Read Only attribute for Unity's Inspector

ReadOnlyAttribute.cs

// ReadOnlyAttribute.cs
// Author: Made Indrayana
// MIT License
// Variables with this attribute will be shown in Inspector but will be grayed out and not editable.

using UnityEngine;
public class ReadOnlyAttribute : PropertyAttribute { }

ReadOnlyAttributeDrawer.cs

// ReadOnlyAttributeDrawer.cs
// Author: Made Indrayana
// MIT License
// Variables with this attribute will be shown in Inspector but will be grayed out and not editable.

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(ReadOnlyAttribute))]
public class ReadOnlyAttributeDrawer : PropertyDrawer
{
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property, label, true);
    }

    // Draw a disabled property field
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        GUI.enabled = false;
        EditorGUI.PropertyField(position, property, label, true);
        GUI.enabled = true;
    }
}