The building of dll file in c# is very essay & simple. But it's difficult for beginner.
No doubt that "Every one among pro was a beginner once."

Let i describe the way.

First open a new project as "ClassLibrary" make a class file which will be build as dll file after compiling.

 public class BasicCalculation
    {
        public Int32 add(Int32 a, Int32 b)
        {
            return (a + b);
        }
        public Int32 substract(Int32 a, Int32 b)
        {
            return (a - b);
        }
        public Int32 multi(Int32 a, Int32 b)
        {
            return (a * b);
        }
        public Int32 divid(Int32 a, Int32 b)
        {
            return (a / b);
        }
    }

Here i make 4 very simple method.

Now we build it as "MyCalculation.dll"

Now we look for the main application.

we must reference our dll file first. The way is Project->Add Reference.
For best way put your dll file in .../bin/debug folder.

then must be use: using MyCalculation;

now for add button the code is following.
 using MyCalculation;
 .....

private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                Int32 x = Convert.ToInt32(textBox1.Text);
                Int32 y = Convert.ToInt32(textBox2.Text);
                // Call dll
                MyCalculation.BasicCalculation madd = new BasicCalculation();//namespace.class
                Int32 z = madd.add(x, y);//class.method
                string ans = Convert.ToString(z);
                textBox3.Text = ans;
            }
            catch
            {
                textBox3.Text = "Wrong Input!";
            }
        }

Now run the program. And you get your recommended out put with the help of dll.
i upload the sample example. Hope you will now make dll file, and say wow it's very essay as it is. :-)

Thank you!