Cannot interpret state of this BitVector32

Question

I am debugging some code right now (VS 2019, .NET Framework 4.7.2), stopped at a breakpoint, using the Immediate Window to evaluate variables. I have a BitVector32 which I am not understanding its state. Here is the content of the IW:

stillInHand.ToString()
"BitVector32{00000000000000000000000000001100}"
stillInHand
{BitVector32{00000000000000000000000000001100}}
    Data: 12
stillInHand[0]
true
stillInHand[1]
false
stillInHand[2]
false
stillInHand[3]
false
stillInHand[4]
true
stillInHand[5]
false
stillInHand[6]
false
stillInHand[7]
false

There have been no calls to any of the Create* methods, and stillInHand was created with the BitVector32(Int32) ctor. Shouldn't indices 2 & 3 be true and all the rest be false?

Answer 1

Actually, this issue is related to the understanding of the index of BitVector32[ ].

First of all, stillInHand[1] doesn’t mean to get the second bit of stillInHand(BitVector32). It represents this action: use 00 00 … 00 01 to perform &(AND) operation with stillInHand(BitVector32).

For an example: stillInHand(BitVector32) is 00 00 00 00 00 … 00 00 00 11 00, and 1 is 00 00 00 00 00 … 00 00 00 00 01. Then perform &(AND) operation.

00 00 00 00 00 … 00 00 00 11 00        12      &(AND)

00 00 00 00 00 … 00 00 00 00 01       `1`

--------------------------------------------

00 00 00 00 00 … 00 00 00 00 00

And you can see that the last bit(focus on the index value 1) changes from 1 to 0, so the result will be false, if you output or see the result of stillInHand[1].

So, for stillInHand[2], you can see

00 00 00 00 00 … 00 00 00 11 00        12      &(AND)

00 00 00 00 00 … 00 00 00 00 10        2

--------------------------------------------

00 00 00 00 00 … 00 00 00 00 00

The second to last bit(focus on the index value 2) changes from 1 to 0, so the result will be false too.

And for stillInHand[8], you can see

00 00 00 00 00 … 00 00 00 11 00        12      &(AND)

00 00 00 00 00 … 00 00 00 10 00        8

--------------------------------------------

00 00 00 00 00 … 00 00 00 10 00

The fourth to last bit(focus on the index value 8) doesn’t change and it remains as 1, so the result will be true.

Actually, if you analyze the source code from here: Reference Source, you can see these codes:

/// <devdoc>
///    <para>Gets or sets a value indicating whether all the specified bits are set.</para>
/// </devdoc>

public bool this[int bit] {
   get {
            return (data & bit) == (uint)bit; 
            //clear other bits (to 0) through the AND operation of the data and mask.
            //If the result of the operation is equal to the mask, return true.
            //Otherwisse, return false.
       }
   set {
            if (value) {
                data |= (uint)bit;
                //data = data OR mask, set the specified bit to “1”
            }
            else {
                data &= ~(uint)bit;
                //data = data AND ~mask, set the specified bit to “0”
            }
        }
}
 

Of course, you can consider it as mask, and this will be easy to understand.