if (someflag == true)
{
if (lblsomelbl.InvokeRequired)
lblsomelbl.Invoke((MethodInvoker) delegate { lblsomelbl.Text = "1234"; });
else
lblsomelbl.Text = "1234";
}
Basically you have to change properties that interact with the UI on the same thread that the UI runs on. Using the Control.Invoke method will do that. Changing things on that other threads "own" is called cross-threading.