Input string was not in correct format at textchange

Change the code a little to look like this:

private void txtRate_TextChanged()
{
   try   
  {
    if(txtRate.Text.Trim() == string.Empty) return;
    decimal qty;   
    decimal price;   
    decimal amt = 0;   
    qty = Convert.ToDecimal(txtQuantity.Text);  
    price = Convert.ToDecimal(txtRate.Text);  
    amt = qty * price;  
    txtAmount.Text = amt.ToString();    
  }   
  catch (Exception ex)   
  {   
    MessageBox.Show(ex.Message);   
  }
}

This way, you return out of txtRate_TextChanged() event each time txtRate is empty.

Problem : TextChanged Event will be fired everytime the Textbox value changes. it will be fired even if you clear the TextBox value and leads to the exception.

Solution 1: you can unsubscribe from the TextChanged Event before clearing the Text Feilds and subscribe to it again after clearing the TextFeilds.so that while clearing the TextBox values it can not call the TextChanged Event as there is no subscribed event handler for TextChanged.

Try This:

txtRate.TextChanged -= new System.EventHandler(txtRate_TextChanged);  

txtProdSerName.Text = "";  
txtQuantity.Text = "";   
txtRate.Text = "";   
txtAmount.Text = ""; 

txtRate.TextChanged += new System.EventHandler(txtRate_TextChanged);

Solution 2: you can create a boolean variable and check if it is really a TextChanged Event.

Step 1 : Create Boolean variable and initialise it to true.
Step 2 : change the Boolean variable value to false before clearing the Textbox values.
Step 3 : change the Boolean variable back to true after clearing the Textbox values.
Step 4 : execute your TextChanged event handler code only if the Boolean variable is true.

Try This:

private bool isTextChangedEvent = true;
private void txtRate_TextChanged(object sender, EventArgs e)   
{
  if(isTextChangedEvent)
  {
    try   
    {   
      decimal qty;   
      decimal price;   
      decimal amt = 0;   
      qty = Convert.ToDecimal(txtQuantity.Text);  
      price = Convert.ToDecimal(txtRate.Text);  
      amt = qty * price;  
      txtAmount.Text = amt.ToString();    
    }   
    catch (Exception ex)   
    {   
      MessageBox.Show(ex.Message);   
    }   
  }
}

while clearing TextBox values:

isTextChangedEvent = false;
txtProdSerName.Text = "";  
txtQuantity.Text = "";   
txtRate.Text = "";   
txtAmount.Text = ""; 
isTextChangedEvent = true;

Problem:

When you change the text it fires the event, and before that you are setting txtQuantity txtrate to "", and in that event you are using txtQuantity txtrate values(which are actually Empty now, so they can not be parsed to decimal) that's why when it tries to change the text of txtRate it finds txtQuantity value is "" (Empty) and throws the exception.

Solution:

Put your this code in a separate method:

private void changeTxtRateValue()
{
    try   
      {   
        decimal qty;   
        decimal price;   
        decimal amt = 0;   
        qty = Convert.ToDecimal(txtQuantity.Text);  
        price = Convert.ToDecimal(txtRate.Text);  
        amt = qty * price;  
        txtAmount.Text = amt.ToString();    
      }   
      catch (Exception ex)   
      {   
        MessageBox.Show(ex.Message);   
      }
}

and call the method only when txtQuantity is not Empty:

private void txtRate_TextChanged(object sender, EventArgs e)   
{
    if(txtQuantity.Text != String.Empty)
        changeTxtRateValue();
}

Best Practise Advice:

Don't use "" to set an string empty, because it is not empty.

Always use String.Empty.

Use this code:

txtProdSerName.Text = String.Empty;  
txtQuantity.Text = String.Empty;   
txtRate.Text = String.Empty;   
txtAmount.Text = String.Empty;

It'll solve your problem.

You are not using the right control in your application. Please use NumericUpDown control instead.