Difference in C# between different getter styles

Yes, both of them are read-only, but there is a difference. In the first one, there's a backing field which is initialized to 0 before the constructor is executed. You can change the value only in the constructor, just like a regular read-only field. The getter itself just returns the value of the field.

In the second one, the getter just returns 0 every time, with no field involved.

So to avoid using any automatically implemented properties or expression-bodied members at all, we have:

First version

private readonly int _number = 0;
public int Number { get { return _number; } }

Second version

public int Number { get { return 0; } }

A clearer example of the difference might be seen like this:

public DateTime CreationTime { get; } = DateTime.UtcNow;
public DateTime CurrentTime => DateTime.UtcNow;

If you create a single object, its CreationTime property will always give the same result - because it's stored in a readonly field, initialized on object construction. However, every time you access the CurrentTime property, that will cause DateTime.UtcNow to be evaluated, so you'll get a potentially different result.