Add ListExtensions.firstOrNull, lastOrNull, singleOrNull.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 - Adds `shuffled` to `IterableExtension`.
 - Shuffle `IterableExtension.sample` results.
+* Add `List.firstOrNull`, `List.lastOrNull`, and `List.singleOrNull` extension methods.
+  They repeat corresponding methods for `Iterable`, but avoid creating iterator objects.
 
 ## 1.18.0
 

lib/src/list_extensions.dart

@@ -286,6 +286,25 @@ extension ListExtensions<E> on List<E> {
       yield slice(i, min(i + length, this.length));
     }
   }
+
+  /// The first element, or `null` if the list is empty.
+  E? get firstOrNull {
+    return isNotEmpty ? this[0] : null;
+  }
+
+  /// The last element, or `null` if the list is empty.
+  E? get lastOrNull {
+    final length = this.length;
+    return length != 0 ? this[length - 1] : null;
+  }
+
+  /// The single element of the list, or `null`.
+  ///
+  /// The value is `null` if the list is empty
+  /// or it contains more than one element.
+  E? get singleOrNull {
+    return length == 1 ? this[0] : null;
+  }
 }
 
 /// Various extensions on lists of comparable elements.

test/extensions_test.dart

@@ -1864,6 +1864,39 @@ void main() {
           expect(() => [1].slices(0), throwsRangeError);
         });
       });
+      group('.firstOrNull', () {
+        test('empty', () {
+          expect(<int>[].firstOrNull, null);
+        });
+        test('single', () {
+          expect([1].firstOrNull, 1);
+        });
+        test('first of multiple', () {
+          expect([1, 3, 5].firstOrNull, 1);
+        });
+      });
+      group('.lastOrNull', () {
+        test('empty', () {
+          expect(<int>[].lastOrNull, null);
+        });
+        test('single', () {
+          expect([1].lastOrNull, 1);
+        });
+        test('last of multiple', () {
+          expect([1, 3, 5].lastOrNull, 5);
+        });
+      });
+      group('.singleOrNull', () {
+        test('empty', () {
+          expect(<int>[].singleOrNull, null);
+        });
+        test('single', () {
+          expect([1].singleOrNull, 1);
+        });
+        test('multiple', () {
+          expect([1, 3, 5].singleOrNull, null);
+        });
+      });
     });
     group('on comparable', () {
       group('.binarySearch', () {