feat: enhance empty collection assertions with detailed failure messages (#3383)

Author: thomhurst

TUnit.Assertions.Tests/ArrayAssertionTests.cs

@@ -59,4 +59,40 @@ public async Task Test_Array_IsSingleElement_String()
         var array = new[] { "single" };
         await Assert.That(array).IsSingleElement();
     }
+
+    [Test]
+    public async Task Test_Array_IsEmpty_Fails_With_Item_Details()
+    {
+        var array = new[] { 1, 2, 3 };
+        var action = async () => await Assert.That(array).IsEmpty();
+
+        var exception = await Assert.That(action).Throws<AssertionException>();
+
+        await Assert.That(exception.Message)
+            .Contains("collection contains items: [1, 2, 3]");
+    }
+
+    [Test]
+    public async Task Test_Array_IsEmpty_Fails_With_String_Item_Details()
+    {
+        var array = new[] { "hello", "world" };
+        var action = async () => await Assert.That(array).IsEmpty();
+
+        var exception = await Assert.That(action).Throws<AssertionException>();
+
+        await Assert.That(exception.Message)
+            .Contains("collection contains items: [hello, world]");
+    }
+
+    [Test]
+    public async Task Test_Array_IsEmpty_Fails_With_Large_Collection_Shows_Limited_Items()
+    {
+        var array = Enumerable.Range(1, 15).ToArray();
+        var action = async () => await Assert.That(array).IsEmpty();
+
+        var exception = await Assert.That(action).Throws<AssertionException>();
+
+        await Assert.That(exception.Message)
+            .Contains("collection contains items: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 5 more...]");
+    }
 }

TUnit.Assertions/Conditions/CollectionAssertions.cs

@@ -41,7 +41,35 @@ protected override Task<AssertionResult> CheckAsync(EvaluationMetadata<TValue> m
                 return Task.FromResult(AssertionResult.Passed);
             }
 
-            return Task.FromResult(AssertionResult.Failed("collection contains items"));
+            // Collection is not empty - collect items for error message
+            var items = new List<object?>();
+            const int maxItemsToShow = 10;
+            var totalCount = 1; // We already have the first item
+
+            // Add first item
+            items.Add(enumerator.Current);
+
+            // Collect remaining items up to the limit
+            while (enumerator.MoveNext())
+            {
+                totalCount++;
+                if (items.Count < maxItemsToShow)
+                {
+                    items.Add(enumerator.Current);
+                }
+            }
+
+            // Build error message
+            var sb = new StringBuilder("collection contains items: [");
+            sb.Append(string.Join(", ", items));
+            if (totalCount > maxItemsToShow)
+            {
+                var remainingCount = totalCount - maxItemsToShow;
+                sb.Append($", and {remainingCount} more...");
+            }
+            sb.Append(']');
+
+            return Task.FromResult(AssertionResult.Failed(sb.ToString()));
         }
         finally
         {