Null reference exception was unhandled by user code error from ArcObjects and c#?

Question

I'm new in programming . i'd like to write a simple code that can create offset from a polyline . i wrote the below code but when run the code i encounter to the error " Null reference exception was unhandled by user code" in line construct.ConstructOffset(pfeaturelayer as IPolyline, 100.3 , missing,missing); how can i solve the problem ?

public partial class copyparallel : Form
    {
        IApplication papp;
        public IHookHelper phookhelper;
         IPolycurve ppolycurve;

        public copyparallel()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {

            IFeatureLayer pfeaturelayer;
            IFeatureClass pfc;

            pfeaturelayer = (IFeatureLayer) phookhelper.FocusMap.get_Layer(0);
           pfc = pfeaturelayer.FeatureClass;

           IConstructCurve construct = new PolylineClass();
           object offset = esriConstructOffsetEnum.esriConstructOffsetMitered;

           object missing = Type.Missing;
           construct.ConstructOffset(pfeaturelayer as IPolyline, 100.3 , missing,missing);

           IFeatureClass poutfc;
           IFeature newf = pfc.CreateFeature();
           newf.Shape = (IGeometry)construct;
           newf.Store();

            }


        }
    }

Answer 1

You'll have to loop through each feature in the feature class to get a polyline instead of trying to cast the entire featurelayer to a polyline.

*To answer your comment, it would look something like this. You'll have to translate into C#

Dim pFCur As IFeatureCursor
Dim pFeat As IFeature

pFCur = pfc.Search(Nothing, False)
pFeat = pFCur.NextFeature
Do Until pFeat Is Nothing
    Dim pPolyline As IPolyline
    pPolyline = pFeat.ShapeCopy

    'your code to construct the offset

    pFeat = pFCur.NextFeature
Loop