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Execute the function I wrote in C # by changing the value (input)

How can I execute the function I wrote inside the c # code when I change the input value?
<input id="quantity2" runat="server" type="number" onserverclick="lod_gheymat" value="" min="1" max="20" />
protected void lod_Price(object sender, EventArgs e)
{
lbl_Plural.Text = quantity2.Value;
}

Use a asp:Textbox instead of an input control with ‘runat=server’.
Then set the property AutoPostback to true, and handle the event ‘OnTextChanged’.
See: https://meeraacademy.com/textbox-autopostback-and-textchanged-event-asp-net/ for an example.

How to get the value of a textbox in code-behind

HTML:
<input type="text" runat="server" value="" placeholder="Search" id="searchB" class="styledTB searchB floatLeft" />
C#:
string strSMain;
protected void Page_Load(object sender, EventArgs e)
{
tbSearchMain = (System.Web.UI.WebControls.TextBox)sender;
strSMail = tbSearchMain.text; // gives me the following error: Exception Details: System.InvalidCastException: Unable to cast object of type 'ASP.site_master' to type 'System.Web.UI.WebControls.TextBox'.
strSMain = searchB.text; //.Text is not an option for me
}
Please help me resolve the issue.
I am creating a web application. And the control is in the MasterPage.

Use this:
<input id="searchB" x:Name="searchB" type="text" runat="server" value="" placeholder="Search" class="styledTB searchB floatLeft" />
The id attribute is used client side. x:Name is used for server side manipulation

I am not sure but no textbox will be sender for Page_Load() function.
Check about DataContext Property and how it works.
http://www.wpf-tutorial.com/data-binding/using-the-datacontext/

How to set html input type text value using ASP.NET C#?

I have an html control which I want to set its value ....
here's the control:
<input runat="server" id="first_name_txt" type="text" placeholder="First Name" />
in code behind, I use:
first_name_txt.Value = String.empty;
but the value of my input control still has the old value like "blah-blah" and not set to "".

Its old question , but may help someone.
You have to use Request.Form to get and call .Value to set the value.
HTML
<input runat="server" id="first_name_txt" type="text" placeholder="First Name" />
CODE BEHIND
//To get value:
string myname=Request.Form["first_name_txt"];
// To set value:
first_name_txt.Value="";

<td>
<input type="text" name="date" value="<%= tdate %>" />
</td>
Code Behind :
protected string tdate { get; set; }
protected void Page_Load(object sender, EventArgs e)
{
this.tdate = DateTime.Now.Day.ToString() + "/" + DateTime.Now.Month.ToString() + "/" + DateTime.Now.Year.ToString();
}

Hello it's not that easy to set data into HTML input, but here is a link that may help you [Link].
1) If it didn't work up to you try to set a value and calling it through Javascript and set the text of this input like the gotten value.
2) You can use the [Div] tag using runat="server", clear it and create a new input with same id,name,etc. but different Text value
Try Step 2 as follow(it worked):
<div id="divTitle" runat="server">
<input type="text" class="input_Text" id="Title" name="Title" />
</div>
divTitle.Controls.Clear();
divTitle.InnerHtml = "<input type='text' class='input_Text' id='Title' name='Title' value='" + ds(0)("Title").ToString() + "' />";
Where ds is a data table that came from select query from database

Try put this in postback
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
first_name_txt.Value = String.empty;
}
}

here is very simple way to do this
{ text_Box.Value = "data";}

hidden variable does not have value after postback asp.net

I have following
<input type="hidden" id="hdnField" name="hdnField"/>
Request.Form.Set("hdnField", x.ToString());
after the page post back the value is not there.
I am new to this, any help would be appreciated.

Source:
You could define a property in your page class and then modify the property value in your code:
protected string HiddenFieldValue { get; set; }
protected void Page_Load(object sender, EventArgs e)
{
if (IsPostBack)
HiddenFieldValue = x.ToString();
else
HiddenFieldValue = x.ToString();
}
Then define the hidden form field like this so that it's value is set to the property value:
<input type='hidden' id='hdnField' value='<%=HiddenFieldValue %>' />
If you only want to set the value form the property during a postback or non-postback you could add the condition as well:
<input type='hidden' id='hdnField' value='<% if(IsPostBack) { %> <%=HiddenFieldValue%> <% } %>' />

You should use asp:HiddenField tag provided by asp rather than using the basic HTML input.
<asp:HiddenField ID="hdnField" Value="" runat="server" ClientIDMode="Static" />
Using this, you can read and write value in c# using hdnField.Value and in jQuery using $('#hdnField').val().

How to get Textbox control in C#

I have a HTML textbox
<script>
$(function () {
$("#datepicker").datepicker({
changeMonth: true,
changeYear: true
});
});
</script>
<input type="text" size="10" name="datepicker" id="datepicker" />
Now i need to set the value of textbox to "abcd";
I tried datepicker.Text = "abcd"; Error is "the name datepicker does not exist in current context"...
I tried finding the Control and assigning the value but still could not do it.
Is there any other way to do it??
Thanks

You'll first need to make your <input> control a server side tag, using the runat="Server" attribute:
<input runat="server" type="text" size="10" name="datepicker" id="datepicker" />
Then, you can modify the value using C# code:
protected void Page_Load(object sender, EventArgs e)
{
datepicker.Text = "New Value"; // Initial value for input field
}
If you want to modify the value on the client side, using jQuery, I'd first suggest making the ID static:
<input runat="server" ClientIDMode="Static" type="text" size="10" name="datepicker" id="datepicker" />
Then you can do:
$("#datepicker").val('New Value');
Anywhere after the page has been loaded.