When you write;
Rectangle pageSize = PageSize.A4
Your pageSize
value can be change as a parameter.
From Named and Optional Arguments
A default value must be one of the following types of expressions:
a constant expression;
an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;
an expression of the form default(ValType), where ValType is a value type.
The expression PageSize.A4
is not classified as a compile-time constant.