using System;

using System.Threading.Tasks;

using System.Collections;

using System.Collections.Generic;

using Microsoft.EntityFrameworkCore;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

    public static async Task UpsertRangeSafe<T>(this DbSet<T> dbSet, IEnumerable<T> items, Func<T,object> findKey, Action<T> onUpdateDbItem) where T : class

    {

        foreach (var item in items)

        {

            var key = findKey(item);

            var dbItem = await dbSet.FindAsync(key).ConfigureAwait(false);

            onUpdateDbItem(dbItem);

        }

        await new UpsertBuilder<T>(dbSet, items)

                    .On(findKey)

                    .OnUpdate(onUpdate)

                    .RunAsync()

                    .ConfigureAwait(false);

    }

}

public interface Builder<T> where T : class

{

}

​

public class UpsertBuilder<T> where T : class

{

    private Func<T,object> _findKey;

    private Func<T> _onUpdate;

    private readonly IEnumerable<T> _items;

    private readonly DbSet<T> _dbSet;

    public UpsertBuilder(DbSet<T> dbSet, IEnumerable<T> items)

    {

        _items = items;

        _dbSet = dbSet;

    }   

    public UpsertBuilder<T> On(Func<T,object> findKey)

    {

        _findKey = findKey;

        return this;

    }   

​

    public UpsertBuilder<T> OnUpdate(Func<T> onUpdate)

    {

        _onUpdate = onUpdate;

        return this;

    }   

    public async Task RunAsync()

    {

        Assert();

        foreach (var item in _items)

        {

            var key = _findKey(item);

            var dbItem = await _dbSet.FindAsync(key).ConfigureAwait(false);

            _onUpdate(dbitem);

        }

    }   

    private void Assert()

    {

        if (_items == null)

        {

            throw new ArgumentNullException(nameof(_items));

        }

        if (_onUpdate == null)

        {

            throw new ArgumentNullException(nameof(_onUpdate));

        }

        if (_findKey == null)

        {

            throw new ArgumentNullException(nameof(_findKey));

        }

    }

}