Null after onClick action - asp.net

Question

I'm starting with asp.net, but i'm not quite sure what am I doing wrong. Actually my first step in asp is to make a calculator but after 2h of looking for answer i've decided to ask you about that.
I have 10 buttons with [0,1,2...9], 4 action buttons [+,-,*,/] and one button to calculate [=]:
int firstNumber, secondNumber;
Button lastClicked;
bool isFirstNumberSet;
protected void Page_Load(object sender, EventArgs e)
{
firstNumber = 0;
isFirstNumberSet = false;
lastClicked = new Button();
}
public void addToTextbox(object sender, EventArgs e)
{
Button number = (Button)sender;
TextBox1.Text += number.Text;
}
public void doSomething(object sender, EventArgs e)
{
if (isFirstNumberSet.Equals(false)) firstNumber = int.Parse(TextBox1.Text);
else secondNumber = int.Parse(TextBox1.Text);
lastClicked = new Button();
lastClicked = (Button)sender;
isFirstNumberSet = true;
TextBox1.Text = "";
}
public void calculate(object sender, EventArgs e)
{
switch (lastClicked.ID)
{
case "ButtonPlus":
TextBox1.Text = (firstNumber + secondNumber).ToString();
isFirstNumberSet = false;
break;
case "ButtonMinus":
TextBox1.Text = (firstNumber - secondNumber).ToString();
isFirstNumberSet = false;
break;
case "ButtonDziel":
TextBox1.Text = (firstNumber / secondNumber).ToString();
isFirstNumberSet = false;
break;
case "ButtonMnoz":
TextBox1.Text = (firstNumber * secondNumber).ToString();
isFirstNumberSet = false;
break;
}
but when i do for example action like 2 + 2, and after that i hit "=". I get an exception about that code:
switch (lastClicked.ID)
It says that lastClicked is null, but how? after hit "+" this should save sender object to this variable. Am i wrong?

Answer 1

Your button variable assignment does not persist between postbacks the way you are expecting. When calculate fires upon your "=" keyhit, lastClicked is now unassigned on this new "round trip," and thus null. When you press one button once, then a second button the next time, there's no memory of the previous hit unless you store the value in a field that's captured in the data sent back to the server.
One approach you could explore involves storing the operation that's been selected into a hidden HTML field on your form, and then interrogate that upon hitting the calculate routine. That eliminates the need to try persisting the "lastClicked" button as you have.
Somewhere in your aspx, within your form, you could add:
<asp:HiddenField id="Operation" runat="server" />
Then, in your codebehind in response to one of your operator buttons:
Operation.Value = "Plus"; // or "+" or whatever is appropriate
That would then get sent back down to the client on the next trip, and then be part of the data sent back to the server when your calculate method is called in response to the "=" button click:
public void calculate(object sender, EventArgs e){
switch(Operation.Value){
case "Plus";
// do the "PLUS" thing
// and so on
... (snip)...
}