在C＃中，为什么在这种情况下使用字段与自动实现的属性会导致不同的行为？

Question

Take the following code from http://csharpindepth.com/Articles/Chapter8/PropertiesMatter.aspx :

using System;

struct MutableStruct
{
    public int Value { get; set; }

    public void SetValue(int newValue)
    {
        Value = newValue;
    }
}

class MutableStructHolder
{
    public MutableStruct Field;
    public MutableStruct Property { get; set; }
}

class Test
{    
    static void Main(string[] args)
    {
        MutableStructHolder holder = new MutableStructHolder();
        holder.Field.SetValue(10);
        holder.Property.SetValue(10);

        Console.WriteLine(holder.Field.Value);
        Console.WriteLine(holder.Property.Value);
    }
}

(There were a couple of comments in the code I missed earlier or didn't read that I've taken out now.)

The claim on the article is that the output is 10 for holder.Field , but 0 for holder.Property , and this has been verified to be accurate. I'm having a bit of trouble seeing why though. An auto-implemented property will have one specific field set aside that it's mapped to, and due to being a struct, Property will immediately have had its backing field fully set and constructed from the beginning. What's the difference?

Answer 1

The line holder.Property returns a copy of MutableStruct . So when you write holder.Property.SetValue(10) you are changing the copy, not the original.

The line holder.Field is an alias for the MutableStruct itself.

It is really hard to not accidentally make copies of structs. Thus we try to not make them mutable.

Answer 2

The difference is the interjection of a getter for the property. It's functionally equivalent to

private MutableStruct _Property;
public MutableStruct GetProperty()
{
    return _Property;
}
public void SetProperty(MutableStruct value)
{
    _Property = value;
}

So when you call

holder.Field.SetValue(10);

you are mutating the struct at Field directly , but when you call

holder.Property.SetValue(10);

you are mutating the copy of the struct returned from GetProperty .