How Can I Efficiently Create a Distinct List Using Lambda Expressions in C#?

Use Lambda expression to customize the Distinct() method

When dealing with enumerable objects, it is often necessary to distinguish unique values. Although the System.Linq namespace provides a Distinct() extension method, it lacks an overload that accepts a Lambda expression to specify object equality.

Problem

Due to the lack of overloading based on Lambda expressions, programmers can only use the IEqualityComparer interface, which needs to implement the Equals() and GetHashCode() methods. This approach can be tedious and cumbersome.

Lambda expression based workaround

While no existing extension method matches the required Lambda syntax, a creative workaround can achieve a similar effect. Using GroupBy() and Select() you can convert enumerable objects:

<code class="language-csharp">IEnumerable<customer> filteredList = originalList
  .GroupBy(customer => customer.CustomerId)
  .Select(group => group.First());</code>

By grouping elements by a specified key and selecting the first element from each group, you effectively create a unique list based on the Lambda expression provided in GroupBy().

Equality problem based on Lambda expression

It is important to note that Lambda expression-based equality comparators may cause unexpected behavior for certain types (such as strings). In order to perform unique operations reliably, an accurate and consistent implementation of GetHashCode() must be ensured.

More insights

.NET Chief Architect Anders Hejlsberg discusses the limitations of the Lambda expression-based Distinct() overload, which is because the hash table inside Distinct() requires a gap between Equals() and GetHashCode() to work properly. compatibility.

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