Why DistinctBy does not contain a definition?

Question

I have a table of product specifications I do not want duplicate elements to be displayed when displaying product specifications. I did this, but the error is displayed. Where did I go wrong?

And displays this error: 'IEnumerable' does not contain a definition

 private static List<ProductPropertiesQueryModel> 
  MapProductProperties(List<ProductProperties> ProductProperties)
    {
       return ProductProperties
        .Select(x=>new ProductPropertiesQueryModel
        {
           ProductId =x.ProductId,
           Color =x.Color,
           Size=x.Size
        }).DistinctBy(r=>r.Color).ToList();                      
    }

Answer 1

DistinctBy came with .NET 6 (New LINQ APIs section). Looks like you are using lower version. You can try one of these below.

1- You can write extension on your own,

public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> listItems, Func<T, TKey> groupingKey)
{
     return listItems.GroupBy(groupingKey).Select(x => x.First());
}

2- Upgrade your .NET version,

3- Use GroupBy() directly,

private static List<ProductPropertiesQueryModel> MapProductProperties(List<ProductProperties> ProductProperties)
{
       return ProductProperties
        .Select(x=>new ProductPropertiesQueryModel
        {
           ProductId =x.ProductId,
           Color =x.Color,
           Size=x.Size
        }).GroupBy(x => x.Color).Select(x => x.First()).ToList();                 
}

4- Use MoreLinq to have DistinctBy()