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Multiple reference to same object


By : Senpai-Will
Date : July 31 2020, 11:00 PM
With these it helps Your premise is wrong. Those are neither two references to the same object (one is a Point, the other a Rectangle), nor does the Rectangle hold a reference to the Point.
This is the code of the constructor (assuming java.awt.Rectangle):
code :
public Rectangle(Point p) {
    this(p.x, p.y, 0, 0);
}


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Protobuf-net object reference deserialization using Dictionary: A reference-tracked object changed reference during dese


By : kuzukuzu
Date : March 29 2020, 07:55 AM
To fix this issue Edit: this should work from the next build onwards simply by marking the type's AsReferenceDefault:
code :
[ProtoContract(AsReferenceDefault=true)]
public class A
{
    // ...
}
[Test]
public void ExecuteHackedViaFields()
{
    // I'm using separate models **only** to keep them clean between tests;
    // normally you would use RuntimeTypeModel.Default
    var model = TypeModel.Create();

    // configure using the fields of KeyValuePair<int,A>
    var type = model.Add(typeof(KeyValuePair<int, A>), false);
    type.Add(1, "key");
    type.AddField(2, "value").AsReference = true;

     // or just remove AsReference on Items
    model[typeof(B)][2].AsReference = false;

    Execute(model);
}
[Test]
public void ExecuteHackedViaSurrogate()
{
    // I'm using separate models **only** to keep them clean between tests;
    // normally you would use RuntimeTypeModel.Default
    var model = TypeModel.Create();

    // or just remove AsReference on Items
    model[typeof(B)][2].AsReference = false;

    // this is the evil bit: configure a surrogate for KeyValuePair<int,A>
    model[typeof(KeyValuePair<int, A>)].SetSurrogate(typeof(RefPair<int, A>));
    Execute(model);
}

[ProtoContract]
public struct RefPair<TKey,TValue> {
    [ProtoMember(1)]
    public TKey Key {get; private set;}
    [ProtoMember(2, AsReference = true)]
    public TValue Value {get; private set;}
    public RefPair(TKey key, TValue value) : this() {
        Key = key;
        Value = value;
    }
    public static implicit operator KeyValuePair<TKey,TValue>
        (RefPair<TKey,TValue> val)
    {
        return new KeyValuePair<TKey,TValue>(val.Key, val.Value);
    }
    public static implicit operator RefPair<TKey,TValue>
        (KeyValuePair<TKey,TValue> val)
    {
        return new RefPair<TKey,TValue>(val.Key, val.Value);
    }
}
private void Execute(TypeModel model)
{
    A a = new A();
    B b = new B();

    b.A = a;

    b.Items.Add(1, a);

    Assert.AreSame(a, b.A);
    Assert.AreSame(b.A, b.Items[1]);

    B deserializedB = (B)model.DeepClone(b);

    Assert.AreSame(deserializedB.A, deserializedB.Items[1]);
}
RuntimeTypeModel.Default.Add(...); // etc

Using multiple models in one view Object reference not set to an instance of an object


By : Jorge Garcia
Date : March 29 2020, 07:55 AM
With these it helps I think you have not initialized CommonViewModel properly. Following code snippent may help you.
code :
 public class HomeController
 {
    private DefaultConnection db = new DefaultConnection();

    public ActionResult Index()
    {
        var viewModel = new CommonViewModel();
        viewModel.users = db.User.ToList();
        viewModel.courses = db.Course.ToList();

        return View(viewModel);
    }        
}

Object reference not set to an instance of an object - Multiple Projects (MVC)


By : vfxjex
Date : March 29 2020, 07:55 AM
hope this fix your issue Since this is such a commonly confusing error to new developers, I've authored a post on my blog to explain what the error means in detail and how to debug it. TL;DR: Object reference not set to an instance of an object is a runtime error (hence why your project compiles fine) that occurs when you're expecting a variable to be an instance of a particular class, but it actually resolves to null at runtime.
This commonly occurs when you're selecting an object from a database, but nothing matches, or you've neglected to initialize a property on your model that requires initialization, like a list. Based on the line of code you've posted, my guess is that either the model itself is null (perhaps because it's coming from the database and you're not checking for null before sending it to the view), or the Enrollments property is null because you've neglected to initialize it, or it's not marked as virtual if your model is an instance of an entity class.
code :
public ActionResult Detail(int id)
{
    var foo = db.Foos.Find(id); // potentially null, if no matching id
    if (foo == null)
    {
        return new HttpNotFoundResult();
    }

    return View(foo);
}
public class Foo
{
    public Foo()
    {
        Bars = new List<Bar>();
    }

    public List<Bar> Bars { get; set; }
}
public class Foo
{
    private List<Bar> bars;
    public List<Bar> Bars
    {
        get
        {
            if (bars == null)
            {
                bars = new List<Bar>();
            }
            return bars;
        }
        set { bars = value; }
    }
}
public class Foo
{
    public List<Bar> Bars { get; set; } = new List<Bars>();
}
public virtual ICollection<Bar> Bars { get; set; }

SQL inserting multiple lines, object reference not set to an instance of an object error


By : Alicia Quintero Sote
Date : March 29 2020, 07:55 AM
seems to work fine Thank you guys for all your help, i finally figured out that the "Object reference not set to an instance of an object." Was actually referring to the line @nick was talking about.
The problem with the code was:

Do multiple reference variables pointing to the same object each take up the same memory footprint as the object


By : user2136204
Date : March 29 2020, 07:55 AM
I wish this help you Java only has references and primitive types. When you assign a reference to a variable it always takes the same amount if memory regardless of what object it references. Typically a reference is 4 bytes but can be 8 bytes for large heaps over 32 GB in size.
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