c# - 这个语句在 C# 中是什么意思？

Question

if ((a & b) == b) 在下面的代码块中是什么意思？

if ((e.Modifiers & Keys.Shift) == Keys.Shift)
{
    lbl.Text += "\n" + "Shift was held down.";
}

为什么不是这样呢？

if (e.Modifiers == Keys.Shift)
{
    lbl.Text += "\n" + "Shift was held down.";
}

Answer 1

如果你看一下Keys enum , 这是 flag enum带有 [FlagsAttribute] 属性。

Use the FlagsAttribute custom attribute for an enumeration only if a bitwise operation (AND, OR, EXCLUSIVE OR) is to be performed on a numeric value.

Define enumeration constants in powers of two, that is, 1, 2, 4, 8, and so on. This means the individual flags in combined enumeration constants do not overlap.

所以 e.Modifiers 可能是多个枚举的组合:

e.Modifiers = Keys.Shift | Keys.Cancel | Keys.Enter

只是非常简单的假设来解释这个概念:

Keys.Shift  : 001 (1)
Keys.Cancel : 010 (2)
Keys.Enter  : 100 (4)

所以:

e.Modifiers = Keys.Shift | Keys.Cancel | Keys.Enter equal 001 | 010 | 100 = 111

条件:

    e.Modifiers & Keys.Shift equal 111 & 001 = 001

意思是:

 e.Modifiers & Keys.Shift == Keys.Shift

如果 e.Modifiers 不包含 Keys.Shift:

e.Modifiers = Keys.Cancel | Keys.Enter (110)

所以结果会是:

e.Modifiers & Keys.Shift equals 110 & 001 = 000 (is not Keys.Shift)

总结一下，这个条件检查e.Modifiers是否包含Keys.Shift