The previous example showed one scenario for using ref variables for return values and local variables: You intended to pass by reference so that callers could modify the returned value. Another common scenario is to use pass-by-reference to avoid copying larger structures. Consider this Point3D structure:
Imagine that this structure is used in an application that creates millions of points, and performs a variety of computations on those points. The size of the Point3D structure means the cost of copying those structures is likely larger than the cost of working with references to them. Let's make some modifications to this class to pass instances by references. You'll learn how to do that safely, and enable optimizations from the compiler.
Start by changing the Origin property to return a reference to the origin:
public static ref Point3D Origin => ref origin;To achieve any performance benefits from this change, modify the code below to declare start as a ref variable and see what happens:
ref var start = ref Point3D.Origin;You can prevent this kind of coding error by declaring the Origin property to return a ref readonly reference:
public static ref readonly Point3D Origin => ref origin;The compiler generates an error on the declaration of start. Add the readonly modifier after ref to fix it. That moves the error later where the code modifies start. Introduce a new variable instead of modifying start.
These two changes show how adding the readonly modifier to ref variables and returns prevent unintended modifications. There are still changes you can make that enable the compiler to perform more optimizations. The Translate method could take its first input as a ref argument, but you don't want to allow modifications. Instead, use the in modifier to disallow any changes to the point or its members:
public static Point3D Translate(in Point3D source, double dX, double dY, double dZ)The primary motivation for adding these features in your code is performance. If you find that copying structures has a negative impact on your application, consider using pass-by-reference and readonly references to minimize the copies.