Referece
Unity
Thanks for GongFaxin, You can download his Code protobuf_for_unity, and Run in Unity 5.3, you can see a lot of Protobuf source code.
Protobuf
Download the tools in Protocol Buffers v3.0.0 Download link
Then, follow the Google Tutorial.
protoc -I=$SRC_DIR --java_out=$DST_DIR $SRC_DIR/addressbook.proto
protoc -I=$SRC_DIR --csharp_out=$DST_DIR $SRC_DIR/addressbook.proto
Then you can get the Java code or C# code.
Java
you will need the Jar: protobuf-java-3.0.0.jar