Equals null c#

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. And yes, any call that potentially return a different value at different times will exhibit this behavior. Add a comment. Active Oldest Votes. Moral of the story here is that properties should behave in a repeatable manner without other side effects like maybe allocating an Id if none is already set It is perfectly possible that you could have two objects with the same Id , but different references.

Improve this answer. Rowland Shaw Rowland Shaw Marc You'd have to be doing something really crazy or just not be thread safe — Rowland Shaw. Rowland Shaw: last one d :D — serhio. Show 3 more comments. Yes null is nothing but internal Pointer with value zero.

WriteLine s1. Should it be avoided when writing code that will handle objects outside the author's control e. Should it always be avoided when checking for null? Is this just hair-splitting? There's no simple answer for this question.

Anyone who says always use one or the other is giving you poor advice, in my opinion. There are actually several different methods you can call to compare object instances.

Given two object instances a and b , you could write:. Equals a,b will by default perform reference equality comparison on reference types and bitwise comparison on value types. From the MSDN documentation:. The default implementation of Equals supports reference equality for reference types, and bitwise equality for value types. Reference equality means the object references that are compared refer to the same object.

Bitwise equality means the objects that are compared have the same binary representation. Note that a derived type might override the Equals method to implement value equality. Value equality means the compared objects have the same value but different binary representations. ReferenceEquals a,b performs reference equality comparison only.

If the types passed are boxed value types, the result is always false. Equals b calls the virtual instance method of Object , which the type of a could override to do anything it wants.

The call is performed using virtual dispatch, so the code that runs depends on the runtime type of a. If the implementation of that operator invokes instance methods on either a or b , it may also depend on the runtime types of the parameters.

Since the dispatch is based on the types in the expression, the following may yield different results:. The instance method Equals is no better here. A user supplied implementation could return whatever it wants, even when comparing to null. But what about the static version of Object. Equals you ask? Can this end up running user code?

Well, it turns out that the answer is YES. The implementation of Object. Equals a,b expands to something along the lines of:. As a consequence, it's possible for the statement: Object.

Equals a,b to run user code when neither of the types in the call are null. Note that Object. Equals a,b does not call the instance version of Equals when either of the arguments is null. In short, the kind of comparison behavior you get can vary significantly, depending on which method you choose to call. One comment here, however: Microsoft doesn't officially document the internal behavior of Object.

Equals a,b. If you need an iron clad gaurantee of comparing a reference to null without any other code running, you want Object. ReferenceEquals :. Types that override Equals Object must also override GetHashCode ; otherwise, hash tables might not work correctly. Equals implementation should return results that are consistent with Equals. If your programming language supports operator overloading and you overload the equality operator for a given type, you must also override the Equals Object method to return the same result as the equality operator.

This helps ensure that class library code that uses Equals such as ArrayList and Hashtable behaves in a manner that is consistent with the way the equality operator is used by application code.

The following guidelines apply to overriding Equals Object for a reference type:. Consider overriding Equals if the semantics of the type are based on the fact that the type represents some value s.

Most reference types must not overload the equality operator, even if they override Equals. However, if you are implementing a reference type that is intended to have value semantics, such as a complex number type, you must override the equality operator.

You should not override Equals on a mutable reference type. This is because overriding Equals requires that you also override the GetHashCode method, as discussed in the previous section. This means that the hash code of an instance of a mutable reference type can change during its lifetime, which can cause the object to be lost in a hash table.

The following guidelines apply to overriding Equals Object for a value type:. If you are defining a value type that includes one or more fields whose values are reference types, you should override Equals Object. The Equals Object implementation provided by ValueType performs a byte-by-byte comparison for value types whose fields are all value types, but it uses reflection to perform a field-by-field comparison of value types whose fields include reference types.

If you override Equals and your development language supports operator overloading, you must overload the equality operator. Equals method avoids boxing the obj argument. If both objA and objB are null , the method returns true. The following example illustrates the Equals Object, Object method and compares it with the ReferenceEquals method. It also enables you to test objects whose value is null for equality.

It compares objA and objB for equality as follows:. It determines whether the two objects represent the same object reference. If they do, the method returns true. This test is equivalent to calling the ReferenceEquals method. In addition, if both objA and objB are null , the method returns true. It determines whether either objA or objB is null.

If so, it returns false. If the two objects do not represent the same object reference and neither is null , it calls objA. Equals objB and returns the result. This means that if objA overrides the Object.

Equals Object method, this override is called. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode. Equals Method Reference Is this page helpful? Please rate your experience Yes No.

Any additional feedback? Namespace: System Assembly: System. Equals method with the following signature:. For more information, see the Relational and type-testing operators section of the C language specification. For more information about equality of record types, see the Equality members section of the records feature proposal note.

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