C#

if (txtdisc.Text == null)
{
    ..
    Double txtdisc22013 = Convert.ToDouble(txtdisc.Text);

Double txtdisc22013 = Convert.ToDouble(null);

 if(string.IsNullorEmpty(txtdic.Text) != true)
 {
   //...
 }

or 

 if(txtdisc.Text != string.Empty)
 {
  //...
 }

Hi
First, like others mentioned, "if" and "else" are conditional statements, they are not loop.
Second, the textbox will return empty if it has no value assigned to it, so checking for null is redundant.
Third, this is a coding horror!

if (txtdisc.Text ==null)  This check is redundant therefore  the calls below will never be executed.
{
   txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;

   Double txtdisc22013 = Convert.ToDouble(txtdisc.Text); Converting a null to a nullable Double will 0.0;

   txttot22013 = txttot22013 - txtdisc22013; (Some value – 0.0)

   txttot.Text = txttot22013.ToString();
}
else
{
   txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;

   txttot.Text = txttot22013.ToString(); The value of "txttot22013" in the else conditions  is no different to the one in if condition.
}

if (txtvat.Text == null)
{
   MessageBox.Show("Enter Vat Amount"); This will never be called.
}
else
{
   Double txtvat22013 = Convert.ToDouble(txtvat.Text); Possible exception.

   txtvat22013 = txtvat22013 / 100;

   txtvat22013 = txtvat22013 * txttot22013;

   Double txtvatinclu22013 = txtvat22013 + txttot22013;

   txtvatinclu.Text = txtvatinclu22013.ToString();
}


If I rewrite your code, it may as follows:

txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;
txttot.Text = txttot22013.ToString();

if (string.IsNullOrEmpty(txtvat.Text))
{
    MessageBox.Show("Enter Vat Amount");
}
else
{
    double txtvat22013;
    if (Double.TryParse(txtvat.Text, txtvat22013))
    {
        Double txtvatinclu22013 = (txtvat22013 * txttot22013 / 100) + txttot22013;

        txtvatinclu.Text = txtvatinclu22013.ToString();
    }else
    {
    MessageBox.Show("Enter Valid Vat Amount");
    }
}

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classy_dog wrote:

1. when do you recommend connection pooling be setup?

classy_dog wrote:

2. Does it make a difference if this connection pooling is a web, desktop, console or some other C# application?

classy_dog wrote:

3. Can you point me to a reference and or some me some code how to setup the connection pooling?

IDbConnection.Close Method -- The Close method rolls back any pending transactions. It then releases the connection to the connection pool, or closes the connection if connection pooling is disabled.

PIEBALDconsult wrote:

My understanding is that the connection is returned to the pool when you close it; you don't need to dispose it immediately.

PIEBALDconsult wrote:

I keep a connection for the life of am application (some of which are Services that run 24/7 for months on end) and open/close as needed.

PIEBALDconsult wrote:

Instantiating the same connections millions of times a day is not a good idea.

jschell wrote:

"Close and Dispose are functionally equivalent

jschell wrote:

You should close it or dispose it when you are done with it in the functional
block

jschell wrote:

is rather insignificant

classy_dog wrote:

1. when do you recommend connection pooling be setup?

classy_dog wrote:

2. Does it make a difference if this connection pooling is a web, desktop, console or some other C# application?

classy_dog wrote:

3. Can you point me to a reference and or some me some code how to setup the connection pooling?