CSHARP-4801: Accessing ChangeStreamDocument.FullDocumentBeforeChange throw if pre-image was already expired (#1183)

Author: sanych-sun

src/MongoDB.Driver.Core/ChangeStreamDocument.cs

@@ -166,6 +166,11 @@ public TDocument FullDocumentBeforeChange
                 // if TDocument is BsonDocument avoid deserializing it again to prevent possible duplicate element name errors
                 if (typeof(TDocument) == typeof(BsonDocument) && BackingDocument.TryGetValue("fullDocumentBeforeChange", out var fullDocumentBeforeChange))
                 {
+                    if (fullDocumentBeforeChange.IsBsonNull)
+                    {
+                        return default;
+                    }
+
                     return (TDocument)(object)fullDocumentBeforeChange.AsBsonDocument;
                 }
                 else

tests/MongoDB.Driver.Core.Tests/ChangeStreamDocumentTests.cs

@@ -326,7 +326,6 @@ public void FullDocument_should_allow_duplicate_elements()
         [Fact]
         public void FullDocument_should_return_null_when_not_present()
         {
-            var value = new BsonDocument("x", 1234);
             var backingDocument = new BsonDocument { { "other", 1 } };
             var subject = CreateSubject(backingDocument: backingDocument);
 
@@ -335,6 +334,32 @@ public void FullDocument_should_return_null_when_not_present()
             result.Should().BeNull();
         }
 
+        [Theory]
+        [InlineData("{ other : 1 }")]
+        [InlineData("{ other : 1, fullDocumentBeforeChange : null }")]
+        public void FullDocumentBeforeChange_should_return_null_when_not_present(string changeDocument)
+        {
+            var backingDocument = BsonDocument.Parse(changeDocument);
+            var subject = CreateSubject(backingDocument: backingDocument);
+
+            var result = subject.FullDocumentBeforeChange;
+
+            result.Should().BeNull();
+        }
+
+        [Theory]
+        [InlineData("{ other : 1, fullDocumentBeforeChange : { } }")]
+        [InlineData("{ other : 1, fullDocumentBeforeChange : { a : 1 } }")]
+        public void FullDocumentBeforeChange_should_return_document_when_present(string changeDocument)
+        {
+            var backingDocument = BsonDocument.Parse(changeDocument);
+            var subject = CreateSubject(backingDocument: backingDocument);
+
+            var result = subject.FullDocumentBeforeChange;
+
+            result.Should().BeOfType<BsonDocument>();
+        }
+
         [Fact]
         public void OperationDescription_should_return_expected_result()
         {