how to set data type in textbox window forms

I’m creating small app for private use. User will type an IP address in a TextBox and test whether it pings correctly.
It doesn’t work because the TextBox is a WinForms Control which can’t be converted into bytes.

Is there any way to set top-down type of data in this TexBox?

System.Text.Encoding.Default.GetBytes(ip_textbox.Text); 
OctetString community = new OctetString("public"); 
AgentParameters param = new AgentParameters(community); 
param.Version = SnmpVersion.Ver1; 
IpAddress agent = new IpAddress(ip_textbox.Text); 
UdpTarget target = new UdpTarget((IPAddress)agent, 161, 2000, 1); 
Pdu pdu = new Pdu(PduType.Get); 
pdu.VbList.Add(".1.3.6.1.4.1.2.1.1.2.0"); 
SnmpV1Packet result = (SnmpV1Packet)target.Request(pdu, param);