C# field offset and length in struct?

I can easily specify offset of fields:

[StructLayout(LayoutKind.Explicit)]
public struct MyMarshaled
{
    [FieldOffset(0)]
    public int Version;
    [FieldOffset(3)]
    public int SubVersion;
}

but how I can specify field lengths in bit representation? For example like this:

[StructLayout(LayoutKind.Explicit)]
public struct MyMarshaled
{
    [FieldOffset(0, lenght=3)]
    public int Version;
    [FieldOffset(3, length=5)]
    public int SubVersion;
}

So, given the byte bitmap 00100001 it will print out struct with value '1' in both fields, i.e 001 is for Version, and 00001 is for SubVersion.