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-= Operator (C# Reference)
The subtraction 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 meaning of the - operator is dependent on the types of x and y (subtraction for numeric operands, delegate removal for delegate operands, and so forth).
The -= operator is also used in C# to unsubscribe from an event. For more information, see How to: Subscribe to and Unsubscribe from Events (C# Programming Guide).