Page Life Cycle In ASP.NET

public partial class PageLifeCycle : System.Web.UI.Page
{
    protected void Page_PreInit(object sender, EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "PreInit";
    }

    protected void Page_Init(object sender, EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "Init";
    }

    protected void Page_InitComplete(object sender, EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "InitComplete";
    }

    protected override void OnPreLoad(EventArgs e)
    {
        // Work and assign values to label.
        // If the page is post back, then label control values will be loaded from view state.
        // E.g.: If you string str = lblName.Text, then str will contain viewstate values.
        lblName.Text += "<br/>" + "PreLoad";
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "Load";
    }

    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "btnSubmit_Click";
    }

    protected void Page_LoadComplete(object sender, EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "LoadComplete";
    }

    protected override void OnPreRender(EventArgs e)
    {
        // Work and assign values to label.
        lblName.Text += "<br/>" + "PreRender";
    }

    protected override void OnSaveStateComplete(EventArgs e)
    {
        // Work and assign values to label.
        // But "SaveStateComplete" values will not be available during post back, i.e., View state.
        lblName.Text += "<br/>" + "SaveStateComplete";
    }

    protected void Page_UnLoad(object sender, EventArgs e)
    {
        // Work and it will not affect label control, view state, and post back data.
        lblName.Text += "<br/>" + "UnLoad";
    }
}

Output

The first time the Page Load is output.

When you click on the Submit Button output.

The first time the page loads with EnableViewState="false".

When you click on the Submit Button output with EnableViewState="false":

Example 2. ViewState Values

Please observe the code comments and output. It will help you to clearly understand the concepts.

public partial class PageLifeCycle : System.Web.UI.Page
{
    protected void Page_PreInit(object sender, EventArgs e)
    {
        // Work and assign values to label.
        // Note: If page is post back or first time call and you have not set any values to ViewState["value"], then
        // Convert.ToString(ViewState["value"]) is always empty.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "PreInit";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected void Page_Init(object sender, EventArgs e)
    {
        // Work and assign values to label.
        // Note: If page is post back or first time call and you have not set any values to ViewState["value"] in previous events, then
        // Convert.ToString(ViewState["value"]) is always empty.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "Init";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected void Page_InitComplete(object sender, EventArgs e)
    {
        // Work and assign values to label.
        // Note: If page is post back or first time call and you have not set any values to ViewState["value"] in previous events, then
        // Convert.ToString(ViewState["value"]) is always empty.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "InitComplete";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected override void OnPreLoad(EventArgs e)
    {
        // Work and assign values to label.
        // Note: If the page is post back and you have set or not set any values to ViewState["value"] in previous events, then
        // Convert.ToString(ViewState["value"]) will always have post back data.
        // E.g.: If you string str = Convert.ToString(ViewState["value"]), then str will contain post back values.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "PreLoad";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        // Work and assign values to label.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "Load";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        // Work and assign values to label.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "btnSubmit_Click";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected void Page_LoadComplete(object sender, EventArgs e)
    {
        // Work and assign values to label.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "LoadComplete";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected override void OnPreRender(EventArgs e)
    {
        // Work and assign values to label.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "PreRender";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected override void OnSaveStateComplete(EventArgs e)
    {
        // Work and assign values to label.
        // But "SaveStateComplete" values will not be available during post back, i.e., View state.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "SaveStateComplete";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }

    protected void Page_UnLoad(object sender, EventArgs e)
    {
        // Work and it will not affect label control values, view state, and post back data.
        ViewState["value"] = Convert.ToString(ViewState["value"]) + "<br/>" + "UnLoad";
        lblName.Text = Convert.ToString(ViewState["value"]);
    }
}

Output

During the first, the Time Page Load is output.

When you click on the Submit Button output:

During the first time, the page loads with EnableViewState="false":

When you click on the Submit Button, the output with EnableViewState="false":

Example 3. ViewState Values

Please observe the code comments and output. It will help you to clearly understand the concepts.

public partial class PageLifeCycle : System.Web.UI.Page
{
    protected void Page_PreInit(object sender, EventArgs e)
    {
        Response.Write("<br/>" + "PreInit");
    }

    protected void Page_Init(object sender, EventArgs e)
    {
        Response.Write("<br/>" + "Init");
    }

    protected void Page_InitComplete(object sender, EventArgs e)
    {
        Response.Write("<br/>" + "InitComplete");
    }

    protected override void OnPreLoad(EventArgs e)
    {
        Response.Write("<br/>" + "PreLoad");
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        Response.Write("<br/>" + "Load");
    }

    protected void Page_LoadComplete(object sender, EventArgs e)
    {
        Response.Write("<br/>" + "LoadComplete");
    }

    protected override void OnPreRender(EventArgs e)
    {
        Response.Write("<br/>" + "PreRender");
    }

    protected override void OnSaveStateComplete(EventArgs e)
    {
        Response.Write("<br/>" + "SaveStateComplete");
    }

    protected void Page_UnLoad(object sender, EventArgs e)
    {
        // Runtime Error: Response is not available in this context.
        // Response.Write("<br/>" + "UnLoad"); // Error
    }
}

Output

Note. If you write Response.Write("<br/>" + "UnLoad"); in the Page_UnLoad event, then it will generate the Runtime Error "Response is not available in this context".