View State

It’s of type StateBag and is used for storing state/data to be used in postback to the same web form.
ViewState being hidden element in form tag rendered, its different for different clients & is submitted to the server only if the form is submitted or posted.
ViewState is a collection of Key-Value pairs, where Key is of type String and Value is of type Object. But only those objects which are marked as Serializable can be stored in ViewState.
ViewState cannot be programmed on the client in javascript because the value of it is encrypted before it is rendered to the browser.

If ViewState of the page is disabled (<%@Page EnableViewState=”False”) than anything added to it is not retained.

Example:1

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>ViewState</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:TextBox runat="server" id="NameField" />
        <asp:Button runat="server" id="SubmitForm" onclick="SubmitForm_Click" text="Submit & set name" />
        <asp:Button runat="server" id="RefreshPage" text="Just submit" />
        <br /><br />
        Name retrieved from ViewState: <asp:Label runat="server" id="NameLabel" />
    </form> 
</body>
</html>

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN">

<title>ViewState</title>

</head>

<body>

<asp:TextBox runat="server" id="NameField" />

<asp:Button runat="server" id="SubmitForm" onclick="SubmitForm_Click" text="Submit & set name" />

<asp:Button runat="server" id="RefreshPage" text="Just submit" />

Name retrieved from ViewState: <asp:Label runat="server" id="NameLabel" />

</form>

</body>

</html>

And the Code Behind:

using System;
using System.Data;
using System.Web;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if(ViewState["NameOfUser"] != null)
            NameLabel.Text = ViewState["NameOfUser"].ToString();
        else
            NameLabel.Text = "Not set yet...";
    }

    protected void SubmitForm_Click(object sender, EventArgs e)
    {
        ViewState["NameOfUser"] = NameField.Text;
        NameLabel.Text = NameField.Text;
    }
}

using System;

using System.Data;

using System.Web;

public partial class _Default : System.Web.UI.Page

{

protected void Page_Load(object sender, EventArgs e)

{

if(ViewState["NameOfUser"] != null)

NameLabel.Text = ViewState["NameOfUser"].ToString();

else

NameLabel.Text = "Not set yet...";

}

protected void SubmitForm_Click(object sender, EventArgs e)

{

ViewState["NameOfUser"] = NameField.Text;

NameLabel.Text = NameField.Text;

}

Example:2

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>ViewState</title>
</head>
<body>
    <form id="form1" runat="server">
        <asp:Button ID=”btnNew“ runat=”server” Text=”New” OnClick=”btnNew_Click” />
        <asp:Button ID=”btnEdit“ runat=”server” Text=”Edit” OnClick=”bnEdit_Click” />
        <asp:Button ID=”btnSave“ runat=”server” Text=”Save” visible=”False” OnClick=”btnSave_Click” />
        <br /><br />
        Name retrieved from ViewState: <asp:Label runat="server" id="NameLabel" />
    </form> 
</body>
</html>

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN">

<title>ViewState</title>

</head>

<body>

<asp:Button ID=”�� runat=”�� Text=”�� OnClick=”btnNew_Click” />

<asp:Button ID=”�� runat=”�� Text=”�� OnClick=”bnEdit_Click” />

<asp:Button ID=”�� runat=”�� Text=”�� visible=”�� OnClick=”btnSave_Click” />

Name retrieved from ViewState: <asp:Label runat="server" id="NameLabel" />

</form>

</body>

</html>

And the Code Behind:

protected void btnNew_Click(. . .)

{
         btnNew.Visible = false;

         btnEdit.Visible = false;

         btnSave.Visible = true;

         ViewState[“state”] = “new”;

}
protected void bnEdit_Click(. . .)

{

         btnNew.Visible = false;

         btnEdit.Visible = false;

         btnSave.Visible = true;

         ViewState[“state”] = “edit”;

}
protected void btnSave_Click(. . .)

{
         if (ViewState[“state”].ToString() == “new”)

              Response.Write(“New record inserted”);

           else

              Response.Write(“Existing record updated”);
}

protected void btnNew_Click(. . .)

{

btnNew.Visible = false;

btnEdit.Visible = false;

btnSave.Visible = true;

ViewState[“state”] = “new”;

}

protected void bnEdit_Click(. . .)

{

btnNew.Visible = false;

btnEdit.Visible = false;

btnSave.Visible = true;

ViewState[“state”] = “edit”;

}

protected void btnSave_Click(. . .)

{

if (ViewState[“state”].ToString() == “new”)

Response.Write(“New record ��);

else

Response.Write(“Existing record ��);

}

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