Type Design: Equals and HashCode

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 
          Dim  ci = DirectCast (obj, ComputerInfo )
          Return  (ci.OSFullName = Me.OSFullName) And
                    (ci.OSPlatform = Me.OSPlatform)
          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 )
             .GetValue(instance, Nothing )) Then 
             Return  False 
          End  If 
        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
       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.

One thought on “Type Design: Equals and HashCode

  1. 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.

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