... Understanding Compound Assignment C# does not allow you to declare any user-defined assignment operators. However, a compound assignment operator (such as +=) is ... when you use its associated compound assignment operator. For example: Hour a = ...; int b = ...; a += a; // same as a = a + a a += b; // same as a = a + b The first compound assignment expression ... the Hour type declares a binary operator+ whose parameters are both Hour. Similarly, the second compound assignment expression (a += b) is also valid because a is of type Hour and b is of type...