C# Struct method doesn't save value if accessed by a property

Question

I need to create a structure that looks like an int (but has an extra field that I need...), so I created a new structure named TestStruct added one method (test()) that I n

Answer 1

When you use Val, the property is returning a copy , it's that copy you modify, then you access the property again which gets another copy.

If you change the struct to a class this problem should go away because if the difference in how classes and structs are treated.

Answer 2

After reading some links, I understand the problem..

I've fix it by changing the test() method to return a new structure with the new value and then instead of doing Val.test() I do Val = Val.test();.

I think a similar situation occurs when we want to replace some chars in a string, we must do something like str = str.replace("xx","yy");

Thank you all for helping me out. I didn't expect to get all those replies so quickly

Answer 3

It's a struct, so it's a value type - it's copied by value, not by reference.

Properties are just syntactic sugar around method calls, Val compiles down to this:

private TestStruct _valBackingField;
public TestStruct get_Val {
    return _valBackingField;
}

When accessing a struct through a property, you are getting a fresh copy every time you access the Getter. When accessing a struct through a field, you are getting the same object every time.

That's why mutable value types are considered evil™.

Answer 4

Your struct is wrong.

For a very large number of reasons, you should never make a mutable struct.

Just like an int or DateTime value is immutable and can never change, so too a specific value of your struct must never change at all.

Instead, you can make functions that return a new, different value .

Here are some reasons that mutable structs are evil:

1. http://ericlippert.com/2008/05/14/mutating-readonly-structs/
2. http://blog.slaks.net/2010/12/when-shouldnt-you-write-ref-this.html
3. http://codeblog.jonskeet.uk/2010/07/27/iterate-damn-you/
4. http://philosopherdeveloper.wordpress.com/2011/03/31/how-i-discovered-a-bug-in-the-c-compiler-part-1/

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To answer the question, Val.test() is equivalent to get_Val().test().
Since structs are value types, `get_Val() (the automatically-generated property getter) returns a copy of the struct.
The original struct in the private backing field is not affected.