hereyougo.cs

using System.Collections.Generic;

public class MyCollection<T> : IEnumerable<T>
{
    private readonly List<T> _collection = new List<T>();
    
    public IEnumerator<T> GetEnumerator() => _collection.GetEnumerator();
    
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

/*
error CS0305: Using the generic type 'IEnumerable<T>' requires 1 type arguments
error CS0305: Using the generic type 'IEnumerator<T>' requires 1 type arguments
error CS0538: 'IEnumerable' in explicit interface declaration is not an interface
error CS0738: 'MyCollection<T>' does not implement interface member 'IEnumerable.GetEnumerator()'. 'MyCollection<T>.GetEnumerator()' cannot implement 'IEnumerable.GetEnumerator()' because it does not have the matching return type of 'IEnumerator'.
*/