Can Lambda Expressions Be Used Directly with `Distinct()` in C#?

Distinct() Methods vs. Lambda Expressions in C#: Exploring Alternatives

The Distinct() extension method in System.Linq provides a convenient way to filter out duplicate elements from an enumerable object. While it can be used without arguments in simple cases, it requires a IEqualityComparer instance to determine object equality. This can be troublesome, especially when using lambda expressions.

Distinct() Is there an overload that accepts lambda expressions? Here’s how to do it:

Custom extension method

You can define your own extension method that takes a lambda expression to compare elements:

<code class="language-csharp">public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> source, Func<T, TKey> keySelector)
{
    return source.GroupBy(keySelector).Select(group => group.First());
}</code>

Usage:

<code class="language-csharp">var distinctValues = myCustomerList.DistinctBy(customer => customer.CustomerId);</code>

Inline IEqualityComparer

While IEqualityComparer cannot be specified directly inline, a custom comparator class can be used as a workaround:

<code class="language-csharp">public class CustomerEqualityComparer : IEqualityComparer<Customer>
{
    public bool Equals(Customer x, Customer y)
    {
        return x.CustomerId == y.CustomerId;
    }

    public int GetHashCode(Customer obj)
    {
        return obj.CustomerId.GetHashCode();
    }
}</code>

Usage:

<code class="language-csharp">var equalityComparer = new CustomerEqualityComparer();
var distinctValues = myCustomerList.Distinct(equalityComparer);</code>

Explanation by Anders Hejlsberg

C# language designer Anders Hejlsberg has solved this problem. He explains that defining an overload of Distinct() that accepts a lambda expression is technically challenging because equal elements must have the same GetHashCode return value if the object is to work properly in the internal hash table. . Therefore, IEqualityComparer must be used.

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