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when i am run the programme its shows the error.....object refence not set

What I have tried:

public void Multiply()
           foreach (GridViewRow row in GridView1.Rows)
               TextBox TextBox1 = (TextBox)row.FindControl("SrNO");
               TextBox TextBox2 = (TextBox)row.FindControl("productname");
               TextBox TextBox3 = (TextBox)row.FindControl("Quntity");
               TextBox TextBox4 = (TextBox)row.FindControl("Rate");
               TextBox TextBox5 = (TextBox)row.FindControl("Total");

             //  string SrNO = TextBox1.Text;
              // string ProductID = TextBox2.Text;
            //   string Quntity = TextBox3.Text;
            //   string Rate = TextBox4.Text;
            //   string TotalAmount = TextBox5.Text;

               int a, b;
               bool isAValid = int.TryParse(TextBox3.Text, out a);
               bool isBValid = int.TryParse(TextBox4.Text, out b);

               if (isAValid && isBValid)
                   TextBox5.Text = (a * b).ToString();
                   TextBox5.Text = "Invalid input";
Updated 2-Oct-20 18:19pm
Richard MacCutchan 2-Oct-20 8:49am
Are you sure your TextBoxes are in the GridView control?
F-ES Sitecore 2-Oct-20 9:06am
What line throws the error?

Details about the error you see: NullReferenceException Class (System) | Microsoft Docs[^]
A NullReferenceException exception is thrown when you try to access a member on a type whose value is null

Possibly because:
1. You've forgotten to instantiate a reference type
2. You've forgotten to dimension an array before initializing it.
3. You get a null return value from a method, and then call a method on the returned type.
4. You're using an expression (for example, you're chaining a list of methods or properties together) to retrieve a value
5. You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws
6. Exception is thrown by a method that is passed null

Following will help you learn about debugging:
Tutorial: Debug Visual Basic code - Visual Studio | Microsoft Docs[^]
First look at the debugger - Visual Studio | Microsoft Docs[^]

When you debug, you will be able to get the exact line where the variable is NULL and error is being raised.
For you I am suspecting error from below portion of code:
TextBox TextBox1 = (TextBox)row.FindControl("SrNO");
TextBox TextBox2 = (TextBox)row.FindControl("productname");
TextBox TextBox3 = (TextBox)row.FindControl("Quntity");
TextBox TextBox4 = (TextBox)row.FindControl("Rate");
TextBox TextBox5 = (TextBox)row.FindControl("Total");
If any one findcontrol fails (because of incorrect case of name or so), followup code expecting the textbox as valid control will bomb while using .Text property. Though, given you seem new and have not shared either error stack trace or entire code, error could be coming from elsewhere.

Try out.
Member 14954114 2-Oct-20 8:23am
noits not working bro plz add the solution
Chris Copeland 2-Oct-20 8:46am
As Sandeep Mewara mentioned, have you tried debugging the code and stepping through it line-by-line to see which variables are null? That's not something we can do for you, you have to do that yourself.
Sandeep Mewara 2-Oct-20 9:14am
@OriginalGriff explains like below:

You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and your IDE will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we can't run your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out! 
BillWoodruff 2-Oct-20 23:42pm
If you quoted this:

You could have even more words-per-week.
BillWoodruff 2-Oct-20 23:23pm
From my point of view, cut-and-paste responses like this, with no direct relevance to the OP's context, add no value to the discussion. They just contribute to the bloat that clogs QA. More words do not necessarily mean more value.
are you sure that each Row is a DataRow ?

// row.RowType == ch .DataRow;

Error check your 'FindControl results:
TextBox TextBox1 = (TextBox)row.FindControl("SrNO");
if(TextBox1 == null)
     throw new NullReferenceException(
Or, wrap the access in a Try-Catch block:
   TextBox TextBox1 = (TextBox)row.FindControl("SrNO");
   TextBox TextBox2 = (TextBox)row.FindControl("productname");
   TextBox TextBox3 = (TextBox)row.FindControl("Quntity");
   TextBox TextBox4 = (TextBox)row.FindControl("Rate");
   TextBox TextBox5 = (TextBox)row.FindControl("Total");
catch (NullReferenceExceptione nre)
    Console.WriteLine($"Control not found: '{row.Name}'");
See: [^], [^]
Otekpo Emmanuel 3-Oct-20 0:24am
If you are having errors that reference not set to an object, then its obvious that that you are using the wrong name to reference a particular TextBox on the GridView control.

So the best and quickest solution would be to check for each TextBox one at a time to see the one that will throw the error
BillWoodruff 3-Oct-20 0:26am
The code I posted shows how to do that.

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