }
}

Answer 3

While Karen's answer is correct, I do want to point out that Entity Framework will automatically hydrate your models with identity fields when you insert. So for example if I have :

class Customer 
{
   public int Id { get; set; }
   public string Name { get; set; }
}

class CustomerService 
{
    int MyMethod()
    {
        var customer = new Customer { Name = "Abc" };

        _customerRepository.Add(customer);

        return customer.Id; //Notice how we never touched this field, but after our insert, EF will populate it for us. 
    }
}

Notice how we don't add or modify the Id field, but after our insert, we can return it fine. While this may not work perfectly in your scenario, I just wanted to show that your add method may not need to return the identity at all, as if you have a reference to the original object, the ID will be set within it.