contract greeter {
    event LogSender(address sender);
    function getSender() constant returns (address) {
       LogSender(msg.sender);
       return msg.sender;
   }
}

try
{
    Web3 web3 = new Web3(new ManagedAccount(txtAddress.Text, txtPassword.Text), "http://localhost:8545");

    GreeterService gs = new GreeterService(web3, txtContractAddress.Text);

    var res = await gs.GetSenderAsyncCall();

    txtLog.AppendText("getSender: " + res + "n");
}
catch(Exception ex)
{
    txtLog.AppendText(ex.Message + "n");
}