Question

I need some clarification. Are these two methods the same or different? I get a little bit confused about when the reference to an object passed in a parameter by value is updated and when a new one is created. I know if that assignment creates a new reference, but what about changing a property? Will both of these methods update the field “_someObjectList” the same way?

 public class SomeObject{
     public Guid UniqueKey { get; set; }
     public object SomeProperty{ get; set; }
 }

 public class SomeObjectListWrapper{

    public SomeObjectListWrapper(List<SomeObject> someObjectList){
        _someObjectList = someObjectList;
    }

    private readonly List<SomeObject> _someObjectList;

    public void ReplaceItemPropertyValue1(Guid itemUniqueKey, object propertyValue)
    {

        List<int> resultIndices = new List<int>();
        for (var i = 0; i < _someObjectList.Count(); i++)
        {
            if (_someObjectList[i].UniqueKey == itemUniqueKey)
                resultIndices.Add(i);
        }

        if (resultIndices.Count != 1)
            throw new Exception(
                "just pretend this is the same exception as Single() throws when it can't find anything");
        _someObjectList[resultIndices[0]].SomeProperty = propertyValue;
    }

    public void ReplaceItemPropertyValue2(Guid itemUniqueKey, object propertyValue)
    {
        _someObjectList.Single(x=>x.UniqueKey==itemUniqueKey).SomeProperty=propertyValue;
    }
}

Answer 1

Because SomeObject is a class (ie. a reference type), both ReplaceItemPropertyValue methods are updating the same object as was inserted into the list and will be retrieved from the list later. (If SomeObject was a struct/value type, the compiler would prevent you from updating an rvalue/return value [1].)

As a minor side-note, your two methods are not actually identical. The Single method raises an exception if there is more than one matching item in the sequence. To properly match the behaviour, use First instead.

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1. “rvalue” is not actually short for “return value,” it just happens that in this case your rvalue is a return value, which is why I specified both options.