[Flags]
public enum enByteEnum : byte
{
a = 0x80,
b = 0x40,
c = 0x20,
d = 0x10,
e = 0x08,
f = 0x04,
g = 0x02,
h = 0x01,
}

enByteEnum test = new enByteEnum();  // "Leere" Enum erzeugen
 
if (Enum.Parse(typeof(enByteEnum), this.combobox1.SelectedItem.ToString(), true).Equals("Wert"))
test |= enByteEnum.a; // Bit setzen
else
test &= ~enByteEnum.a; // Bit entfernen

if (((enByteEnum)(test2) & enByteEnum.a) == enByteEnum.a)
this.txtBox1.Text = "a aktiviert";
else
this.txtBox1.Text = "a deaktiviert";