The principle of least privilege is fundamental to good software engineering. In the context of an app, the principle states that code should be granted the amount of privilege and access needed to accomplish its designated task, but no more. Let’s see how this principle applies to instance variables.

Some instance variables need to be modifiable, and some do not. In Section 8.4, we used keyword const to declare a constant, which must be initialized in its declaration—all objects of the class have the same value for that constant. Suppose, however, we want a constant that can have a different value for each object of a class. For this purpose, C# provides keyword readonly to specify that an instance variable of an object is not modifiable and that any attempt to modify it after the object is constructed is an error. For example,

private readonly int Increment;

declares readonly instance variable Increment of type int. Like a constant, a readonly variable’s identifier uses Pascal Case by convention. Although readonly instance variables can be initialized when they’re declared, this isn’t required. A readonly variable should be initialized by each of the class’s constructors or in the variable’s declaration. Each constructor can assign values to a readonly instance variable multiple times—the variable doesn’t become unmodifiable until after the constructor completes execution. If a constructor does not initialize the readonly variable, the variable uses the same default value as any other instance variable (0 for numeric simple types, false for bool type and null for reference types)—these values actually are set before a constructor executes and can be overwritten by the called constructor.

const members must be assigned values at compile time. Therefore, const members can be initialized only with other constant values, such as integers, string literals, characters and other const members. Constant members with values that cannot be determined at compile time—such as constants that are initialized with the result of a method call—must be declared with keyword readonly, so they can be initialized at execution time. Variables that are readonly can be initialized with more complex expressions, such as an array initializer or a method call that returns a value or a reference to an object.