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Button in asp.net doesn't work sometimes

I have a textbox and a button in my code.The button when clicked should validate the value entered in the textbox.The button most of the times does it but at times doesn't do anything at all.Any help would be appreciated.
<asp:Button ValidationGroup="vgpPno" runat="server" ID="btnSubmitPartNumber" OnClick="btnSubmitPartNumber_Click" SkinID="MSWButton" Text="OK" OnClientClick="if(Page_ClientValidate(){startPLI();}meta:resourcekey="btnSubmitPartNumberResource1" />
<asp:TextBox runat="server" ValidationGroup="vgpPno" SkinID="MSWTextBox"
ID="tbPARTNUMBER" MaxLength="11" meta:resourcekey="tbPARTNUMBERResource1" Width="230" Style="display: inline" CssClass="form-control input-sm"></asp:TextBox>

<asp:Button ValidationGroup="vgpPno" runat="server" ID="btnSubmitPartNumber" OnClick="btnSubmitPartNumber_Click" SkinID="MSWButton" Text="OK"
OnClientClick="if(Page_ClientValidate(){startPLI();}meta:resourcekey='btnSubmitPartNumberResource1'" />

Check Your web Form. If you are using form tag more then one time asp.controls not working properly

How to trigger TextChange event without leaving focus in ASP.NET C#?

I tryed trigger the TextChange event with ajax but it dosnt realy work how i do it. Hope you guys can help me.
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:ScriptManager ID="ScriptManager1" runat="server" AsyncPostBackTimeout="0">
</asp:ScriptManager>
<asp:TextBox ID="TextBox1" runat="server" AutoCompleteType="Disabled" AutoPostBack="True" OnTextChanged="TextBox1_TextChanged"></asp:TextBox>
<asp:Label ID="Label2" runat="server" Text="Label"></asp:Label>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="TextBox1" EventName="TextChanged" />
</Triggers>
</asp:UpdatePanel>

The ScriptManager should appear before the UpdatPanel, not inside of it. And you don't need to define TextBox1 as a trigger. The TextBox being inside the UpdatePanel and the property ChildrenAsTriggers being true by default, the panel will be updated when TextBox1 loses focus after its content has been modified.
UPDATE
I originally misread the question. The following posts describes a method to trigger a postback and refresh the UpdatPanel every time a key is pressed in the TextBox: How do I make a Textbox Postback on KeyUp?.

When page loaded Checkbox only check one time ? ASP.NET ,C#

My simple question is that ,is there any method/way in which 'Checbox' only check one time.I mean when page loaded which contains 'checkbox' ,after page loaded then i select check box and press 'button' then checkbox will be disabled,only one time checkbox will tick.Is there any way?
Thanks.
here is my code :
<label>Allow Null <asp:CheckBox ID="Null" runat="server" /></label>
<label>Primary Key <asp:CheckBox ID="Primary" runat="server" />
<br />
<asp:Button ID="Button1" ValidationGroup="s" onclick="Button1_Click" runat="server" Text="Insert" />
I want when i press "Insert" button then primary checkbox(check its tick or not),if Primary cehckbox tick then next time this will be disabled.Only one time "Primary" checkbox tick.

As the coments suggests ,
from client script you can try this:
$("#checkId").attr("disabled", true);
if you are in asp.net web-forms and the button does postback then you can even disable it from code behind.
Edit
<label>Allow Null <asp:CheckBox ID="Null" runat="server" ClientIdMode="Static" /></label>
<asp:Button ID="Button1" ValidationGroup="s" onclick="Button1_Click" runat="server" OnClientClick="UpdateCheck();" Text="Insert" />
<script>
function updateCheck(){
$("#Null").attr("disabled", true);
}
</script>

asp.net - javascript better way to use onchange with multiple fields

I have a asp.net web page which contains many textboxes.
If any of these boxes I run some javascript via the onchange event of each field.
The javascript basically changes a textbox to 'Y' so I know which section of the page has changed something.
Everything works fine, however I just need to know if there is a way of making the onchange call better/shorter ? Since I have over >100 textboxes, there is a lot of repetitive onchange='CM('3'); on each and every textbox, so I am hoping there is a better way to handle this.
There are three divs, each containing some 35 textboxes, so perhaps there is a way of doing it by DIV, which I don't know.
Any guidance is appreciated.
<asp:TextBox ID="txtDC1_D" runat="server" CssClass="STD_desc160" onchange="CM('3');"></asp:TextBox>
<asp:TextBox ID="txtDC1_C" runat="server" CssClass="STD_cur30" onchange="CM('3');"></asp:TextBox>
<asp:TextBox ID="txtDC1_1" runat="server" CssClass="STD_value55" onchange="CM('3');"></asp:TextBox>
<asp:TextBox ID="txtDC1_2" runat="server" cssClass="STD_value55" onchange="CM('3');"></asp:TextBox>
<asp:TextBox ID="txtDC1_3" runat="server" cssClass="STD_value55" onchange="CM('3');"></asp:TextBox>
<asp:TextBox ID="txtDC1_B" runat="server" CssClass="STD_bassis" onchange="CM('3');"></asp:TextBox>
<br />
<asp:TextBox ID="txtDC2_D" runat="server" CssClass="STD_desc160" onchange="CM('2');"></asp:TextBox>
<asp:TextBox ID="txtDC2_C" runat="server" CssClass="STD_cur30" onchange="CM('2');"></asp:TextBox>
<asp:TextBox ID="txtDC2_1" runat="server" CssClass="STD_value55" onchange="CM('2');" ></asp:TextBox>
<asp:TextBox ID="txtDC2_2" runat="server" cssClass="STD_value55" onchange="CM('2');" ></asp:TextBox>
<asp:TextBox ID="txtDC2_3" runat="server" cssClass="STD_value55" onchange="CM('2');" ></asp:TextBox>
<asp:TextBox ID="txtDC2_B" runat="server" CssClass="STD_bassis" onchange="CM('2');"></asp:TextBox>
<br />

If jQuery is also an option ( which I strongly recommend) , so :
$(function (){
$("input[type=text]").on('change',function (){
if ($(this).val()) $(this).val('Y'); else $(this).val('');
})
});
if not , I will delete this answer.

Requiredfieldvalidator with no postback (Only Javascript)

I would like to know if it is possible to do requiredfieldvalidator purely in javascript.
I do not want my button click which processes the page to perform a postback, as postback completely resets my styling and there is too much to reload on ispostback.
I will however do the postback to the server to update SQL once all the fields have been completed.
<asp:UpdatePanel id="PurGradeUpdate" runat="server" RenderMode="Inline">
<ContentTemplate>
<telerik:RadComboBox ID="PURGrade" runat="server" Width="100" EmptyMessage=" ---" Font-Bold="true"></telerik:RadComboBox>
<asp:RequiredFieldValidator ID="PurGradeValidate" runat="server" Display="Dynamic" ControlToValidate="PURGrade" ErrorMessage="!!!" Font-Italic="true" Font-Bold="true" ForeColor="Red" InitialValue=" ---" EnableClientScript="true"></asp:RequiredFieldValidator>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="PURProduct" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
<telerik:RadButton ID="PurCreate" runat="server" AutoPostBack="false" OnClientClicked="PerformValidation"></telerik:RadButton>
I have the above in an update panel as the comboboxes are populated via SQL read, based on other comboboxes selected index changes.
I would now like to call that "PerformValidation" on my process button click and here the validation must happen, and if not valid, display the errormessage.
Thanks for the assistance.

Ok man i checked it on my VS and this works for sure:
<asp:UpdatePanel id="PurGradeUpdate" runat="server" RenderMode="Inline">
<ContentTemplate>
<asp:DropDownList ID="PURGrade" runat="server" Width="100" Font-Bold="true"></asp:DropDownList>
<asp:RequiredFieldValidator ID="PurGradeValidate" runat="server" Display="Dynamic" ControlToValidate="PURGrade" ErrorMessage="!!!" Font-Italic="true" Font-Bold="true" ForeColor="Red" InitialValue="---" EnableClientScript="true"></asp:RequiredFieldValidator>
<asp:DropDownList ID="PURProduct" runat="server" Width="100" Font-Bold="true"></asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="PURProduct" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>
<Asp:Button ID="PurCreate" runat="server" AutoPostBack="true"></Asp:Button>
In aspx.cs file I added default value:
PURGrade.Items.Insert(0, "---");
So as i told you before you don't need to use custom JavaScript commands to validate on client side cuz asp required field validator runs on client side by default. The only think that can cause problems in your code is custom controls you use. Be sure that your combo box empty value is what is really added to the select list as a default value.