Linq Expressions: Create a max query using a generic DbSet

Question

In order to simplify creating fake data for unit testing, I want to have a function that can create a generic Entity (which I use as an underlying object for a lot of other classes). The entity has an index, and, for generating a new index, I want to simply find the currently highest number and add one.

I am using Index for a lot of other classes, and would like to make this function as generic as possible. My problem is that I don't know how to specify what DbSet to use in my generic function GetMaxID.

This is what I got so far:

private Entity CreateGenericEntity()
{
    return new Entity()
    {
        Guid = Guid.NewGuid(),
        Index = GetMaxID<Entity>(x => x.Index) + 1,
    };
}

private int GetMaxID<TEntity>(Expression<Func<TEntity, int>> expression)
{
    return _repository.Set<TEntity>().Max(expression);
}

_repository has a bunch of different IDbSets properties, such as

public IDbSet<Customers> Customers{ get; set; }

public IDbSet<Orders> Orders{ get; set; }

etc.

Answer 1

I found that I was missing the declaration of TEntity, which was fixed by appending where TEntity : class to my function. I also had to change the expression to accept int? in order to handle the case where the query returns a null value. The complete function looks like this (if anyone is interested)

private int GetMaxID<TEntity>(Expression<Func<TEntity, int?>> expression) where TEntity : class
{
    return _repository.Set<TEntity>().Max(expression) ?? 0;
}