adding empty enumerator

src/Grace/Data/Immutable/ImmutableLinkedList.cs

@@ -124,6 +124,11 @@ public class ImmutableLinkedList<T> : IEnumerable<T>, IReadOnlyList<T>
  /// </summary>
  public static readonly ImmutableLinkedList<T> Empty = new ImmutableLinkedList<T>(default(T), null, 0);
 
+ /// <summary>
+ /// Empty enumerator
+ /// </summary>
+ private static readonly EmptyLinkedListEnumerator EmptyEnumerator = new EmptyLinkedListEnumerator();
+
  /// <summary>
  /// Default constructor
  /// </summary>
@@ -187,7 +192,7 @@ public ImmutableLinkedList<T> AddRange(IEnumerable<T> range)
  /// <returns></returns>
  public IEnumerator<T> GetEnumerator()
  {
- return new LinkedListEnumerator(this);
+ return Count == 0 ? (IEnumerator<T>)EmptyEnumerator : new LinkedListEnumerator(this);
  }
 
  /// <summary>
@@ -267,6 +272,41 @@ public bool Contains(T value)
 
  private string DebuggerDisplayString => $"Count: {Count}";
 
+ /// <summary>
+ /// Empty enumerator
+ /// </summary>
+ private class EmptyLinkedListEnumerator : IEnumerator<T>
+ {
+ /// <summary>Advances the enumerator to the next element of the collection.</summary>
+ /// <returns>true if the enumerator was successfully advanced to the next element; false if the enumerator has passed the end of the collection.</returns>
+ /// <exception cref="T:System.InvalidOperationException">The collection was modified after the enumerator was created. </exception>
+ public bool MoveNext()
+ {
+ return false;
+ }
+
+ /// <summary>Sets the enumerator to its initial position, which is before the first element in the collection.</summary>
+ /// <exception cref="T:System.InvalidOperationException">The collection was modified after the enumerator was created. </exception>
+ public void Reset()
+ {
+
+ }
+
+ /// <summary>Gets the element in the collection at the current position of the enumerator.</summary>
+ /// <returns>The element in the collection at the current position of the enumerator.</returns>
+ public T Current { get; }
+
+ /// <summary>Gets the current element in the collection.</summary>
+ /// <returns>The current element in the collection.</returns>
+ object IEnumerator.Current => Current;
+
+ /// <summary>Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.</summary>
+ public void Dispose()
+ {
+
+ }
+ }
+
  private class LinkedListEnumerator : IEnumerator<T>
  {
  private readonly ImmutableLinkedList<T> _start;