Hi guys. I'm using this article as a reference to use custom collection in propertygrid: LINK When I open the collectioneditor and remove all items then I press OK, I get an exception if null. How can i solve that ? I am using: public T this[int index] { get { if (List.Count == 0) { return default(T); } else { return (T)this.List[index]; } } } as a getter for an item, of course if I have no object how can i restart the whole collection ? this is the whole code /// <summary> /// A generic folder settings collection to use in a property grid. /// </summary> /// <typeparam name="T">can be import or export folder settings.</typeparam> [Serializable] [TypeConverter(typeof(FolderSettingsCollectionConverter)), Editor(typeof(FolderSettingsCollectionEditor), typeof(UITypeEditor))] public class FolderSettingsCollection_New<T> : CollectionBase, ICustomTypeDescriptor { private bool m_bRestrictNumberOfItems; private int m_bNumberOfItems; private Dictionary<string, int> m_UID2Idx = new Dictionary<string, int>(); private T[] arrTmp; /// <summary> /// C'tor, can determine the number of objects to hold. /// </summary> /// <param name="bRestrictNumberOfItems">restrict the number of folders to hold.</param> /// <param name="iNumberOfItems">The number of folders to hold.</param> public FolderSettingsCollection_New(bool bRestrictNumberOfItems = false , int iNumberOfItems = 1) { m_bRestrictNumberOfItems = bRestrictNumberOfItems; m_bNumberOfItems = iNumberOfItems; } /// <summary> /// Add folder to collection. /// </summary> /// <param name="t">Folder to add.</param> public void Add(T t) { if (m_bRestrictNumberOfItems) { if (this.List.Count >= m_bNumberOfItems) { return; } } int index = this.List.Add(t); if (t is WriteDataFolderSettings || t is ReadDataFolderSettings) { FolderSettingsBase tmp = t as FolderSettingsBase; m_UID2Idx.Add(tmp.UID, index); } } /// <summary> /// Remove folder to collection. /// </summary> /// <param name="t">Folder to remove.</param> public void Remove(T t) { this.List.Remove(t); if (t is WriteDataFolderSettings || t is ReadDataFolderSettings) { FolderSettingsBase tmp = t as FolderSettingsBase; m_UID2Idx.Remove(tmp.UID); } } /// <summary> /// Gets ot sets a folder. /// </summary> /// <param name="index">The index of the folder in the collection.</param> /// <returns>A folder object.</returns> public T this[int index] { get { //if (List.Count == 0) //{ // return default(T); //} //else //{ return (T)this.List[index]; //} } } /// <summary> /// Gets or sets a folder. /// </summary> /// <param name="sUID">The UID of the folder.</param> /// <returns>A folder object.</returns> public T this[string sUID] { get { if (this.Count == 0 || !m_UID2Idx.ContainsKey(sUID)) { return default(T); } else { return (T)this.List[m_UID2Idx[sUID]]; } } } /// <summary> /// /// </summary> /// <param name="sUID"></param> /// <returns></returns> public bool ContainsItemByUID(string sUID) { return m_UID2Idx.ContainsKey(sUID); } /// <summary> /// /// </summary> /// <returns></returns> public String GetClassName() { return TypeDescriptor.GetClassName(this, true); } /// <summary> /// /// </summary> /// <returns></returns> public AttributeCollection GetAttributes() { return TypeDescriptor.GetAttributes(this, true); } /// <summary> /// /// </summary> /// <returns></returns> public String GetComponentName() { return TypeDescriptor.GetComponentName(this, true); } /// <summary> /// /// </summary> /// <returns></returns> public TypeConverter GetConverter() { return TypeDescriptor.GetConverter(this, true); } /// <summary> /// /// </summary> /// <returns></returns> public EventDescriptor GetDefaultEvent() { return TypeDescriptor.GetDefaultEvent(this, true); } /// <summary> /// /// </summary> /// <returns></returns> public PropertyDescriptor GetDefaultProperty() { return TypeDescriptor.GetDefaultProperty(this, true); } /// <summary> /// /// </summary> /// <param name="editorBaseType"></param> /// <returns></returns> public object GetEditor(Type editorBaseType) { return TypeDescriptor.GetEditor(this, editorBaseType, true); } /// <summary> /// /// </summary> /// <param name="attributes"></param> /// <returns></returns> public EventDescriptorCollection GetEvents(Attribute[] attributes) { return TypeDescriptor.GetEvents(this, attributes, true); } /// <summary> /// /// </summary> /// <returns></returns> public EventDescriptorCollection GetEvents() { return TypeDescriptor.GetEvents(this, true); } /// <summary> /// /// </summary> /// <param name="pd"></param> /// <returns></returns> public object GetPropertyOwner(PropertyDescriptor pd) { return this; } /// <summary> /// /// </summary> /// <param name="attributes"></param> /// <returns></returns> public PropertyDescriptorCollection GetProperties(Attribute[] attributes) { return GetProperties(); } /// <summary> /// Called to get the properties of this type. /// </summary> /// <returns></returns> public PropertyDescriptorCollection GetProperties() { // Create a collection object to hold property descriptors PropertyDescriptorCollection pds = new PropertyDescriptorCollection(null); // Iterate the list of employees for (int i = 0; i < this.List.Count; i++) { // Create a property descriptor for the employee item and add to the property descriptor collection CollectionPropertyDescriptor_New<T> pd = new CollectionPropertyDescriptor_New<T>(this, i); pds.Add(pd); } // return the property descriptor collection return pds; } public T[] ToArray() { if (arrTmp == null) { arrTmp = new T[List.Count]; for (int i = 0; i < List.Count; i++) { arrTmp[i] = (T)List[i]; } } return arrTmp; } } /// <summary> /// Enable to display data about a collection in a property grid. /// </summary> /// <typeparam name="T">Folder object.</typeparam> public class CollectionPropertyDescriptor_New<T> : PropertyDescriptor { private FolderSettingsCollection_New<T> collection = null; private int index = -1; /// <summary> /// /// </summary> /// <param name="coll"></param> /// <param name="idx"></param> public CollectionPropertyDescriptor_New(FolderSettingsCollection_New<T> coll, int idx) : base("#" + idx.ToString(), null) { this.collection = coll; this.index = idx; } /// <summary> /// /// </summary> public override AttributeCollection Attributes { get { return new AttributeCollection(null); } } /// <summary> /// /// </summary> /// <param name="component"></param> /// <returns></returns> public override bool CanResetValue(object component) { return true; } /// <summary> /// /// </summary> public override Type ComponentType { get { return this.collection.GetType(); } } /// <summary> /// /// </summary> public override string DisplayName { get { if (this.collection[index] != null) { return this.collection[index].ToString(); } else { return null; } } } public override string Description { get { return ""; } } /// <summary> /// /// </summary> /// <param name="component"></param> /// <returns></returns> public override object GetValue(object component) { if (this.collection[index] != null) { return this.collection[index]; } else { return null; } } /// <summary> /// /// </summary> public override bool IsReadOnly { get { return false; } } public override string Name { get { return "#" + index.ToString(); } } /// <summary> /// /// </summary> public override Type PropertyType { get { return this.collection[index].GetType(); } } public override void ResetValue(object component) { } /// <summary> /// /// </summary> /// <param name="component"></param> /// <returns></returns> public override bool ShouldSerializeValue(object component) { return true; } /// <summary> /// /// </summary> /// <param name="component"></param> /// <param name="value"></param> public override void SetValue(object component, object value) { // this.collection[index] = value; } }