/// <summary>
/// removes the element at the beginning of the collection
/// </summary>
/// <returns></returns>
public int RemoveFirst()
{
if (this.first == null)
{
throw new Exception();
}
else if (this.first == this.last)
{
var element = this.first.Value;
this.first = null;
this.last = null;
return(element);
}
else
{
var element = this.first.Value;
this.first = this.first.Next;
this.first.Previous = null;
return(element);
}
}