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Matthew@Home wrote:

but without seeing your existing code, I can't suggest the best way

public class EntityA : IMyInterface
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Active { get; set; }
    public List<DetailA> Details { get; set; }
}

public class EntityB : IMyInterface
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Active { get; set; }
    public List<DetailB> Details { get; set; }
}

public class EntityC : IMyInterface
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Active { get; set; }
    public string SomeOtherProperty { get; set; }
    public string AnotherProperty { get; set; }
    public List<DetailC> Details { get; set; }
}

// Details all pretty much look the same as well, I've got more entities...

public class SharedModel
{
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Active { get; set; }
    public List<SharedDetailModel> Details { get; set; }
}

public TModel GetModels<TModel, TEntity>()
   where TModel : SharedModel, new()
   where TEntity : class, IMyInterface, new()
{
   context.Set<TEntity>()
      .Where(...)
      .Select(e => new TModel
         {
            Id = e.Id,
            Name = e.Name,
            Active = e.Active,
            Details = e.Details.Select(...)
         }).ToList();
}

Expression<Func<TEntity, TModel>> baseSelect = e => new TModel
{
    Id = e.Id,
    Name = e.Name,
    Active = e.Active,
    Details = e.Details.Select(...)
};

// And in inherited class, call GetModels<MyCustomModel, EntityC>().
// Use this as parameter.
Expression<Func<TEntity, TModel>> additionalSelect = e => new MyCustomModel
{
   SomeOtherProperty = e.SomeOtherProperty,
   AnotherProperty = e.AnotherProperty
};

MemberInitExpression baseSelectBody = (MemberInitExpression)baseSelect.Body;
MemberInitExpression additionalSelectBody = (MemberInitExpression)additionalSelect.Body;

ParameterExpression parameter = Expression.Parameter(typeof(TEntity), "e");
IEnumerable<MemberBinding> bindings = baseSelectBody.Bindings.Union(additionalSelectBody.Bindings);
MemberInitExpression init = Expression.MemberInit(Expression.New(typeof(TModel)), bindings);

// The following line does the trick and "stitches" all bindings together with the same parameter reference!
Expression newInit = new ExpressionParameterSwitcher(parameter).Visit(init);

exp = Expression.Lambda<Func<TEntity, TModel>>(newInit, parameter);