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title description ms.date ms.topic f1_keywords helpviewer_keywords author ms.author manager ms.subservice dev_langs monikerRange
CA1013: Overload operator equals on overloading add and subtract
A public or protected type implements the addition or subtraction operators without implementing the equality operator.
11/04/2016
reference
OverrideOperatorEqualsOnOverridingAddAndSubtract
OverrideOperatorEqualsOnOverloadingAddAndSubtract
CA1013
OverloadOperatorEqualsOnOverloadingAddAndSubtract
OverrideOperatorEqualsOnOverloadingAddAndSubtract
OverrideOperatorEqualsOnOverridingAddAndSubtract
CA1013
OverloadOperatorEqualsOnOverloadingAddAndSubtract
mikejo5000
mikejo
mijacobs
code-analysis
CSharp
VB
vs-2019

CA1013: Overload operator equals on overloading add and subtract

Item Value
RuleId CA1013
Category Microsoft.Design
Breaking change Non-breaking

Cause

A public or protected type implements the addition or subtraction operators without implementing the equality operator.

Rule description

When instances of a type can be combined by using operations such as addition and subtraction, you should almost always define equality to return true for any two instances that have the same constituent values.

You cannot use the default equality operator in an overloaded implementation of the equality operator. Doing so will cause a stack overflow. To implement the equality operator, use the Object.Equals method in your implementation. See the following example.

C#

if (Object.ReferenceEquals(left, null))
    return Object.ReferenceEquals(right, null);
return left.Equals(right);

VB

If (Object.ReferenceEquals(left, Nothing)) Then
    Return Object.ReferenceEquals(right, Nothing)
Else
    Return left.Equals(right)
End If

How to fix violations

To fix a violation of this rule, implement the equality operator so that it is mathematically consistent with the addition and subtraction operators.

When to suppress warnings

It is safe to suppress a warning from this rule when the default implementation of the equality operator provides the correct behavior for the type.

Example 1

The following example defines a type (BadAddableType) that violates this rule. This type should implement the equality operator to make any two instances that have the same field values test true for equality. The type GoodAddableType shows the corrected implementation. Note that this type also implements the inequality operator and overrides xref:System.Object.Equals%2A to satisfy other rules. A complete implementation would also implement xref:System.Object.GetHashCode%2A.

:::code language="csharp" source="../snippets/csharp/VS_Snippets_CodeAnalysis/FxCop.Design.AddAndSubtract/cs/FxCop.Design.AddAndSubtract.cs" id="Snippet1":::

Example 2

The following example tests for equality by using instances of the types that were previously defined in this topic to illustrate the default and correct behavior for the equality operator.

:::code language="csharp" source="../snippets/csharp/VS_Snippets_CodeAnalysis/FxCop.Design.TestAddAndSubtract/cs/FxCop.Design.TestAddAndSubtract.cs" id="Snippet1":::

This example produces the following output:

Bad type:  {2,2} {2,2} are equal? No
Good type: {3,3} {3,3} are equal? Yes
Good type: {3,3} {3,3} are == ?   Yes
Bad type:  {2,2} {9,9} are equal? No
Good type: {3,3} {9,9} are == ?   No

See also