DataKeyNames In Databound Controls Must Be A Property?

I use a ListView along with an ObjectDataSource tied to a business object. This object is defined as follows:

public class Employee
{
public int Id;
public string Name;
}

When I try setting the DataKeyNames property of the ListView to Id, ASP.net blows up and says: DataBinding: 'Employee' does not contain a property with the name 'Id'. However, when I change the object to that:

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

It works!!!I couldn't find any documentation about this behavior. Why is it not accepting a simple variable for DataKeyNames and instead it insists on a property?