There is no box conversion or type parameter conversion from 'V' to 'System.IEquatable <V>'

Question

I have the following extension methods that compile successfully and behave as designed.

public static IEnumerable<T> WhereNotEmpty<T>(this IEnumerable<T> source) where T : struct { return source.Where(item => !item.Equals(default(T))); } public static IEnumerable<V> SelectNotEmpty<T, V>(this IEnumerable<T> source, Func<T, V> selector) where V : struct { return source.Select(selector).WhereNotEmpty(); } 

However, to avoid boxing, I added a new general restriction as follows:

 public static IEnumerable<T> WhereNotEmpty<T>(this IEnumerable<T> source) where T : struct, IEquatable<T> { return source.Where(item => !item.Equals(default(T))); } public static IEnumerable<V> SelectNotEmpty<T, V>(this IEnumerable<T> source, Func<T, V> selector) where V : struct, IEquatable<T> { return source.Select(selector).WhereNotEmpty(); // compile error! } 

Now I get a compilation error when SelectNotEmpty calls WhereNotEmpty :

Type 'V' cannot be used as a parameter of type 'T' in a generic type or the method 'MyExtensions.WhereNotEmpty (System.Collections.Generic.IEnumerable). There is no box conversion or type parameter conversion from 'V' to 'System.IEquatable'.

I'm sure I made a stupid mistake, but I do not see this. Can someone point me to me please?

Answer 1

Your limit on V should be

 where V : struct, IEquatable<V> 

The type T in WhereNotEmpty must be IEquatable<T> , and you pass IEnumerable<V> to WhereNotEmpty from SelectNotEmpty after applying the transform.

 public static IEnumerable<V> SelectNotEmpty<T, V>(this IEnumerable<T> source, Func<T, V> selector) where V : struct, IEquatable<V> { return source.Select(selector).WhereNotEmpty(); } 

Answer 2

Try changing:

 public static IEnumerable<V> SelectNotEmpty<T, V>(this IEnumerable<T> source, Func<T, V> selector) where V : struct, IEquatable<T> { return source.Select(selector).WhereNotEmpty(); } 

about

  public static IEnumerable<V> SelectNotEmpty<T, V>(this IEnumerable<T> source, Func<T, V> selector) where V : struct, IEquatable<V> { return source.Select(selector).WhereNotEmpty(); }