Reset a SerializedProperty to it's default value.

At least for scriptable objects you can achieve it like this:

 public static class resetTest
 {
     private static Dictionary<System.Type, SerializedObject> _objectCache = new Dictionary<Type, SerializedObject>();
 
     public static void ResetToDefault(this SerializedProperty prop)
     {
         System.Type type = prop.serializedObject.targetObject.GetType();
         if (!type.IsSubclassOf(typeof(ScriptableObject)))
             return;
 
         SerializedObject serializedCopy = null;
         if (_objectCache.ContainsKey(type))
             serializedCopy = _objectCache[type];
         else
         {
             ScriptableObject serialized2 = ScriptableObject.CreateInstance(type);
             serializedCopy = new SerializedObject(serialized2);
             ScriptableObject.DestroyImmediate(serialized2);
             _objectCache.Add(type, serializedCopy);
         }
 
         prop.serializedObject.CopyFromSerializedPropertyIfDifferent(serializedCopy.FindProperty(prop.propertyPath));
     }
 }

Normally I wouldn't necro a years-old thread but given how popular this question apparently is (judging by the sheer amount of subscribers it has) I decided to share this regardless.

Just ran into this issue with a Readonly attribute I was writing: when I changed the initial values of some parameters in C#, the compiler did its job but on the Unity side, the value didn't change because it was Serialized. It was then that I came across this thread, hoping to find a simple method to reset a SerializedProperty to its default value. This doesn't seem to be the case, and so I devised a method myself. It's a little dirty but it should work.

My approach was to store the default value in the attribute itself using a cast to System.Object, and then use the SerializedProperty.propertyType property to ensure type safety when casting. I've provided both relevant classes below for whoever is interested or for whoever may find it useful - I commit these two classes to the public domain. So, if anyone needs a Readonly attribute, that displays a Serialized field using EditorGUI.HelpBox, then here's a freebie. :)

Firstly, the ReadonlyAttribute class.

 using System;
 using UnityEngine;
 
 [AttributeUsage(AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
 public sealed class ReadonlyAttribute : PropertyAttribute
 {
     // Unity's own MessageType enum is part of UnityEditor, so using that will result in compiler errors when building.
     public enum MessageType
     {
         None = 0,
         Info = 1,
         Warning = 2,
         Error = 3
     }
     public readonly MessageType type;
     public readonly object defaultValue;
 
     // We do not reset if the game is being played. Runtime changes get priority.
     public bool Reset => reset && !Application.isPlaying;
     private readonly bool reset;

     // Use this constructor to enable the reset feature.
     public ReadonlyAttribute(object defaultValue, MessageType type)
     {
         this.type = type;
         this.defaultValue = defaultValue;
         reset = true;
     }
 
     // No constructor with only 'object defaultValue' to avoid ambiguïty between that and the MessageType ctor.
     // After all, MessageType can also be cast to System.Object... so should an instance of MessageType invoke the
     // MessageType ctor or the System.Object ctor?
 
     public ReadonlyAttribute(MessageType type)
     {
         this.type = type;
         defaultValue = null;
         reset = false;
     }
 
     public ReadonlyAttribute() : this(MessageType.Info) { }
 }

Then, the ReadonlyAttribute Drawer. Apologies for the length.

 using UnityEditor;
 using UnityEngine;
 
 [CustomPropertyDrawer(typeof(ReadonlyAttribute))]
 public sealed class ReadonlyAttributeDrawer : PropertyDrawer
 {
     public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
     {
         ReadonlyAttribute _attribute = (ReadonlyAttribute)attribute;
         try
         {
             string text = label.text + ": ";
 
             switch (property.propertyType)
             {
                 case SerializedPropertyType.ArraySize:
                     if (_attribute.Reset)
                     {
                         var value = property.arraySize;
                         text += value;
                         var defaultValue = (int)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.arraySize = defaultValue;
                     }
                     else
                         text += property.boundsIntValue;
 
                     break;
 
                 case SerializedPropertyType.Boolean:
                     if (_attribute.Reset)
                     {
                         var value = property.boolValue;
                         text += value;
                         var defaultValue = (bool)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.boolValue = defaultValue;
                     }
                     else
                         text += property.boolValue;
 
                     break;
 
                 case SerializedPropertyType.Bounds:
                     if (_attribute.Reset)
                     {
                         var value = property.boundsValue;
                         text += value;
                         var defaultValue = (Bounds)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.boundsValue = defaultValue;
                     }
                     else
                         text += property.boundsValue;
 
                     break;
 
                 case SerializedPropertyType.BoundsInt:
                     if (_attribute.Reset)
                     {
                         var value = property.boundsIntValue;
                         text += value;
                         var defaultValue = (BoundsInt)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.boundsIntValue = defaultValue;
                     }
                     else
                         text += property.boundsIntValue;
 
                     break;
 
                 case SerializedPropertyType.Character:
                     if (_attribute.Reset)
                     {
                         var value = (char)property.intValue;
                         text += value;
                         var defaultValue = (char)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.intValue = defaultValue;
                     }
                     else
                         text += (char)property.intValue;
 
                     break;
 
                 case SerializedPropertyType.Color:
                     if (_attribute.Reset)
                     {
                         var value = property.colorValue;
                         text += ColorUtility.ToHtmlStringRGBA(value);
                         var defaultValue = (Color)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.colorValue = defaultValue;
                     }
                     else
                         text += ColorUtility.ToHtmlStringRGBA(property.colorValue);
 
                     break;
 
                 case SerializedPropertyType.Enum:
                     if (_attribute.Reset)
                     {
                         var value = property.enumValueIndex;
                         text += property.enumDisplayNames[value];
                         var defaultValue = (int)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.enumValueIndex = defaultValue;
                     }
                     else
                         text += property.enumDisplayNames[property.enumValueIndex];
 
                     break;
 
                 case SerializedPropertyType.ExposedReference:
                     if (_attribute.Reset)
                     {
                         var value = property.exposedReferenceValue;
                         text += value ? value.name : "Null";
                         var defaultValue = (Object)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.exposedReferenceValue = defaultValue;
                     }
                     else
                         text += property.exposedReferenceValue ? property.exposedReferenceValue.name : "Null";
 
                     break;
 
                 case SerializedPropertyType.FixedBufferSize:
                     // Cannot be written to, so reset is moot.
                     text += property.fixedBufferSize;
                     break;
 
                 case SerializedPropertyType.Float:
                     if (_attribute.Reset)
                     {
                         var value = property.floatValue;
                         text += value;
                         var defaultValue = (float)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.floatValue = defaultValue;
                     }
                     else
                         text += property.floatValue;
 
                     break;
 
                 case SerializedPropertyType.Integer:
                     if (_attribute.Reset)
                     {
                         var value = property.intValue;
                         text += value;
                         var defaultValue = (int)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.intValue = defaultValue;
                     }
                     else
                         text += property.intValue;
 
                     break;
 
                 case SerializedPropertyType.LayerMask:
                     if (_attribute.Reset)
                     {
                         var value = property.intValue;
                         text += LayerMask.LayerToName(value);
                         var defaultValue = (int)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.intValue = defaultValue;
                     }
                     else
                         text += LayerMask.LayerToName(property.intValue);
 
                     break;
 
                 case SerializedPropertyType.ObjectReference:
                     if (_attribute.Reset)
                     {
                         var value = property.objectReferenceValue;
                         text += value ? value.name : "Null";
                         var defaultValue = (Object)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.objectReferenceValue = defaultValue;
                     }
                     else
                         text += property.objectReferenceValue ? property.objectReferenceValue.name : "Null";
 
                     break;
 
                 case SerializedPropertyType.Quaternion:
                     var quat = property.quaternionValue;
                     if (_attribute.Reset)
                     {
                         var defaultValue = (Quaternion)_attribute.defaultValue;
 
                         if (quat != defaultValue)
                             property.quaternionValue = defaultValue;
                     }
                     text += quat + " (euler: " + quat.eulerAngles + ")";
 
                     break;
 
                 case SerializedPropertyType.Rect:
                     if (_attribute.Reset)
                     {
                         var value = property.rectValue;
                         text += value;
                         var defaultValue = (Rect)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.rectValue = defaultValue;
                     }
                     else
                         text += property.rectValue;
 
                     break;
 
                 case SerializedPropertyType.RectInt:
                     if (_attribute.Reset)
                     {
                         var value = property.rectIntValue;
                         text += value;
                         var defaultValue = (RectInt)_attribute.defaultValue;
 
                         // For some bizarre reason, RectInt lacks the == and != operators.
                         if (value.Equals(defaultValue))
                             property.rectIntValue = defaultValue;
                     }
                     else
                         text += property.rectIntValue;
 
                     break;
 
                 case SerializedPropertyType.String:
                     if (_attribute.Reset)
                     {
                         var value = property.stringValue;
                         text += value;
                         var defaultValue = (string)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.stringValue = defaultValue;
                     }
                     else
                         text += property.stringValue;
 
                     break;
 
                 case SerializedPropertyType.Vector2:
                     if (_attribute.Reset)
                     {
                         var value = property.vector2Value;
                         text += value;
                         var defaultValue = (Vector2)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.vector2Value = defaultValue;
                     }
                     else
                         text += property.vector2Value;
 
                     break;
 
                 case SerializedPropertyType.Vector2Int:
                     if (_attribute.Reset)
                     {
                         var value = property.vector2IntValue;
                         text += value;
                         var defaultValue = (Vector2Int)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.vector2IntValue = defaultValue;
                     }
                     else
                         text += property.vector2IntValue;
 
                     break;
 
                 case SerializedPropertyType.Vector3:
                     if (_attribute.Reset)
                     {
                         var value = property.vector3Value;
                         text += value;
                         var defaultValue = (Vector3)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.vector3Value = defaultValue;
                     }
                     else
                         text += property.vector3Value;
 
                     break;
 
                 case SerializedPropertyType.Vector3Int:
                     if (_attribute.Reset)
                     {
                         var value = property.vector3IntValue;
                         text += value;
                         var defaultValue = (Vector3Int)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.vector3IntValue = defaultValue;
                     }
                     else
                         text += property.vector3IntValue;
 
                     break;
 
                 case SerializedPropertyType.Vector4:
                     if (_attribute.Reset)
                     {
                         var value = property.vector4Value;
                         text += value;
                         var defaultValue = (Vector4)_attribute.defaultValue;
 
                         if (value != defaultValue)
                             property.vector4Value = defaultValue;
                     }
                     else
                         text += property.vector4Value;
 
                     break;
 
                 default:
                     EditorGUI.HelpBox(position, "Cannot use ReadonlyAttribute to draw a Serialized Property of type "
                         + property.propertyType.ToString(), MessageType.Error);
                     return;
             }
 
             EditorGUI.HelpBox(position, text, (MessageType)(int)_attribute.type);
         }
         catch(System.Exception e)
         {
             EditorGUI.HelpBox(position, e.Message, MessageType.Error);
         }
     }
 }