GetHashCode on null fields?
How do I deal with null fields in GetHashCode
function?
Module Module1
Sub Main()
Dim c As New Contact
Dim hash = c.GetHashCode
End Sub
Public Class Contact : Implements IEquatable(Of Contact)
Public Name As String
Public Address As String
Public Overloads Function Equals(ByVal other As Contact) As Boolean _
Implements System.IEquatable(Of Contact).Equals
Return Name = other.Name AndAlso Address = other.Address
End Function
Public Overrides Function Equals(ByVal obj As Object) As Boolean
If ReferenceEquals(Me, obj) Then Return True
If TypeOf obj Is Contact Then
Return Equals(DirectCast(obj, Contact))
Else
Return False
End If
End Function
Public Overrides Function GetHashCode() As Int开发者_运维技巧eger
Return Name.GetHashCode Xor Address.GetHashCode
End Function
End Class
End Module
Typically, you check for null and use 0 for that "part" of the hash code if the field is null:
return (Name == null ? 0 : Name.GetHashCode()) ^
(Address == null ? 0 : Address.GetHashCode());
(pardon the C#-ism, not sure of the null check equivalent in VB)
As Jeff Yates suggested, the override in the answer would give the same hash for (name = null, address = "foo") as (name = "foo", address = null). These need to be different. As suggested in link, something similar to the following would be better.
public override int GetHashCode()
{
unchecked // Overflow is fine, just wrap
{
int hash = 17;
hash = hash * 23 + (Name == null ? 0 : Name.GetHashCode());
hash = hash * 23 + (Address == null ? 0 : Address.GetHashCode());
}
return hash;
}
What is the best algorithm for an overridden System.Object.GetHashCode?
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