feat: improve TryGetFirstDescendent logic

Author: Meir017

src/FluentAssertions.Analyzers.Tests/Tips/CollectionTests.cs

@@ -763,6 +763,14 @@ public void CollectionShouldContainSingle_TestAnalyzer_GenericIEnumerableShouldR
         [Implemented]
         public void CollectionShouldHaveElementAt_ElementAtIndexShouldBe_TestAnalyzer(string assertion) => VerifyCSharpDiagnosticCodeBlock(assertion, DiagnosticMetadata.CollectionShouldHaveElementAt_ElementAtIndexShouldBe);
 
+        [DataTestMethod]
+        [AssertionDiagnostic("actual.ElementAt(k).BooleanProperty.Should().Be(expectedItem.BooleanProperty{0});")]
+        [AssertionDiagnostic("actual.ElementAt(6).BooleanProperty.Should().Be(expectedItem.BooleanProperty{0});")]
+        [AssertionDiagnostic("actual.AsEnumerable().ElementAt(k).BooleanProperty.Should().Be(expectedItem.BooleanProperty{0}).And.ToString();")]
+        [AssertionDiagnostic("actual.AsEnumerable().ElementAt(6).BooleanProperty.Should().Be(expectedItem.BooleanProperty{0}).And.ToString();")] 
+        [Implemented]
+        public void CollectionShouldHaveElementAt_ElementAtIndexShouldBe_TestNoAnalyzer(string assertion) => DiagnosticVerifier.VerifyCSharpDiagnosticUsingAllAnalyzers(GenerateCode.GenericIListCodeBlockAssertion(assertion));
+
         [DataTestMethod]
         [AssertionDiagnostic("actual[k].Should().Be(expectedItem{0});")]
         [AssertionDiagnostic("actual[6].Should().Be(expectedItem{0});")]

src/FluentAssertions.Analyzers/Tips/FluentAssertionsAnalyzer.cs

@@ -301,7 +301,7 @@ private static void AnalyzeInvocation(OperationAnalysisContext context, FluentAs
                 return;
             case "OnlyHaveUniqueItems" when assertion.IsContainedInType(metadata.GenericCollectionAssertionsOfT3):
                 {
-                    if (!invocation.TryGetFirstDescendent<IInvocationOperation>(out var invocationBeforeShould)) return;
+                    if (!invocation.TryGetSingleArgumentAs<IInvocationOperation>(out var invocationBeforeShould)) return;
                     switch (invocationBeforeShould.TargetMethod.Name)
                     {
                         case nameof(Enumerable.Select) when IsEnumerableMethodWithPredicate(invocationBeforeShould, metadata):
@@ -370,7 +370,7 @@ private static void AnalyzeInvocation(OperationAnalysisContext context, FluentAs
                 }
             case "Be" when assertion.IsContainedInType(metadata.ObjectAssertionsOfT2):
                 {
-                    if (invocation.TryGetFirstDescendent<IInvocationOperation>(out var invocationBeforeShould))
+                    if (invocation.TryGetSingleArgumentAs<IInvocationOperation>(out var invocationBeforeShould))
                     {
                         switch (invocationBeforeShould.TargetMethod.Name)
                         {