C# 从字节中获取位

Question

I don't know why I am getting wrong bits when reading my byte. I have followed some indications found here, bit it is not working. I show you my code:

byte[] byte22And23 = _packet.ReadBytes(2);
DataFromGTAPackets.Packet8.ControlFlags.rightEngineSmoke = _packet.GetBit(byte22And23[0], 0);
DataFromGTAPackets.Packet8.ControlFlags.leftEngineSmoke = _packet.GetBit(byte22And23[0], 1);
DataFromGTAPackets.Packet8.ControlFlags.rightEngineFire = _packet.GetBit(byte22And23[0], 2);
DataFromGTAPackets.Packet8.ControlFlags.leftEngineFire = _packet.GetBit(byte22And23[0], 3);
DataFromGTAPackets.Packet8.ControlFlags.rightRotorFail = _packet.GetBit(byte22And23[0], 4);
DataFromGTAPackets.Packet8.ControlFlags.leftRotorFail = _packet.GetBit(byte22And23[0], 5);
etc...

public bool GetBit(byte b, int bitNumber)
    {
        bool bit = (b & (1 << bitNumber - 1)) != 0;
        return bit;
    }

My byte 22 has a value of 2, it is: 00000010. And my byte 23 has a value of 0. When I use this code to read byte 22, my 3rd variable (rightEngineFire) is getting "true" (1). It has no sense, it is wrong obviously. But I don't know what is wrong.

Thank you!

Answer 1

You GetBit method considers bit numbers to be 1...32, instead of being 0...31.

Simple test:

bool b1 = GetBit(1, 0); // false
bool b2 = GetBit(1, 1); // true

You should change the method to

public static bool GetBit(byte b, int bitNumber)
{
    bool bit = (b & (1 << bitNumber)) != 0;
    return bit;
}

Or you could write:

DataFromGTAPackets.Packet8.ControlFlags.rightEngineSmoke = _packet.GetBit(byte22And23[0], 1);

But in C-derived languages (and C# is a C-derived language) we count from 0, so the first solution is better (for me).

And as a sidenote:

1 << bitNumber - 1

is quite unreadable, because probably 9 out of 10 programmers don't know/don't remember that the expression means

1 << (bitNumber - 1)

because in the operator precedence table the - comes before the << , so when you mix operators you should use (...) to make clear the priority.

Answer 2

The GetBit() implementation is wrong, you can check with the following test:

[TestMethod]
public void MyTestMethod()
{
    var byte22And23 = new byte[] { 2, 0 };
    var sb = new StringBuilder();
    for (int i = 7; i >=0; i--)
    {
        var r = GetBit(byte22And23[0], i);
        sb.Append((r) ? "1" : "0");
    }

    // result: 00000100
    Assert.AreEqual("00000010", sb.ToString());
}

It looks like there is no need of -1 in GetBit() because you are indexing bits 0-based and not 1-based:

public bool GetBit(byte b, int bitNumber)
{
    // no need of -1 here ------------ˇ
    bool bit = (b & (1 << bitNumber - 1)) != 0;
    return bit;
}

After the modification the tests runs green.