Add numbers in c#

Question

i have a numerical textbox which I need to add it\'s value to another number I have tried this code

String add = (mytextbox.Text + 2)

but

Answer 1

const int addend = 2; 
string myTextBoxText = mytextbox.Text;
var doubleArray = new double[myTextBoxText.ToCharArray().Length];
for (int index = 0; index < myTextBoxText.ToCharArray().Length; index++)
{
    doubleArray[index] = 
        Char.GetNumericValue(myTextBoxText.ToCharArray()[index]) 
        * (Math.Pow(10, (myTextBoxText.ToCharArray().Length - 1) - index));
}
string add  = 
    (doubleArray.Aggregate((term1, term2) => term1 + term2) + addend).ToString();