Question

private void txtOctet1_TextChanged(object sender, EventArgs e)
        {
            double numCheck1;
            if (txtOctet1.Text == "")
            {
            }
            else
            {
                numCheck1 = Convert.ToDouble(txtOctet1.Text);
                if (numCheck1 < 0 | numCheck1 > 255)
                {
                    btnSubnetting.Enabled = false;
                    lblOctet1Error.Text = "Error";
                    lblOctet1Error.BackColor = Color.Red;
                    lblOctet1Error.ForeColor = Color.White;
                }
                else
                {
                    btnSubnetting.Enabled = true;
                    lblOctet1Error.Text = "No Error";
                    lblOctet1Error.BackColor = Color.White;
                    lblOctet1Error.ForeColor = Color.Black;
                }
            }
        }

I have made a decimal to binary converter in C#. This users a class made by me. The user enters their “IP address” in four textboxes (one for each Octet). The above code does work but I don’t want to have to repeat the above code for the other third Octet input textboxes. How would I manage this (if it is possible)

Answer 1

Instead of showing you the solution, let’s play with VS a bit :

1. Introduce two variables that hold the textbox and label references and replace all use below :

private void txtOctet1_TextChanged(object sender, EventArgs e)
{
    double numCheck1;
    TextBox txtToValidate = txtOctet1; // Variable 1
    Label lblError = lblOctet1Error; // Variable 2

/* Select from here in the next step */ 
    if (txtToValidate.Text == "") // Here, txtOctet1 replaced
    {
    }
    else
    {
        numCheck1 = Convert.ToDouble(txtToValidate.Text); // Here, txtOctet1 replaced
        if (numCheck1 < 0 | numCheck1 > 255)
        {
            btnSubnetting.Enabled = false;
            lblError.Text = "Error"; // Here, lblOctet1Error replaced
            lblError.BackColor = Color.Red; // Here, lblOctet1Error replaced
            lblError.ForeColor = Color.White; // Here, lblOctet1Error replaced
        }
        else
        {
            btnSubnetting.Enabled = true;
            lblError.Text = "No Error"; // Here, lblOctet1Error replaced
            lblError.BackColor = Color.White; // Here, lblOctet1Error replaced
            lblError.ForeColor = Color.Black; // Here, lblOctet1Error replaced
        }
    }
/* Select to here in the next step */ 
}

No progress for the moment

2. Select the code you want to reuse

with the mouse or the keyboard, select all code between the two comments place holder I’ve put. Basically, you should have wrapped up the outermost if/else statement.

3.a Extract to a method

Right click the select code, choose Refactor,Extract to a method.

Choose a name for your method, ValidateOctet for example.

Validate. You should have extracted the logic in a custom method :

private void txtOctet1_TextChanged(object sender, EventArgs e)
{
    double numCheck1;
    TextBox txtToValidate = txtOctet1; // Variable 1
    Label lblError = lblOctet1Error; // Variable 2
    ValidateOctet(txtToValidate, lblError);
    }

    private void ValidateOctet(TextBox txtToValidate, Label lblError)
    {
    if (txtToValidate.Text == "") // Here, txtOctet1 replaced
    {
    }
    else
    {
        numCheck1 = Convert.ToDouble(txtToValidate.Text); // Here, txtOctet1 replaced
        if (numCheck1 < 0 | numCheck1 > 255)
        {
            btnSubnetting.Enabled = false;
            lblError.Text = "Error"; // Here, lblOctet1Error replaced
            lblError.BackColor = Color.Red; // Here, lblOctet1Error replaced
            lblError.ForeColor = Color.White; // Here, lblOctet1Error replaced
        }
        else
        {
            btnSubnetting.Enabled = true;
            lblError.Text = "No Error"; // Here, lblOctet1Error replaced
            lblError.BackColor = Color.White; // Here, lblOctet1Error replaced
            lblError.ForeColor = Color.Black; // Here, lblOctet1Error replaced
        }
    }    
}

Still no visible progress

3.b (Optionnal) remove useless variable

I choose to simplify the code by removing the variable we created before. I can call the method directly with the textbox and label reference. Choosing to keep the variable or not is a matter of code styling.

private void txtOctet1_TextChanged(object sender, EventArgs e)
{
    double numCheck1;
    ValidateOctet(txtOctet1, lblOctet1Error);
    }

4. Reuse the method for all textboxes

Simply call ValidateOctet for all couple of Textbox/Label :

private void txtOctet1_TextChanged(object sender, EventArgs e)
{
    double numCheck1;
    ValidateOctet(txtOctet1, lblOctet1Error);
    ValidateOctet(txtOctet2, lblOctet2Error);
    ValidateOctet(txtOctet3, lblOctet3Error);
    ValidateOctet(txtOctet4, lblOctet4Error);
    }

Progress: you have now the logic defined in one place

5. Possible optimization

1. Take a look at other’s answer. Some will probably help you.
2. There is a System.Net.IPAdress class. That provides a set of methods to play with IP Addresses (notably the TryParse method).
3. Don’t know you business requirement, but your code won’t support IP V6. We are at the door of (at least) deployment of IP V6. Maybe you should use only one textbox + the IPAddress class to avoid future limitations.
4. If you intend to have several IP Address fields, you should think about wrapping all the UI and the logic in a reusable UserControl.

Disclaimer

Please not that my answer was not targeted to find the better solution, but help you with using Visual Studio and it’s refactoring features. It’s quite common to start with simple things, to prototype, or playing a bit before the actual implementation. Refactoring tools allows to simply redesign some parts of your code.