apacheGH-38816: [C#] Fix IArrowRecord implementation on StructArray (apache#38827)

### What changes are included in this PR?

Minor bug fix, test for the bug, and a few additional tests which were missed by the previous PR.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

* Closes: apache#38816

Authored-by: Curt Hagenlocher <[email protected]>
Signed-off-by: Curt Hagenlocher <[email protected]>

Author: CurtHagenlocher

csharp/src/Apache.Arrow/Arrays/StructArray.cs

@@ -72,11 +72,11 @@ private IReadOnlyList<IArrowArray> InitializeFields()
 
         IRecordType IArrowRecord.Schema => (StructType)Data.DataType;
 
-        int IArrowRecord.ColumnCount => _fields.Count;
+        int IArrowRecord.ColumnCount => Fields.Count;
 
         IArrowArray IArrowRecord.Column(string columnName, IEqualityComparer<string> comparer) =>
-            _fields[((StructType)Data.DataType).GetFieldIndex(columnName, comparer)];
+            Fields[((StructType)Data.DataType).GetFieldIndex(columnName, comparer)];
 
-        IArrowArray IArrowRecord.Column(int columnIndex) => _fields[columnIndex];
+        IArrowArray IArrowRecord.Column(int columnIndex) => Fields[columnIndex];
     }
 }

csharp/test/Apache.Arrow.Tests/RecordTests.cs

@@ -74,7 +74,25 @@ public void VisitStructAndBatch()
             StructArray level2Array = new StructArray(level2, stringArray.Length, new[] { level1Array }, nulls);
             RecordBatch batch = new RecordBatch(schema, new IArrowArray[] { level2Array }, stringArray.Length);
 
+            var visitor3 = new TestArrayVisitor1();
+            visitor3.Visit(batch);
+            Assert.Equal("111utf8", visitor3.stringBuilder.ToString());
+            var visitor4 = new TestArrayVisitor2();
+            visitor4.Visit(batch);
+            Assert.Equal("322utf8", visitor4.stringBuilder.ToString());
+        }
+
+        [Fact]
+        public void LazyStructInitialization()
+        {
+            StringArray stringArray = new StringArray.Builder().Append("one").AppendNull().AppendNull().Append("four").Build();
+            Field stringField = new Field("column1", StringType.Default, true);
+            StructType structType = new StructType(new[] { stringField });
+            ArrayData structData = new ArrayData(structType, stringArray.Length, 0, 0, new[] { ArrowBuffer.Empty }, new[] { stringArray.Data });
+            IArrowRecord structArray = new StructArray(structData);
 
+            Assert.Equal(1, structArray.ColumnCount);
+            Assert.Equal(structArray.Length, structArray.Column(0).Length);
         }
 
         private class TestTypeVisitor1 : IArrowTypeVisitor, IArrowTypeVisitor<IRecordType>