if you are calling the managed code then
Assembly SampleAssembly = Assembly.LoadFrom(filename);
Type myType = SampleAssembly.GetTypes()[0];
MethodInfo Method = myType.GetMethod("myVoid");
object myInstance = Activator.CreateInstance(myType);
Method.Invoke(myInstance, null);
and if you are calling unmanaged code then
Calling methods from Dll compiled in 'C' from C#[
^]