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I want to use Unity3d develope my game client, but in server side use my own C++ Project,the problem is how can i communicate with my server . Unity3d has Network class , it can connect to a server use tcp , but has not "SendData" function , so my question is ,if i want to send a message to the server, how can i write code , and when i received data from server , how can i devide it to defferent messages.

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I've just started looking at Unity's networking capability, so this isn't authoritative, but might get you started. First, you are not limited to just the Unity Network class. From the docs:

".NET socket libraries can be used for real-time networking, by opening TCP/IP sockets or sending UDP messages."

So on the Unity Client side, you would just write a C# script, that called the various .NET socket functions. Here are two links I found that might be useful:

TCP/IP Client-Server Basics

and

Sockets in C#

Update: for some sample code, see this Answer for TCP Client code