void Button1Click(object sender, EventArgs e) { double d=0; string b=textBox2.Text.ToString(); double a=double.Parse(textBox1.Text); double c=double.Parse(textBox3.Text); if("b"=="+") d=a+c; else if("b"=="-") d=a-c; else if("b"=="*") d=a*c; else if("b"=...