C＃：为什么Networkstream.Read（）能够在没有out / ref关键字的情况下更改缓冲区变量

Question

Why can NetworkStream.Read() write to the byte[]?

byte[] data = new byte[16];

NetworkStream ns = new NetworkStream(socket);
ns.Read(data, 0, data.Length);

//data != new byte[16]

I thought you need a out/ref keyword to write to a variable. like this:

ns.Read(out data, 0, data.Length);

If i try to recreate that method it doesnt work:

public static void testread(byte[] buffer, int size)
{
    byte[] data = new byte[size];
    for (int i = 0; i < data.Length; i++)
    {
        data[i] = 1;
    }
    buffer = data;
}

byte[] data = new byte[16];
testread(data, data.Length);

//data == new byte[16]

But if I add the "out" keyword to testread() does work:

public static void testread(out byte[] buffer, int size)
{
    byte[] data = new byte[size];
    for (int i = 0; i < data.Length; i++)
    {
        data[i] = 1;
    }
    buffer = data;
}

byte[] data = new byte[16];
testread(data, data.Length);

//data != new byte[16]

This proofs that you cant write to a variable without the "out"/"ref" keyword. But how does NetworkStream write to the byte[] without the "out"/"ref" keyword? Scary..

Answer 1

This proofs that you cant write to a variable without the "out"/"ref" keyword.

You need to mentally separate two very different concepts:

• changing the state of an object
• reassigning the value of a variable (parameter, local, field, etc)

(noting that in the case of objects, the latter means changing the object that it refers to)

out is only needed for the second of these. Stream allows the caller to pass in an object (the array), and it writes into the array. No out is needed for this. It is no different to:

void SetCustomerName(Customer obj) { // for class Customer
    obj.Name = "Fred";
}
...
var x = new Customer();
SetCustomerName(x);
Console.WriteLine(x.Name); // prints "Fred"

This updates the object, but doesn't need to change the parameter to do so. It changes the object that the parameter points to .

Answer 2

Most likely because it doesn't do the buffer = data assignment. Instead it reads directly into the buffer passed as the argument, ie if you do buffer[i] = 1 in your loop you emulate it.