Fix KeyNotFoundException when comparing dictionaries

TUnit.Assertions.UnitTests/DictionaryAssertionTests.cs

@@ -1,3 +1,4 @@
+using System.Collections.Immutable;
 using TUnit.Assertions.Extensions;
 
 namespace TUnit.Assertions.UnitTests;
@@ -36,4 +37,13 @@ public async Task String_Dictionary_Contains_Key_IgnoreCase()
 
         await TUnitAssert.That(dictionary).ContainsKey("blah", StringComparer.InvariantCultureIgnoreCase);
     }
+
+    [Test]
+    public async Task Immutable_Dictionary_Does_Not_Contain_Key()
+    {
+        await TUnitAssert.ThrowsAsync<TUnitAssertionException>(async () =>
+            await TUnitAssert.That(ImmutableDictionary<string, int>.Empty.Add("Hello", 1))
+                .IsEquivalentTo(ImmutableDictionary<string, int>.Empty.Add("Hello2", 1))
+        );
+    }
 }

TUnit.Assertions/Compare.cs

@@ -91,6 +91,32 @@ private static IEnumerable<ComparisonFailure> CheckEquivalent<
 
             foreach (var key in keys)
             {
+                if (!actualDictionary.Contains(key))
+                {
+                    yield return new ComparisonFailure
+                    {
+                        Type = MemberType.DictionaryItem,
+                        Actual = $"No entry with key: {key}",
+                        Expected = $"[{key}] = {expectedDictionary[key]}",
+                        NestedMemberNames = [..memberNames, $"[{key}]"]
+                    };
+
+                    yield break;
+                }
+
+                if (!expectedDictionary.Contains(key))
+                {
+                    yield return new ComparisonFailure
+                    {
+                        Type = MemberType.DictionaryItem,
+                        Actual = $"[{key}] = {actualDictionary[key]}",
+                        Expected = $"No entry with key: {key}",
+                        NestedMemberNames = [..memberNames, $"[{key}]"]
+                    };
+
+                    yield break;
+                }
+
                 var actualObject = actualDictionary[key];
                 var expectedObject = expectedDictionary[key];