CSHARP-1660: added support for $strLenCP.

Author: craiggwilson

src/MongoDB.Driver/Linq/Translators/AggregateLanguageTranslator.cs

@@ -310,6 +310,12 @@ private BsonValue TranslateMemberAccess(MemberExpression node)
                 return result;
             }
 
+            if (node.Expression.Type == typeof(string)
+                && TryTranslateStringMemberAccess(node, out result))
+            {
+                return result;
+            }
+
             var expressionType = node.Expression.Type;
             if (node.Expression != null
                 && (expressionType.ImplementsInterface(typeof(ICollection<>))
@@ -701,6 +707,20 @@ private bool TryTranslateDateTimeMemberAccess(MemberExpression node, out BsonVal
             return false;
         }
 
+        private bool TryTranslateStringMemberAccess(MemberExpression node, out BsonValue result)
+        {
+            result = null;
+            var field = TranslateValue(node.Expression);
+            switch (node.Member.Name)
+            {
+                case "Length":
+                    result = new BsonDocument("$strLenCP", field);
+                    return true;
+            }
+
+            return false;
+        }
+
         private bool TryTranslateFirstResultOperator(PipelineExpression node, out BsonValue result)
         {
             var resultOperator = node.ResultOperator as FirstResultOperator;

tests/MongoDB.Driver.Tests/Linq/Translators/AggregateProjectTranslatorTests.cs

@@ -1093,6 +1093,18 @@ public void Should_translate_select_with_an_anonymous_type_then_where_to_map_the
             result.Value.Result.Single().F.Should().Be(33);
         }
 
+        [SkippableFact]
+        public void Should_translate_string_length()
+        {
+            RequireServer.Where(minimumVersion: "3.3.4");
+
+            var result = Project(x => new { Result = x.A.Length });
+
+            result.Projection.Should().Be("{ Result: { \"$strLenCP\": \"$A\" }, _id: 0 }");
+
+            result.Value.Result.Should().Be(7);
+        }
+
         [SkippableFact]
         public void Should_translate_stdDevPop()
         {