CSHARP-4140: Only use $getField when absolutely necessary.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Ast/Expressions/AstGetFieldExpression.cs

@@ -67,17 +67,14 @@ public bool HasSafeFieldName(out string fieldName)
                 constantFieldName.Value is BsonString stringfieldName)
             {
                 fieldName = stringfieldName.Value;
-                if (fieldName.Length > 0 && IsSafeFirstChar(fieldName[0]) && fieldName.Skip(1).All(c => IsSafeSubsequentChar(c)))
+                if (fieldName.Length > 0 && fieldName[0] != '$' && !fieldName.Contains('.'))
                 {
                     return true;
                 }
             }
 
             fieldName = null;
             return false;
-
-            static bool IsSafeFirstChar(char c) => c == '_' || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z');
-            static bool IsSafeSubsequentChar(char c) => IsSafeFirstChar(c) || (c >= '0' && c <= '9');
         }
 
         public override BsonValue Render()

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4140Tests.cs

@@ -0,0 +1,82 @@
+/* Copyright 2010-present MongoDB Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Bson;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4140Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [InlineData("abc")]
+        [InlineData("123")]
+        [InlineData("12$")]
+        [InlineData("!@#")]
+        public void Should_not_use_getField(string fieldName)
+        {
+            var collection = GetCollection<BsonDocument>();
+            CreateCollection(collection, new BsonDocument(fieldName, 123));
+            var queryable = collection.AsQueryable()
+                .Select(x => x[fieldName]);
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, $"{{ $project : {{ _v : '${fieldName}', _id : 0 }} }}");
+
+            var results = queryable.ToList();
+            results.Should().Equal(123);
+        }
+
+        [Theory]
+        [InlineData("")]
+        [InlineData(".")]
+        [InlineData(".a")]
+        [InlineData("a.")]
+        public void Should_use_getField(string fieldName)
+        {
+            var collection = GetCollection<BsonDocument>();
+            CreateCollection(collection, new BsonDocument(fieldName, 123));
+            var queryable = collection.AsQueryable()
+                .Select(x => x[fieldName]);
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, $"{{ $project : {{ _v : {{ $getField : {{ field : '{fieldName}', input : '$$ROOT' }} }}, _id : 0 }} }}");
+
+            var results = queryable.ToList();
+            results.Should().Equal(123);
+        }
+
+        [Theory]
+        [InlineData("$")]
+        [InlineData("$a")]
+        [InlineData("$$a")]
+        [InlineData("$a$")]
+        public void Should_use_getField_with_literal(string fieldName)
+        {
+            var collection = GetCollection<BsonDocument>();
+            CreateCollection(collection, new BsonDocument(fieldName, 123));
+            var queryable = collection.AsQueryable()
+                .Select(x => x[fieldName]);
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, $"{{ $project : {{ _v : {{ $getField : {{ field : {{ $literal : '{fieldName}' }}, input : '$$ROOT' }} }}, _id : 0 }} }}");
+
+            var results = queryable.ToList();
+            results.Should().Equal(123);
+        }
+    }
+}