{"id":3033,"date":"2009-03-25T16:23:00","date_gmt":"2009-03-25T16:23:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/visualstudio\/2009\/03\/25\/how-to-use-linq-methods-to-compare-objects-of-custom-types\/"},"modified":"2019-02-14T15:42:22","modified_gmt":"2019-02-14T23:42:22","slug":"how-to-use-linq-methods-to-compare-objects-of-custom-types","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/visualstudio\/how-to-use-linq-methods-to-compare-objects-of-custom-types\/","title":{"rendered":"How to use LINQ methods to compare objects of custom types"},"content":{"rendered":"

LINQ provides a convenient syntax and many useful methods for operating with collections of objects. However, to be correctly processed by LINQ comparison methods such as <\/font>Distinct<\/span> or <\/font>Intersect<\/span>, a type must satisfy certain requirements.<\/p>\n

<\/font><\/font><\/p>\n

Let’s take a look at the <\/font>Distinct<\/span> method, which returns all distinct objects from a collection.<\/p>\n

<\/font><\/font><\/p>\n

List<<\/span>int<\/span>> numbers = <\/span>new<\/span> List<<\/span>int<\/span>> { 1, 1, 2, 3 };<\/p>\n

<\/span><\/p>\n

var<\/span> distinctNumbers = numbers.Distinct();<\/p>\n

<\/span><\/p>\n

foreach<\/span> (<\/span>var<\/span> number <\/span>in<\/span> distinctNumbers)<\/p>\n

<\/span><\/p>\n

    <\/span>Console.WriteLine(number);<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

The output is: <\/p>\n

<\/font><\/font><\/p>\n

1<\/p>\n

<\/font><\/font><\/span><\/p>\n

2<\/p>\n

<\/font><\/font><\/span><\/p>\n

3<\/p>\n

<\/font><\/font><\/span><\/p>\n

But what if you want to use the <\/font>Distinct<\/span> method for a collection of objects of your own type? For example, like this:<\/p>\n

<\/font><\/font><\/p>\n

class<\/span> Number<\/p>\n

<\/span><\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> int<\/span> Digital { get<\/span>; set<\/span>; }<\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> String<\/span> Textual { get<\/span>; set<\/span>; }<\/p>\n

<\/span><\/p>\n

}<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

class<\/span> Program<\/p>\n

<\/span><\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

    <\/span>static<\/span> void<\/span> Main(string<\/span>[] args)<\/p>\n

<\/span><\/p>\n

    <\/span>{<\/p>\n

<\/span><\/p>\n

       <\/span>List<\/span><Number<\/span>> numbers = new<\/span> List<\/span><Number<\/span>> { <\/p>\n

<\/span><\/p>\n

           <\/span>new<\/span> Number<\/span> { Digital = 1, Textual = “one”<\/span> }, <\/p>\n

<\/span><\/p>\n

           <\/span>new<\/span> Number<\/span> { Digital = 1, Textual = “one”<\/span> } ,<\/p>\n

<\/span><\/p>\n

           <\/span>new<\/span> Number<\/span> { Digital = 2, Textual = “two”<\/span> } ,<\/p>\n

<\/span><\/p>\n

           <\/span>new<\/span> Number<\/span> { Digital = 3, Textual = “three”<\/span> } ,<\/p>\n

<\/span><\/p>\n

           <\/span>};<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

       <\/span>var<\/span> distinctNumbers = numbers.Distinct();<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

       <\/span>foreach<\/span> (var<\/span> number in<\/span> distinctNumbers)<\/p>\n

<\/span><\/p>\n

                   <\/span>Console<\/span>.WriteLine(number.Digital);<\/p>\n

<\/span><\/p>\n

    <\/span>}<\/p>\n

<\/span><\/p>\n

}<\/p>\n

<\/span><\/p>\n

The code compiles, but the output is different:<\/p>\n

<\/font><\/font><\/p>\n

1<\/p>\n

<\/font><\/font><\/span><\/p>\n

1<\/p>\n

<\/font><\/font><\/span><\/p>\n

2<\/p>\n

<\/font><\/font><\/span><\/p>\n

3<\/p>\n

<\/font><\/font><\/span><\/p>\n

Why did that happen? The answer is in the LINQ implementation details. To be correctly processed by the <\/font>Distinct<\/span> method, a type must implement the <\/font>IEquatable<T><\/span> interface and provide its own <\/font><\/span>Equals<\/span> and <\/font><\/span>GetHashCode<\/span> methods.<\/p>\n

<\/font><\/span><\/font><\/p>\n

So, the <\/font><\/span>Number<\/span> class from the previous example should actually look like this:<\/p>\n

<\/font><\/span><\/font><\/p>\n

class<\/span> Number<\/span>: IEquatable<\/span><Number<\/span>><\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> int<\/span> Digital { get<\/span>; set<\/span>; }<\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> String<\/span> Textual { get<\/span>; set<\/span>; }<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> bool<\/span> Equa\nls(Number<\/span> other)<\/p>\n

<\/span><\/p>\n

    <\/span>{<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>\/\/ Check whether the compared object is null.<\/p>\n

<\/span><\/span><\/p>\n

        <\/span>if<\/span> (Object<\/span>.ReferenceEquals(other, null<\/span>)) return<\/span> false<\/span>;<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>\/\/ Check whether the compared object references the same data.<\/p>\n

<\/span><\/span><\/p>\n

        <\/span>if<\/span> (Object<\/span>.ReferenceEquals(this<\/span>, other)) return<\/span> true<\/span>;<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>\/\/ Check whether the objects’ properties are equal.<\/p>\n

<\/span><\/span><\/p>\n

        <\/span>return<\/span> Digital.Equals(other.Digital) &&<\/p>\n

<\/span><\/p>\n

               <\/span>Textual.Equals(other.Textual);<\/p>\n

<\/span><\/p>\n

    <\/span>}<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

    <\/span>\/\/ If Equals returns true for a pair of objects,<\/p>\n

<\/span><\/span><\/p>\n

    <\/span>\/\/ GetHashCode must return the same value for these objects.<\/p>\n

<\/span><\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> override<\/span> int<\/span> GetHashCode()<\/p>\n

<\/span><\/p>\n

    <\/span>{<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>\/\/ Get the hash code for the Textual field if it is not null.<\/p>\n

<\/span><\/span><\/p>\n

        <\/span>int<\/span> hashTextual = Textual == null<\/span> ? 0 : Textual.GetHashCode();<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>\/\/ Get the hash code for the Digital field.<\/p>\n

<\/span><\/span><\/p>\n

        <\/span>int<\/span> hashDigital = Digital.GetHashCode();<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>\/\/ Calculate the hash code for the object.<\/p>\n

<\/span><\/span><\/p>\n

        <\/span>return<\/span> hashDigital ^ hashTextual;<\/p>\n

<\/span><\/p>\n

    <\/span>}<\/p>\n

<\/span><\/p>\n

}<\/span><\/p>\n

<\/span><\/p>\n

But what if you cannot modify the type? What if it was provided by a library and you have no way of implementing the <\/font>IEquatable<T><\/span> interface in this type? The answer is to create your own equality comparer and pass it as a parameter to the <\/font>Distinct<\/span> method. <\/p>\n

<\/font><\/font><\/p>\n

The equality comparer must implement the <\/font>IEqualityComparer<T><\/span> interface and, again, provide <\/font><\/span>GetHashCode<\/span> and <\/font><\/span>Equals<\/span> methods.<\/p>\n

<\/font><\/span><\/font><\/p>\n

Here is how the equality comparer for the original <\/font><\/span>Number<\/span> class might look:<\/p>\n

<\/font><\/span><\/font><\/p>\n

class<\/span> NumberComparer<\/span> : IEqualityComparer<\/span><Number<\/span>><\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> bool<\/span> Equals(Number<\/span> x, Number<\/span> y)<\/p>\n

<\/span><\/p>\n

   <\/span> <\/span>{<\/p>\n

<\/span><\/p>\n

        <\/span>if<\/span> (Object<\/span>.ReferenceEquals(x, y)) return<\/span> true<\/span>;<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

        <\/span>if<\/span> (Object<\/span>.ReferenceEquals(x, null<\/span>) || <\/p>\n

<\/span><\/p>\n

            <\/span>Object<\/span>.ReferenceEquals(y, null<\/span>))<\/p>\n

<\/span><\/p>\n

                <\/span>return<\/span> false<\/span>;<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

            <\/span>return<\/span> x.Digital == y.Digital && x.Textual == y.Textual;<\/p>\n

<\/span><\/p>\n

    <\/span>}<\/p>\n

<\/span><\/p>\n

<\/p>\n

 <\/p>\n

<\/span><\/p>\n

    <\/span>public<\/span> int<\/span> GetHashCode(Number<\/span> number)<\/p>\n

<\/span><\/p>\n

    <\/span>{<\/p>\n

<\/span><\/p>\n

        <\/span>if<\/span> (Object<\/span><\/span><\/p><\/p>\n","protected":false},"excerpt":{"rendered":"

LINQ provides a convenient syntax and many useful methods for operating with collections of objects. However, to be correctly processed by LINQ comparison methods such as Distinct or Intersect, a type must satisfy certain requirements. Let’s take a look at the Distinct method, which returns all distinct objects from a collection. List<int> numbers = new […]<\/p>\n","protected":false},"author":13,"featured_media":255385,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[155],"tags":[3,1383],"class_list":["post-3033","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-visual-studio","tag-net-framework","tag-c"],"acf":[],"blog_post_summary":"

LINQ provides a convenient syntax and many useful methods for operating with collections of objects. However, to be correctly processed by LINQ comparison methods such as Distinct or Intersect, a type must satisfy certain requirements. Let’s take a look at the Distinct method, which returns all distinct objects from a collection. List<int> numbers = new […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/posts\/3033","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/users\/13"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/comments?post=3033"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/posts\/3033\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/media\/255385"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/media?parent=3033"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/categories?post=3033"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/visualstudio\/wp-json\/wp\/v2\/tags?post=3033"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}