kuskmen kuskmen - 3 years ago 73
C# Question

Odd return syntax statement

I know this may sound strange but I don't know even how to search this syntax in internet and also I am not sure what exactly means.

So I've watched over some MoreLINQ code and then I noticed this method

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
{
if (source == null) throw new ArgumentNullException(nameof(source));
if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));

return _(); IEnumerable<TSource> _()
{
var knownKeys = new HashSet<TKey>(comparer);
foreach (var element in source)
{
if (knownKeys.Add(keySelector(element)))
yield return element;
}
}
}


What is this odd return statement?
return _();
?

Answer Source

This is C# 7.0 which supports inline functions....

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
       this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
    {
        if (source == null) throw new 
           ArgumentNullException(nameof(source));
        if (keySelector == null) throw 
             new ArgumentNullException(nameof(keySelector));

        // This is basically executing _AnonymousFunction()
        return _AnonymousFunction(); 

        // This is a new inline method, 
        // return within this is only within scope of
        // this method
        IEnumerable<TSource> _AnonymousFunction()
        {
            var knownKeys = new HashSet<TKey>(comparer);
            foreach (var element in source)
            {
                if (knownKeys.Add(keySelector(element)))
                    yield return element;
            }
        }
    }

Current C# with Func<T>

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
       this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
    {
        if (source == null) throw new 
           ArgumentNullException(nameof(source));
        if (keySelector == null) throw 
             new ArgumentNullException(nameof(keySelector));

        Func<IEnumerable<TSource>> func = () => {
            var knownKeys = new HashSet<TKey>(comparer);
            foreach (var element in source)
            {
                if (knownKeys.Add(keySelector(element)))
                    yield return element;
            }
       };

        // This is basically executing func
        return func(); 

    }

The trick is, _() is declared after it is used, which is perfectly fine.

Pratical use of inline functions

Above example is just a demonstration of how inline method can be used, but most likely if you are going to invoke method just once, then it is of no use.

Many times we have repeated code in method, lets look at this example..

  public void ValidateCustomer(Customer customer){

      if( string.IsNullOrEmpty( customer.FirstName )){
           string error = "Firstname cannot be empty";
           customer.ValidationErrors.Add(error);
           ErrorLogger.Log(error);
           throw new ValidationError(error);
      }

      if( string.IsNullOrEmpty( customer.LastName )){
           string error = "Lastname cannot be empty";
           customer.ValidationErrors.Add(error);
           ErrorLogger.Log(error);
           throw new ValidationError(error);
      }

      ... on  and on... 
  }

I could optimize this with...

  public void ValidateCustomer(Customer customer){

      _validate(string value, string error){
           if(!string.IsNullOrWhitespace(value)){

              // i can easily reference customer here
              customer.ValidationErrors.Add(error);

              ErrorLogger.Log(error);
              throw new ValidationError(error);                   
           }
      }

      _validate(customer.FirstName, "Firstname cannot be empty");
      _validate(customer.LastName, "Lastname cannot be empty");
      ... on  and on... 
  }
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