Symmetric encryption algorithm function

Question

To start off, I am still learning object oriented programming. Ok, I have a combo box with the different types of symmetric algorithms.

private void Form3_Load(object sender, EventArgs e)
{
    openencrypt();
    comboBox1.Items.Add("AES");
    comboBox1.Items.Add("DES");
    comboBox1.Items.Add("Rijndael");
    comboBox1.Items.Add("RC2");
    comboBox1.Items.Add("Triple DES");
    comboBox1.SelectedIndex = 0;
}

Then I have my encrypt function checking what types they are.

byte[] hpass;
string nFilepath = Set.nfilepath;
FileStream Open = new FileStream(oFilepath, FileMode.Open, FileAccess.Read);
FileStream Save = new FileStream(nFilepath, FileMode.OpenOrCreate, FileAccess.Write);
SHA512 sh512 = new SHA512Managed();
hpass = sh512.ComputeHash(Encoding.ASCII.GetBytes(textBox1.Text));
PasswordDeriveBytes pdb = new PasswordDeriveBytes(hpass, hash);

if (comboBox1.SelectedIndex.Equals(0))
{
    Aes alg = Aes.Create();
    alg.Key = pdb.GetBytes(32);
    alg.IV = pdb.GetBytes(16);
}
if (comboBox1.SelectedIndex.Equals(1))
{
    DES alg = DES.Create();
    alg.Key = pdb.GetBytes(32);
    alg.IV = pdb.GetBytes(16);
}
if (comboBox1.SelectedIndex.Equals(2))
{
    Rijndael alg = Rijndael.Create();
    alg.Key = pdb.GetBytes(32);
    alg.IV = pdb.GetBytes(16);
}

But when I don't want to have to put a crypto stream in each if statement. So is there a way to offload the checking to a function and return a symmetricalgorithm type? With the Key and IV? Am I going about this completely wrong?## Heading ##

Answer 1

A more object oriented approach would be:

Create an algorithm interface to be displayed in your combo box:

public interface IAlgorithmItem
{
    SymmetricAlgorithm CreateAlgorithm();

    string DisplayName { get; }
}

Then, create a new class for each desired algorithm:

public class AesAlgorithm : IAlgorithmItem
{
    public AesAlgorithm()
    {
    }

    public SymmetricAlgorithm CreateAlgorithm()
    {
        return Aes.Create();
    }

    public string DisplayName
    {
        get { return "AES"; }
    }
}

public class RijndaelAlgorithm : IAlgorithmItem
{
    public SymmetricAlgorithm CreateAlgorithm()
    {
        return Rijndael.Create(); 
    }

    public string DisplayName
    {
        get { return "Rijndael"; }
    }
}

// ...

Then, you can create a new list of items:

var listItems = new List<IAlgorithmItem>() { new AesAlgorithm(), new RijndaelAlgorithm() };

Then you can bind your combo box to this list:

comboBox1.DataSource = listItems;
comboBox1.DisplayMember = "DisplayName";

Later, you can reference the selected item:

var algorithmItem = (IAlgorithmItem)comboBox1.SelectedItem;
var algorithm = algorithmItem.CreateAlgorithm();

EDIT: Updated with Will's suggestion of using an interface rather than an abstract base class. EDIT 2: Updated to use a create method rather than property, as the result of the operation will create a new algorithm each time it is accessed.