messagebox in asp.net facing exception

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System.Web.HttpContext.Current.Response.Write("<script>alert('Item Already Exists')</script>");

in asp.net c# i am using this or showung messagebox but its giving a exception that is
Object reference not set to an instance of an object.

I also used

ScriptManager.RegisterClientScriptBlock(this,this.GetType(), "alert", "alert('Your Message');", true);

but i think its requried control event because of this.GetType() and i am not using control for this it is a simple user defined function in which I want to show messagebox,
plz tell me where i did make mistek. thank you.

Posted 30-Aug-15 3:22am

Member 11922776

Updated 30-Aug-15 3:23am

v2

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Abhipal Singh 30-Aug-15 9:31am

System.Web.HttpContext.Current.Response
one of the object in the above statement is null because of which you are getting the error. Debug and see which one is null.

Member 11922776 30-Aug-15 9:53am

means is there need to write script at page.aspx soure code? or <script>alert('Item Already Exists')</script> this is enough. because i did not written at source file.

F-ES Sitecore 30-Aug-15 10:36am

Your code works for me so there is something about the context you're not telling us.

Also just so you know, the code isn't showing a messagebox from server code, it is generating javascript that will show a message when the page has finished rendering, so you can't use this as a "click to continue" thing.

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 30-Aug-15 11:26am

Can you debug and see where exactly you saw this exception?

Member 11922776 31-Aug-15 0:29am

Keyword 'this' is not valid in a static property, static method, or static field initializer this error is fired when execute this line ScriptManager.RegisterClientScriptBlock(this,this.GetType(), "alert", "alert('Your Message');", true);

and about below line

System.Web.HttpContext.Current.Response.Write("<script>alert('Item Already Exists')</script>");

i did not use any variable. what is alert('Item Already Exists') is alert is user define fuction and must be define esle where?

and at this line exception is at:

System.Web.HttpContext.Current <= at this point its show null.

thanx for u r value able time.

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 0:43am

Are you trying to do this inside a Web Method?

Member 11922776 31-Aug-15 0:55am

yes inside a Web Method
public static void QueryFunModbusRTU()

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 1:05am

Refer my answer.

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-08-30T09:47:00

You did not show where the exception with the message "Object reference not set to an instance of an object" is thrown.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object".

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx,
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx.

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx.

Good luck,

—SA

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) · Answer 2 · 2015-08-30T19:02:00

Solution 2

Quote:
yes inside a Web Method
public static void QueryFunModbusRTU()

That is the reason why it is coming.

Do one thing. Don't do like this to show an alert. WebMethods are designed to take some paramter and return you something after some operation. So, after all your operation, just return a value or something. Then in client side Ajax success, read that value and show an alert. This is idea and perfect way.

Posted 30-Aug-15 19:02pm

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ)

Comments

Member 11922776 31-Aug-15 1:06am

sorry i m not getting, may you post the code plz. thanx

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 1:08am

I am saying that don't put that code inside the web method. You might have a Ajax call to call this web method. So inside the ajax success event, show the alert. As simple as that.

Member 11922776 31-Aug-15 1:19am

please tell me how to do that.
and Ajax call?

Member 11922776 31-Aug-15 1:35am

i find it. but i did't want to use any event like clink. I want to show message through web-methode only. is there any other way to do this?

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 2:20am

That is a wrong approach. You can;t do that. As I said, the simplest way is to do it inside ajax success event.

Member 11922776 31-Aug-15 2:36am

simply i took label and show message in it. but still its firing a error of
"Object reference not set to an instance of an object."

i am accessing label from different webform. like this

Trends fc = new Trends();
string str = "Check device";
fc.Label3.Text = str;

where Trends is a other from in which label3 declared.

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 2:40am

As I said this is wrong approach. You can't access controls inside the WebMethod.

Why are you not trying the way I suggested? Please show the alert inside the ajax success.

Member 11922776 31-Aug-15 2:46am

please tell me ajax success. how to use.

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 3:18am

Refer - Call ASP.Net Page Method using jQuery AJAX Example

Member 11922776 31-Aug-15 5:50am

hi,

there are two web methods 1)Createthread 2)QueryFunModbusRTU which are call on click event of public void Reconfigure_Done_Editing_click(object sender, EventArgs e)

first Createthread() this web method is called and then this method call second web method which is QueryFunModbusRTU now in this method contain if else and i want show message in else part.

i tried to implemet javascript/jquery/ajax call for alert message, but question is this javascript function also required click event to execute we can't call the javascript from web method i think, if we can then plz tell how? or any other solution plz tell same.

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 31-Aug-15 6:49am

How are you calling the WebMethod? You have a Ajax call right? Please post that code here.

Member 11922776 1-Sep-15 1:24am

[source code.aspx]
<div class="popup_Buttons">
<input id="Reconfigure_Done_Editing" value="Done" type="button" style="visibility: hidden; width: 10px;" />
<asp:Button ID="Button5" runat="server" Text="Done Editing" OnClick="Reconfigure_Done_Editing_click" />
<input id="Reconfigure_panel_Button_Cancel" value="Cancel" type="button" />
</div>

.cs code:

#region Reconfiguration
public void Reconfigure_Done_Editing_click(object sender, EventArgs e)
{

Createthread();
}

[Web Method]
public static void CreateThread()
{
QueryFunModbusRTU();
}

#region Modbus_RTU
[Web Method]
public static void QueryFunModbusRTU()
{
if()
{

}
else
{
*HERE I WANT TO SHOW MESSAGEBOX
}
}

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 1-Sep-15 4:19am

Why don't you directly call the method "QueryFunModbusRTU"? And after that call, just show an alert in JavaScript.

Member 11922776 2-Sep-15 8:29am

i can't, because all depend on each other, must follows flow.

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 2-Sep-15 15:25pm

I don't see any dependencies in your code.

Member 11922776 3-Sep-15 0:07am

to call QueryFunModbusRTU method directly i have to use any event like button click that's what you said,Right? to call this by click event i have to fire click_event directly. but i have to first execute Reconfigure_Done_Editing_click method,then CreateThread() without user interference and then QueryFunModbusRTU() this one. is u want to say that call click event of button in method Createthread() then problem is again same how to call event in web-method?

Tadit Dash (ତଡିତ୍ କୁମାର ଦାଶ) 3-Sep-15 13:51pm

That's why I gave you the link to check, but I guess you have not. Never mind.

Here is the link - Call ASP.Net Page Method using jQuery AJAX Example

See carefully that on a button click, it is directly calling the WebMethod through ajax. That is what you need to do.