CSHARP-3977: Refactor GroupForLinq3 to return an explicit type. (#693)

Author: JamesKovacs

src/MongoDB.Driver/GroupForLinq3Result.cs

@@ -0,0 +1,57 @@
+/* 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;
+
+namespace MongoDB.Driver
+{
+    /// <summary>
+    /// Represents the return result from PipelineDefinitionBuilder.GroupForLinq3 method.
+    /// </summary>
+    /// <typeparam name="TInput">The type of the input documents.</typeparam>
+    /// <typeparam name="TValue">The type of the values.</typeparam>
+    /// <typeparam name="TOutput">The type of the output documents.</typeparam>
+    public class GroupForLinq3Result<TInput, TValue, TOutput>
+    {
+        internal GroupForLinq3Result(PipelineStageDefinition<TInput, IGrouping<TValue, TInput>> groupStage, PipelineStageDefinition<IGrouping<TValue, TInput>, TOutput> projectStage)
+        {
+            GroupStage = groupStage;
+            ProjectStage = projectStage;
+        }
+
+        /// <summary>
+        /// The resulting group stage.
+        /// </summary>
+        public PipelineStageDefinition<TInput, IGrouping<TValue, TInput>> GroupStage { get; }
+
+        /// <summary>
+        /// The resulting project stage.
+        /// </summary>
+        public PipelineStageDefinition<IGrouping<TValue, TInput>, TOutput> ProjectStage { get; }
+
+        /// <summary>
+        /// Deconstructs this class into its components.
+        /// </summary>
+        /// <param name="groupStage">The group stage.</param>
+        /// <param name="projectStage">The project stage.</param>
+        public void Deconstruct(
+            out PipelineStageDefinition<TInput, IGrouping<TValue, TInput>> groupStage,
+            out PipelineStageDefinition<IGrouping<TValue, TInput>, TOutput> projectStage)
+        {
+            groupStage = GroupStage;
+            projectStage = ProjectStage;
+        }
+    }
+}

src/MongoDB.Driver/PipelineStageDefinitionBuilder.cs

@@ -735,15 +735,15 @@ public static PipelineStageDefinition<TInput, TOutput> Group<TInput, TValue, TOu
         /// <param name="translationOptions">The translation options.</param>
         /// <returns>The stage.</returns>
         /// <remarks>This method can only be used with LINQ3 but that can't be verified until Render is called.</remarks>
-        public static (PipelineStageDefinition<TInput, IGrouping<TValue, TInput>> GroupStage, PipelineStageDefinition<IGrouping<TValue, TInput>, TOutput> ProjectStage) GroupForLinq3<TInput, TValue, TOutput>(
+        public static GroupForLinq3Result<TInput, TValue, TOutput> GroupForLinq3<TInput, TValue, TOutput>(
             Expression<Func<TInput, TValue>> value,
             Expression<Func<IGrouping<TValue, TInput>, TOutput>> group,
             ExpressionTranslationOptions translationOptions = null)
         {
             Ensure.IsNotNull(value, nameof(value));
             Ensure.IsNotNull(group, nameof(group));
             var stages = new Linq.Linq3Implementation.GroupExpressionStageDefinitions<TInput, TValue, TOutput>(value, group);
-            return (stages.GroupStage, stages.ProjectStage);
+            return new GroupForLinq3Result<TInput, TValue, TOutput>(stages.GroupStage, stages.ProjectStage);
         }
 
         /// <summary>