Nuin null null

As we learned in the previous section, operations involving null are extremely fast and easy to perform at run-time. So far we assumed working with reference types. The reason for this is simple: null doesn't exist for value types. As we have seen previously, a reference is a pointer to a memory-address that stores a value e. If a reference points to null , then no value is associated with it. On the other hand, a value is, by definition, the value itself. There is no pointer involved.

A value type is stored as the value itself. Therefore the concept of null doesn't exist for value types. The following picture demonstrates the difference. On the left side you can see again the memory in case of variable name being a reference pointing to "Bob".

The right side shows the memory in case of variable name being a value type. As we can see, in case of a value type, the value itself is directly stored at the address A0A1 which is associated with variable name.

There would be much more to say about reference versus value types, but this is out of the scope of this article. Please note also that some programming languages support only reference types, others support only value types, and some e.

C and Java support both of them. The concept of null exists only for reference types. It doesn't exist for value types. Suppose we have a type person with a field emailAddress. Suppose also that, for a given person which we will call Alice, emailAddress points to null. As we have seen already, what we can assert is that no value is associated with emailAddress.

But why is there no value? What is the reason of emailAddress pointing to null? If we don't know the context and history, then we can only speculate. The reason for null could be:. In practice we often know the application and context. We intuitively associate a precise meaning to null. In a simple and flawless world, null would simply mean that Alice actually doesn't have an email address.

When we write code, the reason why a reference points to null is often irrelevant. We just check for null and take appropriate actions. For example, suppose that we have to write a loop that sends emails for a list of persons.

The code in Java could look like this:. We just acknowledge the fact that there is no email address, log a warning, and continue. If a reference points to null then it always means that there is no value associated with it.

In most cases, null has a more specific meaning that depends on the context. In this case, returning null or an empty list is ambiguous. Does it mean that the patient doesn't have allergies, or does it mean that an allergy test has not yet been performed?

These are two semantically very different cases that must be handled differently. Or else the outcome might be life-threatening. Just suppose that the patient has allergies, but an allergy test has not yet been done and the software tells the doctor that 'there are no allergies'.

Hence we need additional information. We need to know why the function returns null. It would be tempting to say: Well, to differentiate, we return null if an allergy test has not yet been performed, and we return an empty list if there are no allergies.

The different semantics for returning null versus returning an empty list would need to be well documented. And as we all know, comments can be wrong i. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 11 years, 2 months ago. Active 10 years, 2 months ago.

Viewed 14k times. I have an object of type Foo. Foo has an Id int a Is the code bellow "good"? Regarding the last question: it could happen. Let's say a property returns DateTime. Ticks , then it could be that x and y refer the same instance, but since the property getter would be called twice it could return different results.

Good lateral thinking but you surely would not set that Id of your object, would you? This can happen by return a Guid. NewGuid as well. Aliostad: I was thinking beyond the property name.

Age property value to We have set emp. You can see in the out put window, we are able to fetch Name and Age property value correctly using, However Address properties are displayed as empty. Lets see why this happened here? Name and emp? Age evaluated to valid values. In case of emp. City , emp? State , emp? Country and emp? For example, In below case, Age property is a value type and emp?. Age returns int? So while assigning it to a variable then you have to declare the variable as nullable type.

In below case we have declared int? For example, when we invoke emp?. If emp is NULL then no further call in the expression will occur. If you club null conditional operator?. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.

Privacy policy. This example shows how to handle possible null values in source collections. If a source collection is null or contains an element whose value is null , and your query doesn't handle null values, a NullReferenceException will be thrown when you execute the query. In the previous example, the where clause filters out all null elements in the categories sequence. This technique is independent of the null check in the join clause.