You can use explicit field layouts for that:
[StructLayout(LayoutKind.Explicit)]
public struct SampleUnion
{
[FieldOffset(0)] public float bar;
[FieldOffset(4)] public int killroy;
[FieldOffset(4)] public float fubar;
}
Untested. The idea is that two variables have the same position in your struct. You can of course only use one of them.
More informations about unions in struct tutorial