Unable to Modify struct Members

问题:

I'm not at all new to programming, but there seems to be a hole in my understanding of C# structs.

Can anyone explain why the following code prints out "Dist1: 0, Dist2: 0"?

struct Distance
{
    public void SetFeet(int feet) { Value = feet; }
    public void SetMiles(float miles) { Value = (int)(miles * 5280f); }
    public int GetFeet() { return Value; }
    public float GetMiles() { return Value / 5280f; }
    private int Value;
}

class Distances
{
    public Distance Dist1 { get; set; }
    public Distance Dist2 { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        Distances distances = new Distances();
        distances.Dist1.SetFeet(1000);
        distances.Dist2.SetFeet(2000);

        Console.WriteLine("Dist1: {0}, Dist2: {1}",
            distances.Dist1.GetMiles(),
            distances.Dist2.GetMiles());

        Console.ReadLine();
    }
}

回答1:

struct are value types - so when you are accessing distances.Dist1.SetFeet you basically are accessing a copy... see for example at MSDN http://msdn.microsoft.com/en-us/library/aa288471%28v=vs.71%29.aspx

[EDIT after comment]
OTOH if you do distances.Dist1 = new Distance ().SetFeet (1000); AND change the return of SetFeet from void to Distance it should work. Alternatively make Distance a class.

For a reference on how to build structs in a way that they work as expected see the DateTime struct in the framework - http://msdn.microsoft.com/en-us/library/system.datetime.aspx
[/EDIT after comment]

回答2:

Getters and setters -- how properties are accessed -- still function like methods in this regard. That is,

distances.Dist1.SetFeet(1000);

is "equivalent" to

distances.GetDist1().SetFeet(1000);

The "copy" of the structure (value) is made when it is returned from the getter (or passed to the setter). If Dist1 were a member variable this would not be the case and would work "as expected".

Happy coding.