overriding equals

Gilbert Consellado

public class User{
public int UserID {get;set;}
public string Username {get;set;}

publict override bool Equals(object obj)
{
if(!(obj is User)) return false;
return (obj as User).UserID == UserID;
}

public override int GetHashCode()
{
//UserID is form the database
return UserID;
}

public override ToString()
{
return Username;
}
}

base on the code above, when i create to object from the database, comparing them will return false even they are the same. example:

User u1 = new User(){UserID = 1, Username = "FirstUser"};
User u2 = new User(){UserID = 1, Username = "FirstUser"};

var b = u1 == u2;

variable b is false; I thought they will be the same, is there i missing there? I will appreciate for any advice will be given Thank you.

Brisingr Aerowing

You need to override the == and != operators.

public static bool operator ==(User lhs, User rhs)
{
if(lhs == null && rhs == null) return true;
if(lhs == null && rhs != null) return false;
if(lhs != null && rhs == null) return false;
return lhs.UserID == rhs.UserID;
}

public static bool operator !=(User lhs, User rhs)
{
return !(lhs == rhs);
}

Just insert these into the class and you will be able to use == and != to check for equality and inequality. Note that if you override one, C# forces you to override the other. There are several other operators you can override. You can get all the nitty gritty details on MSDN[^].

What do you get when you cross a joke with a rhetorical question? The metaphorical solid rear-end expulsions have impacted the metaphorical motorized bladed rotating air movement mechanism. Do questions with multiple question marks annoy you???

Brisingr Aerowing

You might also want to check for null being passed in to the Equals method. i.e. insert the following line into the beginning of the method:

if(other == null) return false;

What do you get when you cross a joke with a rhetorical question? The metaphorical solid rear-end expulsions have impacted the metaphorical motorized bladed rotating air movement mechanism. Do questions with multiple question marks annoy you???

Gilbert Consellado

thank you for the quick reply. I thought overriding equals will be enough, By the way, one more question base on the code above, which is most efficient using the "Equals" method or the equals operator? Thank you

F ES Sitecore

http://blogs.msdn.com/b/csharpfaq/archive/2004/03/29/when-should-i-use-and-when-should-i-use-equals.aspx[^]

Richard Deeming

As you're dealing with a class, not a struct, you can simplify the operator to:

public static bool operator ==(User lhs, User rhs)
{
return Equals(lhs, rhs);
}

public static bool operator !=(User lhs, User rhs)
{
return !Equals(lhs, rhs);
}

The static Equals method[^] will perform the null checks for you, and then call the instance Equals method on the first parameter:

public static bool Equals(Object objA, Object objB)
{
if (objA==objB) {
return true;
}
if (objA==null || objB==null) {
return false;
}
return objA.Equals(objB);
}

At first glance, it might look like the first line of that method would cause a stack overflow, since your == operator is calling a method that calls the == operator. However, within the Equals method, it's calling the == operator on the Object type, which just tests for reference equality.

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Gilbert Consellado

Thank you, for all the hint, i finally got it now