I am trying to populate Amount text field auto

Question

I have setup already

 

"When i select the item the Rate populates automaticaly" 

 

Trying to Engineer?

 

As ItemName and Rate is populated in if i enter the Quantity Manualy with Leave event of Quantity Text box it should pouplate the Amount of the item automaticaly i have write the code but its not happening i put the breakpoint its going to

 

try

{

amountValue.Text = Convert.ToString(Convert.ToInt32(salePrice.Text) * Convert.ToInt32(qtyValue.Text));

}

 

but after that its entering to 

 

catch(Exception ex)

{

MessageBox.Show("Error");

 

and giving me Error dialog box 

 

try please let me know if i am making any mistake.