Obtain non-explicit field offset

I have the following class:

[StructLayout(LayoutKind.Sequential)]
class Class
{
    public int Field1;
    public byte Field2;
    public short? Field3;
    public bool Field4;
}

How can I get the byte offset of Field4 starting from the start of the class data (or object header)?
To illustrate:

Class cls = new Class();
fixed(int* ptr1 = &cls.Field1) //first field
fixed(bool* ptr2 = &cls.Field4) //requested field
{
    Console.WriteLine((byte*)ptr2-(byte*)ptr1);
}

The resulting offset is, in this case, 5, because the runtime actually moves the generic Field3's data to the end of the type (and pads it).

How can I retrieve this offset? Is there any .NET method used for that, or do I have to write my own, taking all the exceptions into account (type size, padding, explicit offsets, etc.)?