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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 cannot be overloaded directly, but user-defined types can overload the - operator (see operator).

// cs_operator_subtraction_assignment.cs
using System;
class MainClass
    static void Main() 
        int a = 5;
        a -= 6;