Error: "Cannot modify the return value" c#

Question

This is because Point is a value type (struct).

Because of this, when you access the Origin property you're accessing a copy of the value held by the class, not the value itself as you would with a reference type (class), so if you set the X property on it then you're setting the property on the copy and then discarding it, leaving the original value unchanged. This probably isn't what you intended, which is why the compiler is warning you about it.

If you want to change just the X value, you need to do something like this:

Origin = new Point(10, Origin.Y);

Using a backing variable won't help. The Point type is a Value type.

You need to assign the whole Point value to the Origin property:-

Origin = new Point(10, Origin.Y);

The problem is that when you access the Origin property what is returned by the get is a copy of the Point structure in the Origin properties auto-created field. Hence your modification of the X field this copy would not affect the underlying field. The compiler detects this and gives you an error since this operation is entirely useless.

Even if you used your own backing variable your get would look like:-

get { return myOrigin; }

You'd still be returning a copy of the Point structure and you'd get the same error.

Hmm... having read your question more carefully perhaps you actually mean to modify the backing variable directly from within your class:-

myOrigin.X = 10;

Yes that would be what you would need.

By now you already know what the source of the error is. In case a constructor doesn't exist with an overload to take your property (in this case X), you can use the object initializer (which will do all the magic behind the scenes). Not that you need not make your structs immutable, but just giving additional info:

struct Point
{
    public int X { get; set; }
    public int Y { get; set; }
}

class MyClass
{
    public Point Origin { get; set; }
}

MyClass c = new MyClass();
c.Origin.X = 23; //fails.

//but you could do:
c.Origin = new Point { X = 23, Y = c.Origin.Y }; //though you are invoking default constructor

//instead of
c.Origin = new Point(23, c.Origin.Y); //in case there is no constructor like this.

This is possible because behind the scenes this happens:

Point tmp = new Point();
tmp.X = 23;
tmp.Y = Origin.Y;
c.Origin = tmp;

This looks like a very odd thing to do, not at all recommended. Just listing an alternate way. The better way to do is make struct immutable and provide a proper constructor.

Aside from debating the pros and cons of structs versus classes, I tend to look at the goal and approach the problem from that perspective.

That being said, if you don't need to write code behind the property get and set methods (as in your example), then would it not be easier to simply declare the Origin as a field of the class rather than a property? I should think this would allow you to accomplish your goal.

struct Point
{
    public int X { get; set; }
    public int Y { get; set; }
}

class MyClass
{
    public Point Origin;
}

MyClass c = new MyClass();
c.Origin.X = 23;   // No error.  Sets X just fine

I think a lot of people are getting confused here, this particular issue is related to understanding that value type properties return a copy of the value type (as with methods and indexers), and value type fields are accessed directly. The following code does exactly what you are trying to achieve by accessing the property's backing field directly (note: expressing a property in its verbose form with a backing field is the equivalent of an auto property, but has the advantage that in our code we can access the backing field directly):

class Program
{
    static void Main(string[] args)
    {
        var myClass = new MyClass();
        myClass.SetOrigin();
        Debug.Assert(myClass.Origin.X == 10); //succeeds
    }
}

class MyClass
{
    private Point _origin;
    public Point Origin
    { 
        get => _origin; 
        set => _origin = value; 
    }

    public void SetOrigin()
    {
        _origin.X = 10; //this works
        //Origin.X = 10; // fails with CS1612;
    }
}

The error you are getting is an indirect consequence of not understanding that a property returns a copy of a value type. If you are returned a copy of a value type and you do not assign it to a local variable then any changes you make to that copy can never be read and therefore the compiler raises this as an error since this cannot be intentional. If we do assign the copy to a local variable then we can change the value of X, but it will only be changed on the local copy, which fixes the compile time error, but will not have the desired effect of modifiying the Origin property. The following code illustrates this, since the compilation error is gone, but the debug assertion will fail:

class Program
{
    static void Main(string[] args)
    {
        var myClass = new MyClass();
        myClass.SetOrigin();
        Debug.Assert(myClass.Origin.X == 10); //throws error
    }
}

class MyClass
{
    private Point _origin;
    public Point Origin
    { 
        get => _origin; 
        set => _origin = value; 
    }

    public void SetOrigin()
    {
        var origin = Origin;
        origin.X = 10; //this is only changing the value of the local copy
    }
}