A basic addition using C# in Visual studio
Step 1: Open Visual Studio -> Create a New Project.
Step 2: Select Windows form App and Hit Next.
Step 3: Give a name of your choice, select the folder and click Create, wait a few moments while its created.
Step 4: Add Three labels and three text boxes, from Toolbox. if you cannot see Toolbox, Click View and select Toolbox.
Step 5: Place the following code by double clicking in the "button", or select the Design view
{
textBox3.Text = Convert.ToString(Convert.ToInt64(textBox1.Text) + Convert.ToInt64(textBox2.Text));
}
Step 6: Hit Enter to run

