Develop Reference

ASP.NET: add postback to anchor

In my javascript file, I got an ajax to get all list and iterate these data and append <a id='userID' class='btn'>Assign ID<> to my list.
So, how do a add postback to these anchor and redirect it inside my method in the server. Below is my code but didn't work. When I click the achor button, it just redirect/refresh to the same page without doing any changes and didn't show the text.
<a id='uniqueID' class='btn assignID' href='javascript:void(0);' onclick='javascript:__doPostBack('uniqueID','')'>Assign ID</a>
protected void Action_assignID(object sender, EventArgs e)
{
// assign ID action
Response.Write("Pass");
}

You should be changed your button to:
<a id='uniqueID' class='btn assignID' href='javascript:void(0);' onclick="javascript:__doPostBack('uniqueID','Assign ID')">Assign ID</a>
And it's a good idea to implement the IPostBackEventHandler interface in your codebehind as below:
public partial class WebForm : Page, IPostBackEventHandler
{
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack)
{
}
}
public void RaisePostBackEvent(string eventArgument)
{
// do somethings at here
}
}
Hope this help!

The __doPostBack method really doesn't do anything special except, well... perform a POST operation back to the same page with two specific form arguments.
The first parameter is the __EVENTTARGET and the second parameter is the __EVENTARGUMENT.
The magic all happens in ASP.Net where it automagically wires up your controls to event handlers, but since you are creating these entirely in JavaScript the server doesn't know that those controls exist.
However, you can manually grab these values and do something with them.
//Client Side JavaScript:
__doPostBack('my-event', '42');
//Code Behind
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack)
{
var target = Request.Params["__EVENTTARGET"];
var args = Request.Params["__EVENTARGUMENT"];
Target.Text = target; // 'my-event'
Argument.Text = args; // '42'
}
}

Use of Page_PreRender to execute C# code when page controls are loaded

I have the following code :
protected void Page_Load(object sender, EventArgs e)
{
string runat = "runat=\"server\"";
}
protected void Page_PreRender(object sender, EventArgs e)
{
//some code
}
<iframe <%=runat%>></iframe>
I need to be sure the code within Page_PreRender executes only when the variable runat has been initialized & the iframe control is ready for rendering.
Unfortunately it does not work. I also tried Page_PreRenderComplete and it does not work.
Does anyone have an idea to fix this problem ?
Thanks!

Create a PlaceHolder in your markup and then add the iframe programmatically to the PlaceHolder as a LiteralControl, like this:
protected void Page_Load(object sender, EventArgs e)
{
PlaceHolder1.Controls.Add(new LiteralControl("<iframe src='something.aspx'></iframe>"));
}

ASP.NET call onclick function with parameter

In my code a user can like a post when they click on the following code:
<li><%# Eval("Likes") %></li>
This will run the function LikePost in the codebehind:
public void LikePost(object sender, EventArgs e)
{
//like post whit given id using a database query
}
but how can I give that function a parameter, because it needs the postid from the post that the user is going to like.

Instead of an HTML link, use an asp:LinkButton which has a CommandArgument property. Something like this:
<asp:LinkButton
ID="LinkButton1"
Text='<%#Eval("Likes")%>'
CommandArgument='<%#Eval("ID")%>'
OnCommand="LikePost"
CssClass="icon fa-heart"
runat="server"/>
Then in your code-behind the signature takes a CommandEventArgs:
public void LikePost(Object sender, CommandEventArgs e)
{
// e.CommandArgument should contain the desired value
}

How to pass an attribute value from ImageButton to code behind

On page load I add a new attribute (UploadStatus) to an ImageButton (FileUpload_result) from code behind. Now on button click I want to retrieve the value of the added attribute. How can I do that?
public string UploadStatus = "testing";
protected void Page_Load(object sender, EventArgs e)
{ FileUpload_result.Attributes.Add("UploadStatus", UploadStatus); }
<asp:ImageButton runat="server" ID="FileUpload_result" OnClick="FileUpload_Click" ImageUrl="icon-ok.png" UploadStatus="testing" />
protected void FileUpload_Click(object sender, EventArgs e)
{ // How can I get the value of the added attribute UploadStatus? The value is testing in this case}

In your FileUpload_Click method you can access the attribute like this.
((ImageButton)sender).Attributes["UploadStatus"]

Something like FileUpload_result.Attributes[""UploadStatus"] might get it. https://msdn.microsoft.com/en-us/library/kkeesb2c%28v=vs.140%29.aspx

Why my Query string is not working?

I am facing problem in query string. Following is my asp code
<asp:Label ID="Lable1" runat="server" Text="" ></asp:Label>
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
C# code:
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
Lable1.Text += Request.QueryString["refresh"] ;
Response.Redirect("QueryString1t.aspx?refresh=" + 1 + "");
}
Up to my knowledge Lable1 text should change on button click every time. Lable1 text should not show any thing on page load . on button click it should be like for first click 1 for second click 11 and so on..
But it is not showing as my expectation .So tell me please where i am wrong?

You are redirecting after setting label text, wrong approach.
Try this: -
protected void Page_Load(object sender, EventArgs e)
{
Lable1.Text = Request.QueryString["refresh"];
}
protected void Button1_Click(object sender, EventArgs e)
{
Response.Redirect("QueryString1t.aspx?refresh=" +
string.IsNullOrEmpty(Request.QueryString["refresh"]) ? 0 :
Convert.ToInt32(Request.QueryString["refresh"]) + 1 + "");
}
OR this: -
protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
Lable1.Text = string.IsNullOrEmpty(Lable1.Text) ? "0" :
(Convert.ToInt32(Lable1.Text) + 1).ToString();
}

Have a look at this:
http://msdn.microsoft.com/en-us/library/ms178472.aspx
Particularly the fourth and fifth events in the cycle.
If you want the text to be updated as you are looking for there, then you need to put that code in the page load, not the click event handler.
In brief, you have to think of it like this: Every time you do a redirect, you lose viewstate,
http://msdn.microsoft.com/en-us/library/ms972976.aspx
which is the means by which ASP.NET keeps track of your updates to the controls, like in your event handler. So your update is immediately lost.
The next piece of code you will hit is your load event, so that is where you need to set the label's text property.
How do I persist the value of a label through a response.redirect?