Automatic Property Value Not Updated with Struct

Question

Take the following `struct` class: ``` public struct SingleWraper { private double _myValue; public double MyValue { get { return _myValue; } set { _myValue = value; } } public void SetMyValue(double myValue) { _myValue = myValue; } } public struct DoubleWraper { public SingleWraper SingWraper { get; set; } public void SetMyValue(double singleVa) { SingWraper.SetMyValue(singleVa); } } ``` Run the following test: ``` [Test] public void SetMyValue() { var singleWraper = new DoubleWraper(); singleWraper.SetMyValue(10); Assert.AreEqual(10,singleWraper.SingWraper.MyValue); } ``` It fails. However, if you don't use automatic property for `DoubleWraper`, i.e, you expand the field as below: ``` public struct DoubleWraper { private SingleWraper _singWraper; public SingleWraper SingWraper { get { return _singWraper; } set { _singWraper = value; } } public void SetMyValue(double singleVa) { _singWraper.SetMyValue(singleVa); } } ``` Then the test will pass. Why this is so?

Accepted Answer

It is here:

```
_singWraper.SetMyValue(singleVa);
```

vs:

```
SingWraper.SetMyValue(singleVa);
```

In the second, you access a property, therefore you clone the struct; essentially this is the same as:

```
var clonedAndIndependentValue = SingWraper; // getter
clonedAndIndependentValue.SetMyValue(singleVa);
```

Note we've updated a different struct value; contrast to field access, which talks to the existing struct value.

Yet another example of why mutable structs are evil. DON'T DO IT. Make the struct immutable (no `Set*` methods, property setters, or other shenanigans). Or use a class.

Automatic Property Value Not Updated with Struct

Solution

Problem