Linq Union do not work - linq

Question

I am trying to use Linq Union to add additional record into result but Union do not work. Maybe someone could point me in right direction.
public class ProductView
{
public int Id { get; set; }
public bool Active { get; set; }
public string Name { get; set; }
public int ProductTypeId { get; set; }
public int UserCount { get; set; }
}
void Main()
{
var product = Products.Select(p => new ProductView
{
Id = p.Id,
Active = p.Active,
Name = p.Name,
ProductTypeId = p.ProductTypeId,
UserCount = 1
}).ToList();
//The new item is not jointed to the result above
product.Union(new[] {
new ProductView
{
Id = 9999,
Active = true,
Name = "Test",
ProductTypeId=0,
}
});
product.Dump();
}

Answer 1

You need to store the output:
var product2 = product.Union(new[] {
new ProductView
{
Id = 9999,
Active = true,
Name = "Test",
ProductTypeId=0,
}
});
product2.Dump();
In addition to this, overriding the Equals behaviour would be useful - as you probably want to check equality using just the Id field?
For example, if you don't override the Equals behaviour, then you will get Object reference equals like this:
void Main()
{
var list = new List<Foo>()
{
new Foo() { Id = 1},
new Foo() { Id = 2},
new Foo() { Id = 3},
};
var list2 = new List<Foo>()
{
new Foo() { Id = 1},
new Foo() { Id = 2},
new Foo() { Id = 3},
};
var query = list.Union(list2);
query.Dump();
}
// Define other methods and classes here
public class Foo
{
public int Id {get;set;}
}
produces six items!
But if you change Foo to:
public class Foo
{
public int Id {get;set;}
public override bool Equals(object obj)
{
if (obj == null || !(obj is Foo)) return false;
var foo= (Foo)obj;
return this.Id == foo.Id;
}
public override int GetHashCode()
{
return this.Id.GetHashCode();
}
}
then you will get 3 items - which is probably what you are expecting.

Answer 2

You need to override Equals and GetHashCode in ProductView if you want to use Union in a meaningful way(other than comparing by refence).
public class ProductView
{
public int Id { get; set; }
public bool Active { get; set; }
public string Name { get; set; }
public int ProductTypeId { get; set; }
public int UserCount { get; set; }
public override bool Equals(object obj)
{
if (obj == null || !(obj is ProductView)) return false;
ProductView pv2 = (ProductView)obj;
return this.Id == pv2.Id;
}
public override int GetHashCode()
{
return this.Id.GetHashCode();
}
}
You could also implement an IEqualityComparer<ProductView> in a similar way and use it for this overload of Union.