You could do something like append the real string as a byte[ ] to the end of the message and then use the bytes where the string reference was in the struct’s byte[ ] to store the length of the string. Then on the receiving end you would deserialize back the struct, set the MyStruct.someSrtring = new string(bytes appended to the message).

byte[] ToBytes(string ThisString)
{
byte[] StringBytes = System.Text.Encoding.UTF8.GetBytes(ThisString);
return StringBytes;
}

string FromBytes(byte[] TheseBytes)
{
string ThisString = new String(System.Text.Encoding.UTF8.GetChars(TheseBytes));
return ThisString;
}

..Of course, you may have to do some byte reversals depending on your data transmission format (such as over networks).

field 1 = bit 15 to 13
field 2 = bit 12 to 10
field 3 = bit 9 to 0

etc…

PD: Sorry my shitty English…