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CA2218: Override GetHashCode on overriding Equals

  • Article
Property Value
Rule ID CA2218
Title Override GetHashCode on overriding Equals
Category Usage
Fix is breaking or non-breaking Non-breaking
Enabled by default in .NET 9 As suggestion

Cause

A public type overrides System.Object.Equals but does not override System.Object.GetHashCode.

Rule description

GetHashCode returns a value, based on the current instance, that's suited for hashing algorithms and data structures such as hash tables. Two objects that are the same type and are equal must return the same hash code to ensure that instances of the following types work correctly:

Note

This rule only applies to Visual Basic code. The C# compiler generates a separate warning, CS0659.

How to fix violations

To fix a violation of this rule, provide an implementation of GetHashCode. For a pair of objects of the same type, ensure that the implementation returns the same value if your implementation of Equals returns true for the pair.

When to suppress warnings

Do not suppress a warning from this rule.

Class example

The following example shows a class (reference type) that violates this rule.

' This class violates the rule.
Public Class Point

    Public Property X As Integer
    Public Property Y As Integer

    Public Sub New(x As Integer, y As Integer)
        Me.X = x
        Me.Y = y
    End Sub

    Public Overrides Function Equals(obj As Object) As Boolean

        If obj = Nothing Then
            Return False
        End If

        If [GetType]() <> obj.GetType() Then
            Return False
        End If

        Dim pt As Point = CType(obj, Point)

        Return X = pt.X AndAlso Y = pt.Y

    End Function

End Class

The following example fixes the violation by overriding GetHashCode().

' This class satisfies the rule.
Public Class Point

    Public Property X As Integer
    Public Property Y As Integer

    Public Sub New(x As Integer, y As Integer)
        Me.X = x
        Me.Y = y
    End Sub

    Public Overrides Function GetHashCode() As Integer
        Return X Or Y
    End Function

    Public Overrides Function Equals(obj As Object) As Boolean

        If obj = Nothing Then
            Return False
        End If

        If [GetType]() <> obj.GetType() Then
            Return False
        End If

        Dim pt As Point = CType(obj, Point)

        Return Equals(pt)

    End Function

    Public Overloads Function Equals(pt As Point) As Boolean
        Return X = pt.X AndAlso Y = pt.Y
    End Function

    Public Shared Operator =(pt1 As Point, pt2 As Point) As Boolean
        Return pt1.Equals(pt2)
    End Operator

    Public Shared Operator <>(pt1 As Point, pt2 As Point) As Boolean
        Return Not pt1.Equals(pt2)
    End Operator

End Class

See also