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POWhy does using anonymous type work and using an explicit type not in a GroupBy?
 

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  1. POWhy does using anonymous type work and using an explicit type not in a GroupBy?
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    <p>I have a problem where I want a group type to be strongly typed but if I do it doesn't group correctly. See the code below...</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; namespace ConsoleApplication35 { class Program { static void Main(string[] args) { List&lt;Foo&gt; foos = new List&lt;Foo&gt;(); foos.Add(new Foo() { Key = "Test" }); foos.Add(new Foo() { Key = "Test" }); foos.Add(new Foo() { Key = "Test" }); var groups = foos.GroupBy&lt;Foo, dynamic&gt;(entry =&gt; new { GroupKey = entry.Key }); Console.WriteLine(groups.Count()); groups = foos.GroupBy&lt;Foo, dynamic&gt;(entry =&gt; new GroupingKey() { GroupKey = entry.Key }); Console.WriteLine(groups.Count()); } public class Foo { public string Key { get; set; } } public class GroupingKey { public string GroupKey { get; set; } } } } </code></pre> <p>The output:</p> <pre><code>1 3 Press any key to continue . . . </code></pre> <p>I would expect the result to be the same regardless of using an explicit type nor not i.e. should only be one group with 3 items not 3 groups with 1 item. What is going on here?</p> <p><em><strong>Update</em></strong> I added an IEqualityComparer and it works now! See below:</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; namespace ConsoleApplication35 { class Program { static void Main(string[] args) { List&lt;Foo&gt; foos = new List&lt;Foo&gt;(); foos.Add(new Foo() { Key = "Test" }); foos.Add(new Foo() { Key = "Test" }); foos.Add(new Foo() { Key = "Test" }); var groups = foos.GroupBy&lt;Foo, dynamic&gt;(entry =&gt; new //GroupingKey() { GroupKey = entry.Key }); Console.WriteLine(groups.Count()); groups = foos.GroupBy&lt;Foo, GroupingKey&gt;(entry =&gt; new GroupingKey() { GroupKey = entry.Key }, new GroupingKeyEqualityComparer()); Console.WriteLine(groups.Count()); } public class Foo { public string Key { get; set; } } public class GroupingKey { public string GroupKey { get; set; } } public class GroupingKeyEqualityComparer : IEqualityComparer&lt;GroupingKey&gt; { #region IEqualityComparer&lt;GroupingKey&gt; Members public bool Equals(GroupingKey x, GroupingKey y) { return x.GroupKey == y.GroupKey; } public int GetHashCode(GroupingKey obj) { return obj.GroupKey.GetHashCode(); } #endregion } } } </code></pre> <p>Output:</p> <pre><code>1 1 Press any key to continue . . . </code></pre> <p>This pretty much confirms the answer given by JaredPar!</p>
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    1. USJim
    1. USJim
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      1. POWhy does using anonymous type work and using an explicit type not in a GroupBy?
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      1. POWhy does using anonymous type work and using an explicit type not in a GroupBy?
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      1. VTUpMod
    1. CODoes it behave if you change the second `groups` to a different variable name and instead of using `<Foo, dynamic>` use a `<Foo, GroupingKey>`? It's possible that the disconnect is the generic typing in this instance.
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      1. POWhy does using anonymous type work and using an explicit type not in a GroupBy?
      1. USJoel Etherton
    2. CO@Joel - actually the dynamic thing didn't seem to make a difference until I added IEqualityComparer and it forced me to change it so the types aligned.
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      1. POWhy does using anonymous type work and using an explicit type not in a GroupBy?
      1. USJim
 

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