Insert data into open file binarywriter/filestream

You could use BinaryReader to read bytes (and BinaryWriter to write them to a second file) until you reach the insertion point.

You could then use the appropriate BinaryWriter method to make the insertion (Write(Int16) for a short) and then carry on reading the existing bytes and writing them to the second file until you get to the end.

You'd then need to delete the first file and rename the second file to the first.

This is not as neat but has the advantage that you don't need to read the entire file into memory first.

I'll give this a go...

Is putting each byte into a list the only way or are there alternatives?

To insert a short, you can first convert it to a byte array (2 elements, of course) using BitConverter.GetBytes(Int16) and then insert those bytes, one after the other, into the List.

As it's a List of bytes, the index into the List will be the same as the offset into the file - whether this is expressed as decimal or hexadecimal.

So, to insert 65 at offset 0x1F, you'd do:

bytes.Insert(0x1F, (byte)65);

I don't mind reading it all into memory, it's only going to be around 4mb.

If I read it in byte by byte into a list, what if I want to add a short?.

Also, by reading it into a list as suggested, and then going to index 5 to insert, what if I only know the hex/decimal offset where I want to insert rather than the lists index?