&= Operator (C# Reference)
The AND assignment operator.
An expression using the &= assignment operator, such as
x &= y
is equivalent to
x = x & y
except that x is only evaluated once. The & operator performs a bitwise logical AND operation on integral operands and logical AND on bool operands.
The &= operator cannot be overloaded directly, but user-defined types can overload the binary & operator (see operator).