gistfile1.cs

[ToolboxItemAttribute(false)]
    public class HelloWorldWebPart : WebPart {

        TextBox _firstName;
        TextBox _lastName;
        Button _sayHello;
        Label _result;

        protected override void CreateChildControls() {
            _firstName = new TextBox() { ID = "txtFirstName" };
            this.Controls.Add(_firstName);

            _lastName = new TextBox() { ID = "txtLastName" };
            this.Controls.Add(_lastName);

            _sayHello = new Button() { ID = "btnSayHello", Text = "Say Hello" };
            _sayHello.Click += new EventHandler(_sayHello_Click);
            this.Controls.Add(_sayHello);

            _result = new Label() { ID = "lblResult" };
            this.Controls.Add(_result);
        }

        void _sayHello_Click(object sender, EventArgs e) {
            _result.Text = "Hello " + _firstName.Text + " " + _lastName.Text;
        }
    }