Assign sharedMaterials from assets with editor script.

Apologize for my poor English, forgive me please. I'm working on a hex tile map editor, and I need to assign materials to tiles. I found out I cannot assign the materials one by one, like what other people do on their post. The assigned material never shows up.(not even in the inspector) But if I assign an array, the problem solved. I suspect it has something to do with assign by value, but nor sure about it. I'd like to hear some explanation. thanks!

 //              this region doesn't work:
 //                Material temp;
 //                temp = AssetDatabase.LoadAssetAtPath("Assets/Materials/"+tData.TerrainDBIndex+"top.mat",typeof(Material))as Material;
 //                renderer.sharedMaterials[0] = temp;
 //                temp = AssetDatabase.LoadAssetAtPath("Assets/Materials/"+tData.TerrainDBIndex+"side.mat",typeof(Material))as Material;
 //                renderer.sharedMaterials[1] = temp;
 
 //                this region work: 
                 Material[] temp;
                 temp = new Material[2];
                 temp[0]    = AssetDatabase.LoadAssetAtPath("Assets/Materials/"+tData.TerrainDBIndex+"top.mat",typeof(Material))as Material;
                 temp[1] =AssetDatabase.LoadAssetAtPath("Assets/Materials/"+tData.TerrainDBIndex+"side.mat",typeof(Material))as Material;
                 renderer.sharedMaterials = temp;