I found out a solution to put numbers in 2 textboxes, using a button to add these numbers and show the result in a label in C++ doing this. So a conversion to string is made as oogabooga said also:
I am using little of both C# and C++ to see the differences. My application is very dependant of fast calculations and thats why I really cant let go of C++ yet.
double Number1 = double::Parse(textBox3->Text);
double Number2 = double::Parse(textBox4->Text);
double result = 0;
result = Number1 + Number2;
resultlabel->Text = result.ToString();