Question

I keep having null pointer exception when i add to header.headerItems.Add(d2); in which my headerItems is an observable collection of type Data

Anyone can tell me why is this ?

 private void addData(Header header)
 {
        Data d2 = new Data("pol");
        header.headerItems.Add(d2);
 }

my Method which call this method is just as below. In which header is part of a Packet class.

addData(packet.header);

My header class

public class Header
    {
    private ObservableCollection<Data> headerCollection;
    private String sheadervalue;
    public Header(String headervalue)
    {
        sheadervalue = headervalue;
    }

    public ObservableCollection<Data> headerItems
    { get { return headerCollection; } }

    public string headervalue
    {
        get { return sheadervalue; }
        set { sheadervalue = headervalue; }
    }
}

My data class

  public class Data
{
    private String sDatavalue;

    public Data(String datavalue)
    {
        this.sDatavalue = datavalue;

    }

    public string datavalue
    {
        get { return sDatavalue; }
        set { sDatavalue = datavalue; }
    }

}

Answer 1

problem is you accessing field of null object (header). do as below

private void addData(Header header)
 {
    if(null == header)
    {
       // if header null what you can do... e.g log header is null 
       return; 
    }
    if(null == header.headerItems)
    {
        header.headerItems = ....; // create new header item collection 
    }

    Data d2 = new Data("pol");
    header.headerItems.Add(d2);
 }