Good type design dictates that the Equals should always be overridden. This is actually a FXCop violation but 99.99% of types that I see, never has it (or GetHashCode). Overriding Equals is pretty easy.
Public Overloads Function Equals(ByVal obj As [Object] ) As Boolean
Try
Dim ci = DirectCast (obj, ComputerInfo )
Return (ci.OSFullName = Me.OSFullName) And (ci.OSPlatform = Me.OSPlatform)
Catch
Return False
End Try
End Function
The issue with overriding Equals like shown in the code above, is maintenance. Any developer that adds a property will have to remember to add that property to the statement above. Good luck keeping this up to date. I wrote the following method to help with this issue.
Public Function DoesObjectEqualInstance(ByVal obj As [Object] , ByVal instance As Object ) As Boolean
For Each prop As PropertyInfo In obj.GetType.GetProperties()
If Not prop.GetValue(obj, Nothing ) .Equals(instance.GetType.GetProperty(prop.Name) .GetValue(instance, Nothing )) Then
Return False
End If
Next
Return True
End Function
Now your Equals statement should look like this:
Public Overloads Function Equals(ByVal obj As [Object] ) As Boolean
Return General.DoesObjectEqualInstance(Me , obj)
End Function
GetHashCode should also be overridden in the same manner for the same reasons. Here is the code and an example:
Public Function GetInstanceHashCode(ByVal instance As Object ) As Int32
Dim hash As Int32
For Each prop As PropertyInfo In instance.GetType.GetProperties()
Dim value = prop.GetValue(instance, Nothing )
hash = hash Xor value.GetHashCode
Next
Return hash
End Function
Public Overloads Overrides Function GetHashCode() As Integer
Return General.GetInstanceHashCode(Me )
End Function
Note: This code uses reflection which is not as fast if you coded it yourself, but it’s much more maintainable.
Tip By: David McCarter
This code can be found in the dotNetTips.Utility open source project.
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Bill Wagner explains another reason for overriding HashCode in the case where you’re using your type as a key in dictionaries. The default hash code for reference types potentially violates some of the requirements for a dictionary key. You have to be very careful when using reference types as dictionary keys, and probably should avoid it if you haven’t thought it through pretty carefully.