こちらとか｡
https://dotnetcodr.com/2016/12/11/messaging-through-memory-mapped-files-in-net-c-2/
http://devlights.hatenablog.com/entry/20101123/p1


文字列の大きさを多めに取っておくのが嫌なら､ｻｲｽﾞも書いておく｡

 # Write
 byte[] Buffer = Encoding.UTF8.GetBytes("Hello");
 MemoryMappedFile mmf = MemoryMappedFile.CreateNew("test", 1000);
 MemoryMappedViewAccessor accessor = mmf.CreateViewAccessor();
 accessor.Write(0, (UInt16)Buffer.Length);
 accessor.WriteArray(sizeof(UInt16), Buffer, 0, Buffer.Length);


 # Read
 MemoryMappedFile mmf = MemoryMappedFile.CreateNew("test", 1000);
 MemoryMappedViewAccessor accessor = mmf.CreateViewAccessor();
 ushort Size = accessor.ReadUInt16(0);
 byte[] Buffer = new byte[Size];
 accessor.ReadArray(sizeof(UInt16), Buffer, 0, Buffer.Length); 
 MessageBox.Show(Encoding.UTF8.GetString(Buffer));

http://stackoverflow.com/questions/10806518/write-string-data-to-memorymappedfile

でも､文字列より後に続くﾃﾞｰﾀがあると位置が変わっちゃうので､大きめに取っておく方が無難｡
MMF に置いておくﾃﾞｰﾀを､まるっと構造体で持っておいた方が位置を気にしなくて良いので楽なのは､C# も同じです｡