StateBag.cs实现代码 对理解实现自定义状态管理很有帮助。

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/**
* Namespace: System.Web.UI
* Class:     StateBag
*
* Author:  Gaurav Vaish
* Maintainer: gvaish@iitk.ac.in
* Implementation: yes
* Contact: <gvaish@iitk.ac.in>
* Status:  100%
*
* (C) Gaurav Vaish (2001)
*/

using System;
using System.Web;
using System.Collections;
using System.Collections.Specialized;

namespace System.Web.UI
{
public sealed class StateBag : IStateManager, IDictionary, ICollection, IEnumerable
{
private bool marked;
private HybridDictionary bag;

public StateBag (bool ignoreCase)
{
Initialize (ignoreCase);
}

public StateBag ()
{
Initialize (false);
}

private void Initialize (bool ignoreCase)
{
marked = false;
bag = new HybridDictionary (ignoreCase);
}

public int Count {
get { return bag.Count; }
}


public object this [string key] {
get {
if (key == null || key.Length == 0)
throw new ArgumentException (HttpRuntime.FormatResourceString ("Key_Cannot_Be_Null"));

object val = bag [key];

if (val is StateItem)
return ((StateItem) val).Value;

return null; //
}

set { Add (key, value); }
}

object IDictionary.this [object key] {
get { return this [(string) key] as object; }

set { Add ((string) key, value); }
}

public ICollection Keys {
get { return bag.Keys; }
}

public ICollection Values {
get { return bag.Values; }
}

public StateItem Add (string key, object value)
{
if (key == null || key.Length == 0)
throw new ArgumentException (HttpRuntime.FormatResourceString ("Key_Cannot_Be_Null"));

StateItem val = bag [key] as StateItem; //don't throw exception when null
if(val == null) {
if(value != null || marked) {
val = new StateItem (value);
bag.Add (key, val);
}
}
else if (value == null && !marked)
bag.Remove (key);
else
val.Value = value;

if (val != null && marked) {
val.IsDirty = true;
}

return val;
}

public void Clear ()
{
bag.Clear ();
}

public IDictionaryEnumerator GetEnumerator ()
{
return bag.GetEnumerator ();
}

public bool IsItemDirty (string key)
{
object o = bag [key];

if (o is StateItem)
return ((StateItem) o).IsDirty;

return false;
}

public void Remove (string key)
{
bag.Remove (key);
}

/// <summary>
/// Undocumented
/// </summary>
public void SetItemDirty (string key, bool dirty)
{
if (bag [key] is StateItem)
((StateItem) bag [key]).IsDirty = dirty;
}

internal bool IsTrackingViewState {
get { return marked; }
}

internal void LoadViewState (object state)
{
if(state!=null) {
Pair pair = (Pair) state;
ArrayList keyList = (ArrayList) (pair.First);
ArrayList valList = (ArrayList) (pair.Second);

int valCount = valList.Count;
for(int i = 0; i < keyList.Count; i++) {
if (i < valCount)
Add ((string) keyList [i], valList [i]);
else
Add ((string) keyList [i], null);
}
}
}

internal object SaveViewState ()
{
if(bag.Count > 0) {
ArrayList keyList = null, valList = null;

foreach (string key in bag.Keys) {
StateItem item = (StateItem) bag [key];

if (item.IsDirty) {
if (keyList == null) {
keyList = new ArrayList ();
valList = new ArrayList ();
}

keyList.Add (key);
valList.Add (item.Value);
}
}

if (keyList!=null)
return new Pair (keyList, valList);
}
return null;
}

internal void TrackViewState()
{
marked = true;
}

IEnumerator IEnumerable.GetEnumerator ()
{
return GetEnumerator ();
}

void IStateManager.LoadViewState (object savedState)
{
LoadViewState (savedState);
}

object IStateManager.SaveViewState ()
{
return SaveViewState ();
}

void IStateManager.TrackViewState ()
{
TrackViewState ();
}

bool IStateManager.IsTrackingViewState {
get { return IsTrackingViewState; }
}

void ICollection.CopyTo (Array array, int index)
{
Values.CopyTo (array, index);
}

bool ICollection.IsSynchronized {
get { return false; }
}

object ICollection.SyncRoot
{
get { return this; }
}

void IDictionary.Add (object key, object value)
{
Add ((string) key, value);
}

void IDictionary.Remove (object key)
{
Remove ((string) key);
}

bool IDictionary.Contains (object key)
{
return bag.Contains ((string) key);
}

bool IDictionary.IsFixedSize {
get { return false; }
}

bool IDictionary.IsReadOnly {
get { return false; }
}

#if NET_2_0
public void SetDirty ()
{
foreach (string key in bag.Keys)
SetItemDirty (key, true);
}
#endif

}
}
下面实现自定义状态管理
    public class Address:IStateManager
    {
        private bool _isTrackingViewState;
        private StateBag _viewState;

       protected StateBag ViewState
        {
            get
            {
                if (_viewState == null)
                {
                    _viewState = new StateBag(false);
                    if (_isTrackingViewState) ((IStateManager)_viewState).TrackViewState();
                }
                return _viewState;
            }
        }
      bool IStateManager.IsTrackingViewState
        {
            get
            {
                return _isTrackingViewState;
            }
        }

        void IStateManager.LoadViewState(object savedState)
        {
            if (savedState != null)
            {
                ((IStateManager)ViewState).LoadViewState(savedState);
            }
        }

        object IStateManager.SaveViewState()
        {
            object savedState = null;
            if (_viewState != null)
            {
                savedState =
                   ((IStateManager)_viewState).SaveViewState();
            }
            return savedState;
        }

        void IStateManager.TrackViewState()
        {
            _isTrackingViewState = true;
           
            if (_viewState != null)
            {
                ((IStateManager)_viewState).TrackViewState();
            }
        }

        #endregion

        internal void SetDirty()
        {
            _viewState.SetDirty(true);
        }
   [
       Category("Behavior"),
       DefaultValue(""),
       Description("城市"),
       NotifyParentProperty(true),
       ]
        public String City  //主要这里的ViewState 为我们自定义的
        {
            get { return ViewState["City"] != null ? (string)ViewState["City"] : String.Empty; }
            set { ViewState["City"] = value; }
        }
  }