Apply a material to all models in a folder.

ModelMaterialSetterEditor.cs

using UnityEngine;
using UnityEditor;

namespace WizardsCode.Models {
    /// <summary>
    /// A custom Unity Editor window that allows users to find and process 3D models in a specified folder,
    /// applying a selected material to all renderers within the models.
    /// 
    /// This is useful when an asset is published in which the models do not have materials applied. For things
    /// like terrain details this is problematic and since there are usually many models for details the manual
    /// process of applying the material can be tedious. This editor window automates that process in a single click.
    /// 
    /// However, it is currently limited to models that are in the same folder as the material. That is, it will only
    /// truly be useful when the models are using a material atlas.
    /// </summary>
    public class ModelMaterialSetterEditor : EditorWindow
    {
        /// <summary>
        /// The folder path where the models are located. This is selected by the user through the editor interface.
        /// </summary>
        private string folderPath;

        /// <summary>
        /// The material to be applied to the models' renderers. This is selected by the user through the editor interface.
        /// </summary>
        private Material selectedMaterial;

        /// <summary>
        /// Displays the "Model Material Finder" window in the Unity Editor.
        /// </summary>
        [MenuItem("Wizards Code/Model Material Finder")]
        public static void ShowWindow()
        {
            GetWindow<ModelMaterialSetterEditor>("Model Material Finder");
        }

        /// <summary>
        /// Draws the GUI for the editor window, allowing the user to select a folder, choose a material,
        /// and process the models in the folder.
        /// </summary>
        private void OnGUI()
        {
            EditorGUILayout.LabelField("Select Folder and Material", EditorStyles.boldLabel);

            // Folder path field
            EditorGUILayout.BeginHorizontal();
            EditorGUILayout.LabelField("Folder Path", GUILayout.Width(75));
            folderPath = AssetDatabase.GUIDToAssetPath(
                AssetDatabase.AssetPathToGUID(AssetDatabase.GetAssetPath(
                    EditorGUILayout.ObjectField(
                        AssetDatabase.LoadAssetAtPath<Object>(folderPath), typeof(DefaultAsset), false) as DefaultAsset))
            );
            EditorGUILayout.EndHorizontal();

            if (string.IsNullOrEmpty(folderPath))
            {
                EditorGUILayout.HelpBox("Please select a folder containing the models.", MessageType.Warning);
                return;
            }

            // Material field
            selectedMaterial = (Material)EditorGUILayout.ObjectField("Material", selectedMaterial, typeof(Material), false);
            if (selectedMaterial == null)
            {
                EditorGUILayout.HelpBox("Please select a material to apply to the models.", MessageType.Warning);
                return;
            }
            EditorGUILayout.Space();

            if (GUILayout.Button("Process Models"))
            {
                if (string.IsNullOrEmpty(folderPath) || selectedMaterial == null)
                {
                    EditorUtility.DisplayDialog("Error", "Please select a folder and a material before processing.", "OK");
                }
                else
                {
                    ProcessModels();
                }
            }
        }

        void ProcessModels()
        {     
            int count = 0;
            string[] guids = AssetDatabase.FindAssets("t:Model", new[] { folderPath });
            foreach (string guid in guids)
            {
                string assetPath = AssetDatabase.GUIDToAssetPath(guid);
                EditorUtility.DisplayProgressBar("Processing Models", $"Processing {assetPath}", (float)count / guids.Length);
                
                GameObject model = AssetDatabase.LoadAssetAtPath<GameObject>(assetPath);
                if (model != null)
                {
                    Renderer[] renderers = model.GetComponentsInChildren<Renderer>();
                    foreach (var renderer in renderers)
                    {
                        renderer.sharedMaterial = selectedMaterial;
                    }
                    EditorUtility.SetDirty(model);
                    count++;
                }
            }
            EditorUtility.ClearProgressBar();
            if (count > 0) {
                EditorUtility.DisplayDialog("Processing Complete", $"{count} of {guids.Length} models were processed and updated with the selected material.", "OK");
                AssetDatabase.SaveAssets();
                AssetDatabase.Refresh();
            }
            else {
                EditorUtility.DisplayDialog("No Models Found", "No models were found in the selected folder.", "OK");
            }
        }
    }
}