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I have joined two objects so that I can access properties of both objects from a single object. I did the join using LINQ then I tried to filter the result using the .Where() and .Single() methods.
I got the error "Object reference not set to an instance of an object." at runtime.
Please take a look at my code below and help explain what I did incorrectly.
public class Product
{
[Display(Name = "ID")]
public int ID { get; set; }
[Display(Name = "Name")]
public string Name { get; set; }
[Display(Name = "Price")]
public decimal Price { get; set; }
[Display(Name = "CategoryID")]
public int CategoryID { get; set; }
}
public class Category
{
[Display(Name = "ID")]
public int ID { get; set; }
[Display(Name = "Name")]
public string Name { get; set; }
}
public void MyMethod(string id)
{
List<Product> _products = new List<Product>();
List<Category> _categories = new List<Category>();
_products = psc.FindAllProducts();
_categories = psc.FindAllCategories();
var _productCategories =
from c in _categories
join p in _products
on c.ID equals p.CategoryID
select new {
p.ID, p.Name, p.Price,
p.Quantity, p.CategoryID,
_categoryID = c.ID,
_categoryName = c.Name
};
var selectedCat = _productCategories
.Where
(x => x._categoryID.ToString() == id)
.SingleOrDefault()._categoryName;
}
modified 25-Jul-16 0:28am.
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I don't see a default if the select fails!
Never underestimate the power of human stupidity
RAH
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public Void MyMethod(string id)
{
List<Product> _products = new List<Product>();
List<Category> _categories = new List<Category>();
_products = psc.FindAllProducts().ToList();
_categories = psc.FindAllCategories().ToList();
ProductCategory selectedCategory = new ProductCategory();
StringBuilder errors = new StringBuilder();
try
{
var _productCategories =
from c in _categories join p in _products
on c.ID equals p.CategoryID
select new
{
p.ID, p.Name, p.CategoryID,
_categoryID = c.ID, _categoryName = c.Name
};
try
{
var selectedCat = _productCategories
.Where(x => x._categoryID == Convert.ToInt32(id))
.SingleOrDefault()._categoryName;
foreach (var pc in _productCategories)
{
if (Convert.ToInt32(id) == pc.CategoryID)
{
selectedCategory.ProductID = pc.ID;
selectedCategory.Name = pc.Name;
selectedCategory.CategoryID = pc._categoryID;
selectedCategory.CategoryName = pc._categoryName;
}
}
}
catch (Exception ex)
{
errors.Append(ex).AppendLine("<br> <br>");
}
}
catch (Exception ex)
{
errors.Append(ex).AppendLine("<br"> <br>");
}
}
modified 25-Jul-16 1:02am.
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I suspect the problem is that the category ID you've passed in doesn't exist. As a result, the SingleOrDefault method returns null, and you then try to retrieve the _categoryName property from that null reference.
If you want the code to fail if the category ID doesn't exist, change the code to use Single instead. Otherwise, you'll need to test the value returned from SingleOrDefault to see if it's null.
You can combine the Where and Single[OrDefault] calls, as both Single[OrDefault] and First[OrDefault] have overloads which accept a filter.
And there's no need to create the two List<T> instances at the start, as you immediately overwrite them on the following lines.
List<Product> _products = psc.FindAllProducts();
List<Category> _categories = psc.FindAllCategories();
var _productCategories =
from c in _categories
join p in _products
on c.ID equals p.CategoryID
select new {
p.ID, p.Name, p.Price,
p.Quantity, p.CategoryID,
_categoryID = c.ID,
_categoryName = c.Name
};
var selectedProductCategory = _productCategories.SingleOrDefault(x => x._categoryID.ToString() == id);
var selectedCat = selectedProductCategory == null ? null : selectedProductCategory._categoryName;
var selectedCat = _productCategories.SingleOrDefault(x => x._categoryID.ToString() == id)?._categoryName;
var selectedCat = _productCategories.Single(x => x._categoryID.ToString() == id)._categoryName;
Also, are you sure there's only ever one product in a given category? If there's more than one, Single[OrDefault] will throw an exception.
And I'm not sure why you're passing the category ID as a string, when it's defined as an int in the model?
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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Thank you so much for your reply. You were right that the CategoryID passed in from a form did not exist.
That's why I got the null exception when I ran my LINQ query.
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Let's assume we have something like this:
public class Person : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName]string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
*1) Copy of the subscribers really needed?
A lot of time in forums like CP I read we need to make a copy of the subscribers before invoke the handlers. Argumentation: This because theoretically the subscribers can unsubscribe in between...
My doubts, respectively I see only arguments why not to copy:
a.) In a some lot (in most?) of MSDN examples, no copy will be made. So, when MSDN does not do it why should I do it?
b.) In case someone unsubscribes he is not longer interested in the event. Why should I notify him then longer?
c.) My opinion is, that it can be dangerous to inform a subscriber while he unsubscribed and therefore does not expect notifications any longer.
[Edit] Thank you BillWoodruff 
Ok, a problem I see now in case I do not make a copy:
In bewteen (if PropertyChanged != null) and invoking PropertyChanged(...) it is possible that one does unsubscribe. Probably this Point is solved in c#6 by using (PropertyChanged?.Invoke(...)
But still "c.)" above is critical I think...(?) and the subscriber has to be aware that even after unsubscribing he can get notifications.
Comments / corrections / notes will be very appreciated.
Thank you in advance.
modified 19-Jan-21 21:04pm.
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I think you may find Jon Skeet's comments on this relevant: [^], and his information on using an extension method, and the null-conditional operator in C#6 ... interesting.
I don't pretend to understand the thread-safety and/or multiple-subscriber issues involved: for that we need Richard Deeming or Richard MacCutchan to step-up
«There is a spectrum, from "clearly desirable behaviour," to "possibly dodgy behavior that still makes some sense," to "clearly undesirable behavior." We try to make the latter into warnings or, better, errors. But stuff that is in the middle category you don’t want to restrict unless there is a clear way to work around it.» Eric Lippert, May 14, 2008
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Thank you for this. I just edited the question a Little bit
modified 19-Jan-21 21:04pm.
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I'm glad that's helpful; perhaps it's a bias of mine that I think copying a pointer is "cheap" ?
«There is a spectrum, from "clearly desirable behaviour," to "possibly dodgy behavior that still makes some sense," to "clearly undesirable behavior." We try to make the latter into warnings or, better, errors. But stuff that is in the middle category you don’t want to restrict unless there is a clear way to work around it.» Eric Lippert, May 14, 2008
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BillWoodruff wrote: we need Richard Deeming or Richard MacCutchan to step-up
That's very kind but misguided I'm afraid. I could possibly give an answer, but I would need to do quite a bit of research and testing first.
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I know it is your modesty that makes you suggest I may be kindly misguided to consider you (and brother Deeming) as guides non-pareil in this esoteric area, and I appreciate that quality as much as I do the depth of your technical knowledge Richard MacCutchan wrote: I could possibly give an answer
Duchess to Alice (Alice’s Adventures in Wonderland, Chapter 9) That’s nothing to what I could say if I chose.
«There is a spectrum, from "clearly desirable behaviour," to "possibly dodgy behavior that still makes some sense," to "clearly undesirable behavior." We try to make the latter into warnings or, better, errors. But stuff that is in the middle category you don’t want to restrict unless there is a clear way to work around it.» Eric Lippert, May 14, 2008
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BillWoodruff wrote: know it is your modesty
 But thank you anyway.
And, to be honest I could not have produced as good an explanation as Pete's below.
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BillWoodruff wrote: we need Richard Deeming or Richard MacCutchan to step-up
Or Pete, who nailed the explanation below. But your link also answers the question, albeit in much greater detail.
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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Imagine your system has an event with one subscriber. This subscriber is on a background thread. Now, consider the case where the very last action before context switching over to the thread your subscriber is on is to check to see if you have any subscriptions. That's fine as the null check will tell you that you have a subscriber.
Okay, you're now processing on this other thread and the first thing it does is to remove the event subscription. You now have nothing hooked up to the event but when you context switch back, it's going to continue as though there are subscriptions. That's why you take the copy.
This space for rent
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Thank you for your Feedback.
Yes I see this Point meanwhile. On the other Hand when the subscriber unsubscribes he still has to be aware to get notified and this fact (I hope I'm right on this, that it is a fact) I never found in a discussion.
Time Publisher Subscriber
---- --------- ---------
1 Take a copy -
2 - Unsubscribe
3 Invoke "Check for race condition needed before proceeding"
modified 19-Jan-21 21:04pm.
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It's not the unsubscribed side that is the problem. If you think about it from the point of view of why you test the event in the first place, it gives you the hint that you could end with functionally identical code between not testing the subscription and throwing an exception because nothing is listening to the event and testing it and throwing an exception because you have ended up in a situation where nothing is listening to the event.
This space for rent
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Pete O'Hanlon wrote: That's why you take the copy.
Really appreciate your explanation, Pete ! From what you say, I infer that when you do something like:
var handler = PropertyChanged;
Then an actual copy of the underlying handler is made ? Not just a "duplicate of the pointer" ? My struggle to form some kind of practically useful "mental model" of EventHandlers and InvocationQueue ... continues, although I don't have any real problem using them, right now ... albeit my usage does not include dealing with possible threading issues, yet. What saved my read-end was learning that you can effectively switch a specific EventHandler instance in/out by doing a -= remove call which, praise be, does not throw an error if the EventHandler == null.
From now on, I believe I can summon your presence by simply invoking the names of the two Richards
«There is a spectrum, from "clearly desirable behaviour," to "possibly dodgy behavior that still makes some sense," to "clearly undesirable behavior." We try to make the latter into warnings or, better, errors. But stuff that is in the middle category you don’t want to restrict unless there is a clear way to work around it.» Eric Lippert, May 14, 2008
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As I understand it, it's a "duplicate of the pointer", not a deep clone of the delegate. When you add or remove a handler, the "pointer" is replaced with a new instance, because delegates are immutable.
Delegates are immutable; once created, the invocation list of a delegate does not change.
...
Combining operations, such as Combine and Remove, do not alter existing delegates. Instead, such an operation returns a new delegate that contains the results of the operation, an unchanged delegate, or null.
Again, Jon Skeet has some good information: C# in Depth: Delegates and Events[^]
"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer
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I had the same question and so I made a test. In this test I made a copy of the handler as suggested var handler = PropertyChanged;, then let remove the handler from the subscriber (-=) and see, handler still holds the copied subscription(s). So here copy means more Clone I think.
I hope it helps
modified 19-Jan-21 21:04pm.
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The delegate is immutable. So handler -= someHandler; is not modifying or updating the original delegate, it is creating a new delegate which is then stored in handler. So the copy-of-the-handler is indeed a copy of the reference to the (immutable) delegate.
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Thank you very much for the this reply 
modified 19-Jan-21 21:04pm.
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I can't find simple solution for the case below.
How can I get time of some action in my code, and display it simpe through Console.WriteLine() in format [Date] [Time] [Action]?
For example I need time when "Ford" is created:
Car c= new Car();
c.createCar("Ford");
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c.Created = DateTime.Now
Never underestimate the power of human stupidity
RAH
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