Select a Part of the Line Segment

I have a line feature, i have to select a part of the line feature.

For example there is a line feature of 100meters. I want to select only 50meters of that line or any length less than 100meters.

I tried the following methods,
I created geometry of the feature, cut the line to the required length and depicted it as a ielement on the map.
This gives unusual results like sometimes the line appears perfectly and sometimes the line is drawn randomly. Hence i want to know whether there is any way to select a part of the line segment only using ifeatureselection or alternative methods.

I am developing in C#.net

thank you