string Passwordeller = "";
char delPasseller=Convert.ToChar(" ") ;
Random randomNumber = new Random();

for (int i = 0; i < 7; i++)
{
int intNum = randomNumber.Next(48, 57);
int intBig = randomNumber.Next(65, 90);
int intSmall = randomNumber.Next(97, 122);
int intRand = randomNumber.Next(1, 3);

switch (intRand)
{
case 1:
delPasseller = Convert.ToChar(intNum);
break;
case 2:
delPasseller = Convert.ToChar(intBig);
break;
case 3:
delPasseller = Convert.ToChar(intSmall);
break;
}
Passwordeller += delPasseller.ToString();
}
TextBoxLosen.Text = Passwordeller.Trim();