Combine multiple meshes into single mesh

MeshCombinerHelper.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

#if UNITY_EDITOR
using UnityEditor;

[InitializeOnLoad()]
class MeshCombinerHelperTag
{
    static MeshCombinerHelperTag()
    {
        icon = EditorGUIUtility.ObjectContent(null, typeof(Mesh)).image;

        EditorApplication.hierarchyWindowItemOnGUI -= OnHierarchyItem;
        EditorApplication.hierarchyWindowItemOnGUI += OnHierarchyItem;
        EditorApplication.RepaintHierarchyWindow();
    }

    static Texture icon;

    static void OnHierarchyItem(int instanceID, Rect instanceRect)
    {
        var obj = EditorUtility.InstanceIDToObject(instanceID);
        var gameObject = obj as GameObject;
        if (!gameObject)
            return;

        var combiner = gameObject.GetComponent<MeshCombinerHelper>();
        if (!combiner)
            return;

        var width = 16;
        var rightAlignedRect = new Rect(
            instanceRect.xMax - width,
            instanceRect.yMin,
            width,
            instanceRect.height + 1
        );
        GUI.Label(rightAlignedRect, icon);
    }
}

#endif

public class MeshCombinerHelper
    : MonoBehaviour
{
    public void Start()
    {
        var meshFilters = GetComponentsInChildren<MeshFilter>();
        var perMaterialCombines = new Dictionary<Material, List<CombineInstance>>();
        var index = 0;

        while (index < meshFilters.Length)
        {
            var meshRenderer = meshFilters[index].gameObject.GetComponent<MeshRenderer>();
            var meshMaterial = meshRenderer.sharedMaterial;

            if (!perMaterialCombines.ContainsKey(meshMaterial))
                perMaterialCombines.Add(meshMaterial, new List<CombineInstance>());

            var combines = perMaterialCombines[meshMaterial];
            var combine = new CombineInstance();

            combine.mesh = meshFilters[index].sharedMesh;
            combine.transform = Matrix4x4.Inverse(transform.localToWorldMatrix) * meshFilters[index].transform.localToWorldMatrix;

            combines.Add(combine);

            meshFilters[index].gameObject.SetActive(false);
            index++;
        }

        var count = 0;
        foreach (var entry in perMaterialCombines)
        {
            var obj = gameObject;
            if (count > 0)
            {
                obj = new GameObject(string.Format("MeshCombine{0}", count));
                obj.transform.SetParent(transform);
                obj.transform.ResetLocalTransforms();
            }

            var meshFilter = obj.AddComponent<MeshFilter>();
            var meshRenderer = obj.AddComponent<MeshRenderer>();

            meshRenderer.sharedMaterial = entry.Key;
            meshFilter.mesh = new Mesh();
            meshFilter.mesh.CombineMeshes(entry.Value.ToArray());

            obj.SetActive(true);

            count++;
        }
    }
}