ArcEngine+c#,求角平分线?

<P>我想由三点坐标来做角平分线,用IConstructLine 的ConstructAngleBisector是否合适?</P>
<P>代码如下,但输出时有错误:未将对象引用设置到对象的实例。错在那里呢,请指教.</P>
<P>private void button1_Click(object sender, System.EventArgs e)<BR>  {this.mx1 = Convert.ToDouble(this.textBox1.Text.Trim() );<BR>  this.my1 = Convert.ToDouble(this.textBox2.Text.Trim() );<BR>   this.mx2 = Convert.ToDouble( this.textBox3.Text.Trim() );;<BR>   this.my2 = Convert.ToDouble(this.textBox4.Text.Trim() );<BR>   this.mx3 = Convert.ToDouble(this.textBox5.Text.Trim() );<BR>   this.my3 = Convert.ToDouble(this.textBox6.Text.Trim());<BR>   IPoint pt1=new PointClass();<BR>   IPoint pt2=new PointClass();<BR>   IPoint pt3=new PointClass();<BR>   pt1.X=mx1;<BR>   pt1.Y=my1 ;<BR>   pt2.X=mx2 ;<BR>   pt2.Y=my2 ;<BR>   pt3.X=mx3 ;<BR>   pt3.Y=my3 ;<BR>   IConstructLine pline =new LineClass() ;<BR>   //ILine pl= new LineClass() ;<BR>            pline.ConstructAngleBisector (pt2,pt1,pt3,20,false) ;<BR>             <BR>   <BR>  this._pMapCtrl.DrawShape( pline as IGeometry, ref this.oPtSym );<BR>  }</P>

<P>错误信息是:</P>
<P>未处理的“System.Runtime.InteropServices.COMException”类型的异常出现在 mscorlib.dll 中。</P>
<P>其他信息: The supplied geometry does not have a geometry type that is supported for drawing<BR></P>

<P>问题似乎是出在 drawshap 这个方法上，我最近也遇到了这样的问题</P>

这里最好对IConstructLine进行检测