StateItem 类

定义

表示视图状态信息在 Web 请求间持续时保存在 StateBag 类中的项。Represents an item that is saved in the StateBag class when view state information is persisted between Web requests. 此类不能被继承。This class cannot be inherited.

public ref class StateItem sealed
public sealed class StateItem
type StateItem = class
Public NotInheritable Class StateItem
继承
StateItem

示例

下面的代码示例使用 StateItem 类的 ValueIsDirty 属性来保存简单自定义 ASP.NET 服务器控件类的状态 StateBagSampleThe following code example uses the Value and IsDirty properties of the StateItem class to save the state of a simple custom ASP.NET server control class, StateBagSample. 当页发布到服务器时,IsDirty 属性将检查该项是否已被修改。When the page has posted to the server, the IsDirty property checks whether the item has been modified. 状态值通过访问 Value 属性来显示。The state values are displayed by accessing the Value property.

// Create a namespace that contains a class, MyItem,
// that implements the IStateManager interface and 
// another, MyControl, that overrides its own view-state
// management methods to use those of MyItem.
using System;
using System.Web;
using System.Web.UI;
using System.Collections;
using System.Security.Permissions;

namespace StateBagSample
{
    // Create a class that implements IStateManager so that
    // it can manage its own view state.   
    [AspNetHostingPermission(SecurityAction.Demand,
       Level = AspNetHostingPermissionLevel.Minimal)]
    public sealed class MyItem : IStateManager
    {
        private string _message;

        // The StateBag object that allows you to save
        // and restore view-state information.
        private StateBag _viewstate;

        // The constructor for the MyItem class.
        public MyItem(string mesg)
        {
            _message = mesg;
            _viewstate = new StateBag();
            _viewstate.Add("message", _message);
        }

        // Create a Message property that reads from and writes
        // to view state. If the set accessor writes the message
        // value to view state, the StateBag.SetItemDirty method
        // is called, telling view state that the item has changed. 
        public string Message
        {
            get
            {
                return (string)_viewstate["message"];
            }
            set
            {
                _message = value;
                _viewstate.SetItemDirty("message", true);
            }
        }

        // Implement the LoadViewState method. If the saved view state
        // exists, the view-state value is loaded to the MyItem control. 
        void IStateManager.LoadViewState(object savedState)
        {
            _message = (string)_viewstate["message"];
            if (savedState != null)
                ((IStateManager)_viewstate).LoadViewState(savedState);
        }

        // Implement the SaveViewState method. If the StateBag
        // that stores the MyItem class's view state contains
        // a value for the message property and if the value
        // has changed since the TrackViewState method was last 
        // called, all view state for this class is deleted, 
        // using the StateBag.Clear method,and the new value is added.
        object IStateManager.SaveViewState()
        {
            // Check whether the message property exists in 
            // the ViewState property, and if it does, check
            // whether it has changed since the most recent
            // TrackViewState method call.
            if (!((IDictionary)_viewstate).Contains("message") || _viewstate.IsItemDirty("message"))
            {
                _viewstate.Clear();
                // Add the _message property to the StateBag.
                _viewstate.Add("message", _message);
            }
            return ((IStateManager)_viewstate).SaveViewState();
        }


        // Implement the TrackViewState method for this class by
        // calling the TrackViewState method of the class's private
        // _viewstate property.
        void IStateManager.TrackViewState()
        {
            ((IStateManager)_viewstate).TrackViewState();
        }

        // Implement the IsTrackingViewState method for this class 
        // by calling the IsTrackingViewState method of the class's
        // private _viewstate property. 
        bool IStateManager.IsTrackingViewState
        {
            get
            {
                return ((IStateManager)_viewstate).IsTrackingViewState;
            }
        }

        // Create a function that iterates through the view-state
        // values stored for this class and returns the
        // results as a string.
        public string EnumerateViewState()
        {
            string keyName, keyValue;
            string result = String.Empty;
            StateItem myStateItem;
            IDictionaryEnumerator myDictionaryEnumerator = _viewstate.GetEnumerator();
            while (myDictionaryEnumerator.MoveNext())
            {
                keyName = (string)myDictionaryEnumerator.Key;
                myStateItem = (StateItem)myDictionaryEnumerator.Value;
                keyValue = (string)myStateItem.Value;
                result = result + "<br>ViewState[" + keyName + "] = " + keyValue;
            }
            return result;
        }
    }
    // This class contains an instance of the MyItem class as 
    // private member. It overrides the state management methods 
    // of the Control class, since it has to invoke state 
    // management methods of MyItem whenever its own
    // view state is being saved, loaded, or tracked.
    [AspNetHostingPermission(SecurityAction.Demand,
       Level = AspNetHostingPermissionLevel.Minimal)]
    public sealed class MyControl : Control
    {
        private MyItem myItem;
        public MyControl()
            : base()
        {
            myItem = new MyItem("Hello World!");
        }
        // Override the LoadViewState method of the Control class.
        protected override void LoadViewState(object savedState)
        {
            if (savedState != null)
            {
                object[] myState = (object[])savedState;
                if (myState[0] != null)
                    base.LoadViewState(myState[0]);
                if (myState[1] != null)
                    ((IStateManager)myItem).LoadViewState(myState[1]);
            }
        }
        // Override the TrackViewState method of the Control class
        // to call the version of this method that was 
        // implemented in the MyItem class.
        protected override void TrackViewState()
        {
            base.TrackViewState();
            if (myItem != null)
                ((IStateManager)myItem).TrackViewState();
        }

        // Override the SaveViewState method of the Control class to
        // call the version of this method that was implemented by
        // the MyItem class.
        protected override object SaveViewState()
        {
            object baseState = base.SaveViewState();
            object itemState = (myItem != null) ? ((IStateManager)myItem).SaveViewState() : null;
            object[] myState = new object[2];
            myState[0] = baseState;
            myState[1] = itemState;
            return myState;
        }

        public void SetMessage(string mesg)
        {
            myItem.Message = mesg;
        }

        public string GetMessage()
        {
            return myItem.Message;
        }

        // Display the contents of Message and ViewState properties. 
        protected override void Render(HtmlTextWriter output)
        {
            // Track changes to  view state before rendering.
            TrackViewState();
            output.Write("Message: " + myItem.Message);
            output.Write("<br>");
            output.Write("<br>Enumerating the view state of the custom control<br>");
            output.Write(myItem.EnumerateViewState());
        }
    }
}
' Create a namespace that contains a class, MyItem,
' that implements the IStateManager interface and 
' another, MyControl, that overrides its own view-state
' management methods to use those of MyItem.
Imports System.Web
Imports System.Web.UI
Imports System.Collections
Imports System.Security.Permissions

Namespace StateBagSampleVB

    ' Create a class that implements IStateManager so that
    ' it can manage its own view state.   

    <AspNetHostingPermission(SecurityAction.Demand, _
       Level:=AspNetHostingPermissionLevel.Minimal)> _
    Public NotInheritable Class MyItem
        Implements IStateManager
        Private _message As String

        ' The StateBag object that allows you to save
        ' and restore view-state information.
        Private _viewstate As StateBag


        ' The constructor for the MyItem class.
        Public Sub New(ByVal mesg As String)
            _message = mesg
            _viewstate = New StateBag()
            _viewstate.Add("message", _message)
        End Sub

        ' Create a Message property that reads from and writes
        ' to view state. If the set accessor writes the message
        ' value to view state, the StateBag.SetItemDirty method
        ' is called, telling view state that the item has changed. 

        Public Property Message() As String
            Get
                Return CStr(_viewstate("message"))
            End Get
            Set(ByVal value As String)
                _message = value
                _viewstate.SetItemDirty("message", True)
            End Set
        End Property

        ' Implement the LoadViewState method. If the saved view state
        ' exists, the view-state value is loaded to the MyItem 
        ' control. 
        Sub LoadViewState(ByVal savedState As Object) Implements IStateManager.LoadViewState
            _message = CStr(_viewstate("message"))
            If Not (savedState Is Nothing) Then
                CType(_viewstate, IStateManager).LoadViewState(savedState)
            End If
        End Sub
        ' Implement the SaveViewState method. If the StateBag
        ' that stores the MyItem class's view state contains
        ' a value for the message property and if the value
        ' has changed since the TrackViewState method was last 
        ' called, all view state for this class is deleted, 
        ' using the StateBag.Clear method,and the new value is added.
        Function SaveViewState() As Object Implements IStateManager.SaveViewState
            ' Check whether the message property exists in 
            ' the ViewState property, and if it does, check
            ' whether it has changed since the most recent
            ' TrackViewState method call.
            If Not CType(_viewstate, IDictionary).Contains("message") OrElse _viewstate.IsItemDirty("message") Then
                _viewstate.Clear()
                ' Add the _message property to the StateBag.
                _viewstate.Add("message", _message)
            End If
            Return CType(_viewstate, IStateManager).SaveViewState()
        End Function 'IStateManager.SaveViewState


        ' Implement the TrackViewState method for this class by
        ' calling the TrackViewState method of the class's private
        ' _viewstate property.
        Sub TrackViewState() Implements IStateManager.TrackViewState
            CType(_viewstate, IStateManager).TrackViewState()
        End Sub
        ' Implement the IsTrackingViewState method for this class 
        ' by calling the IsTrackingViewState method of the class's
        ' private _viewstate property. 

        ReadOnly Property IsTrackingViewState() As Boolean Implements IStateManager.IsTrackingViewState
            Get
                Return CType(_viewstate, IStateManager).IsTrackingViewState
            End Get
        End Property

        ' Create a function that iterates through the view-state
        ' values stored for this class and returns the
        ' results as a string.
        Public Function EnumerateViewState() As String
            Dim keyName, keyValue As String
            Dim result As String = [String].Empty
            Dim myStateItem As StateItem
            Dim myDictionaryEnumerator As IDictionaryEnumerator = _viewstate.GetEnumerator()
            While myDictionaryEnumerator.MoveNext()
                keyName = CStr(myDictionaryEnumerator.Key)
                myStateItem = CType(myDictionaryEnumerator.Value, StateItem)
                keyValue = CStr(myStateItem.Value)
                result = result + "<br>ViewState[" + keyName + "] = " + keyValue
            End While
            Return result
        End Function 'EnumerateViewState
    End Class

    ' This class contains an instance of the MyItem class as 
    ' private member. It overrides the state management methods 
    ' of the Control class, since it has to invoke state 
    ' management methods of MyItem whenever its own
    ' view state is being saved, loaded, or tracked.

    <AspNetHostingPermission(SecurityAction.Demand, _
       Level:=AspNetHostingPermissionLevel.Minimal)> _
    Public NotInheritable Class MyControl
        Inherits Control
        Private myItem As MyItem

        Public Sub New()
            myItem = New MyItem("Hello World!")
        End Sub

        ' Override the LoadViewState method of the Control class.
        Protected Overrides Sub LoadViewState(ByVal savedState As Object)
            If Not (savedState Is Nothing) Then
                Dim myState As Object() = CType(savedState, Object())
                If Not (myState(0) Is Nothing) Then
                    MyBase.LoadViewState(myState(0))
                End If
                If Not (myState(1) Is Nothing) Then
                    CType(myItem, IStateManager).LoadViewState(myState(1))
                End If
            End If
        End Sub
        ' Override the TrackViewState method of the Control class
        ' to call the version of this method that was 
        ' implemented in the MyItem class.
        Protected Overrides Sub TrackViewState()
            MyBase.TrackViewState()
            If Not (myItem Is Nothing) Then
                CType(myItem, IStateManager).TrackViewState()
            End If
        End Sub

        ' Override the SaveViewState method of the Control class to
        ' call the version of this method that was implemented by
        ' the MyItem class.
        Protected Overrides Function SaveViewState() As Object
            Dim baseState As Object = MyBase.SaveViewState()
            Dim itemState As Object
            If Not (myItem Is Nothing) Then
                itemState = CType(myItem, IStateManager).SaveViewState()
            Else
                itemState = Nothing
            End If

            Dim myState(1) As Object
            myState(0) = baseState
            myState(1) = itemState
            Return myState
        End Function 'SaveViewState


        Public Sub SetMessage(ByVal mesg As String)
            myItem.Message = mesg
        End Sub


        Public Function GetMessage() As String
            Return myItem.Message
        End Function 'GetMessage


        ' Display the contents of Message and ViewState properties. 
        Protected Overrides Sub Render(ByVal output As HtmlTextWriter)
            ' Track changes to  view state before rendering.
            TrackViewState()
            output.Write(("Message: " + myItem.Message))
            output.Write("<br>")
            output.Write("<br>Enumerating the view state of the custom control<br>")
            output.Write(myItem.EnumerateViewState())
        End Sub
    End Class
End Namespace 'StateBagSampleVB

注解

视图状态是指页面或 ASP.NET 服务器控件的属性值的累计,并发送到隐藏字段中请求的浏览器。View state is the accumulation of a page's or an ASP.NET server control's property values and is sent to the requesting browser in a hidden field.

可以使用 Item[String] 属性或 Add 方法,将 StateItem 对象显式添加到 ASP.NET 服务器控件的 StateBag 对象中。You can explicitly add StateItem objects to an ASP.NET server control's StateBag object using either the Item[String] property or the Add method. 然后,StateBag 跟踪其存储的所有项的更改。The StateBag then tracks changes to all the items that it stores. StateItem 对象所做的任何更改都反映在其 IsDirty 属性中。Any changes to a StateItem object are reflected in its IsDirty property. 在服务器控制处理的 "保存" 视图状态阶段,就在控件呈现到页面之前,会通过调用 SaveViewState 方法来保存这些更改。These changes are saved by a call to the SaveViewState method during the save view state phase of server control processing, just before the control is rendered to the page. 有关详细信息,请参阅ASP.NET Web 服务器控件For more information, see ASP.NET Web Server Controls.

属性

IsDirty

获取或设置一个值,该值指示 StateItem 对象是否已修改。Gets or sets a value indicating whether the StateItem object has been modified.

Value

获取或设置存储在 StateBag 对象中的 StateItem 对象的值。Gets or sets the value of the StateItem object that is stored in the StateBag object.

方法

Equals(Object)

确定指定的对象是否等于当前对象。Determines whether the specified object is equal to the current object.

(继承自 Object)
GetHashCode()

用作默认哈希函数。Serves as the default hash function.

(继承自 Object)
GetType()

获取当前实例的 TypeGets the Type of the current instance.

(继承自 Object)
MemberwiseClone()

创建当前 Object 的浅表副本。Creates a shallow copy of the current Object.

(继承自 Object)
ToString()

返回一个表示当前对象的 string。Returns a string that represents the current object.

(继承自 Object)

适用于

另请参阅