graphics - Missing using directive or assembly reference in WPF in C# -

i trying draw triangle in wpf in c# learn how use tool. if check last statement, can't run program because of it. think missing using directive or maybe assembly reference ...

these using directives employing:

using system; using system.collections.generic; using system.linq; using system.text; using system.windows; using system.windows.controls; using system.windows.data; using system.windows.input; using system.windows.media; using system.windows.media.imaging; using system.windows.navigation; using system.windows.shapes; using system.windows.media.media3d; 

here code:

system.windows.media.media3d.point3d point0 = new point3d(-0.5, 0, 0); system.windows.media.media3d.point3d point1 = new point3d(0.5, 0.5, 0.3); system.windows.media.media3d.point3d point2 = new point3d(0, 0.5, 0);   system.windows.media.media3d.meshgeometry3d trianglemesh = new meshgeometry3d();  trianglemesh.positions.add(point0); trianglemesh.positions.add(point1); trianglemesh.positions.add(point2);  int n0 = 0; int n1 = 1; int n2 = 2;  trianglemesh.triangleindices.add(n0); trianglemesh.triangleindices.add(n1); trianglemesh.triangleindices.add(n2);  system.windows.media.media3d.vector3d norm = new vector3d(0, 0, 1); trianglemesh.normals.add(norm); trianglemesh.normals.add(norm); trianglemesh.normals.add(norm);  system.windows.media.media3d.material frontmaterial = new diffusematerial(new solidcolorbrush(colors.blue));  system.windows.media.media3d.geometrymodel3d trianglemodel = new geometrymodel3d(trianglemesh, frontmaterial);  trianglemodel.transform = new transform3dgroup();  system.windows.media.media3d.modelvisual3d visualmodel = new modelvisual3d(); visualmodel.content = trianglemodel;  this.mainviewport.children.add(visualmodel); // here have error ! 

in xaml file overwrite grid this, , black triangle appear. tested it.

<grid>         <viewport3d name="mainviewport" cliptobounds="true">             <viewport3d.camera>                 <perspectivecamera    farplanedistance="100"   lookdirection="-11,-10,-9"   updirection="0,1,0"   nearplanedistance="1"    position="11,10,9"    fieldofview="70" />             </viewport3d.camera>             <modelvisual3d>                 <modelvisual3d.content>                     <directionallight      color="white"      direction="-2,-3,-1" />                 </modelvisual3d.content>             </modelvisual3d>         </viewport3d> </grid>