Addressed comments.

Microsoft.Azure.Cosmos/src/Linq/CosmosLinqQuery.cs

@@ -16,6 +16,7 @@ namespace Microsoft.Azure.Cosmos.Linq
  using Microsoft.Azure.Cosmos.Serializer;
  using Microsoft.Azure.Cosmos.Tracing;
  using Newtonsoft.Json;
+ using Debug = System.Diagnostics.Debug;
 
  /// <summary>
  /// This is the entry point for LINQ query creation/execution, it generate query provider, implements IOrderedQueryable.
@@ -109,11 +110,10 @@ public IEnumerator<T> GetEnumerator()
  }
 
  FeedIterator<T> localFeedIterator = this.CreateFeedIterator(false, out ClientOperation clientOperation);
-
- if (clientOperation != ClientOperation.None)
- {
- throw new InvalidOperationException($"Linq expression cannot be converted to query directly since it involves client side operation : {clientOperation}");
- }
+ Debug.Assert(
+ clientOperation == ClientOperation.None,
+ "CosmosLinqQuery Assert!",
+ $"Unexpected client operation. Expected 'None', Received '{clientOperation}'");
 
  while (localFeedIterator.HasMoreResults)
  {
@@ -151,33 +151,33 @@ public override string ToString()
  public QueryDefinition ToQueryDefinition(IDictionary<object, string> parameters = null)
  {
  LinqQuery linqQuery = DocumentQueryEvaluator.Evaluate(this.Expression, this.linqSerializationOptions, parameters);
-
- if (linqQuery.ClientOperation != ClientOperation.None)
- {
- throw new InvalidOperationException($"Linq expression cannot be converted to query directly since it involves client side operation : {linqQuery.ClientOperation}");
- }
+ ClientOperation clientOperation = linqQuery.ClientOperation;
+ Debug.Assert(
+  clientOperation == ClientOperation.None,
+ "CosmosLinqQuery Assert!",
+  $"Unexpected client operation. Expected 'None', Received '{clientOperation}'");
 
  return QueryDefinition.CreateFromQuerySpec(linqQuery.SqlQuerySpec);
  }
 
  public FeedIterator<T> ToFeedIterator()
  {
  FeedIterator<T> iterator = this.CreateFeedIterator(true, out ClientOperation clientOperation);
- if (clientOperation != ClientOperation.None)
- {
- throw new InvalidOperationException($"This operation does not support LINQ expression that contains client operation {clientOperation}");
- }
+ Debug.Assert(
+  clientOperation == ClientOperation.None,
+ "CosmosLinqQuery Assert!",
+  $"Unexpected client operation. Expected 'None', Received '{clientOperation}'");
 
  return new FeedIteratorInlineCore<T>(iterator, this.container.ClientContext);
  }
 
  public FeedIterator ToStreamIterator()
  {
  FeedIterator iterator = this.CreateStreamIterator(true, out ClientOperation clientOperation);
- if (clientOperation != ClientOperation.None)
- {
- throw new InvalidOperationException($"This operation does not support LINQ expression that contains client operation {clientOperation}");
- }
+ Debug.Assert(
+  clientOperation == ClientOperation.None,
+ "CosmosLinqQuery Assert!",
+  $"Unexpected client operation. Expected 'None', Received '{clientOperation}'");
 
  return new FeedIteratorInlineCore(iterator, this.container.ClientContext);
  }
@@ -203,10 +203,10 @@ internal async Task<Response<T>> AggregateResultAsync(CancellationToken cancella
  Headers headers = new Headers();
 
  FeedIteratorInlineCore<T> localFeedIterator = this.CreateFeedIterator(isContinuationExpected: false, clientOperation: out ClientOperation clientOperation);
- if (clientOperation != ClientOperation.None)
- {
- throw new InvalidOperationException($"This operation does not support LINQ expression that contains client operation {clientOperation}");
- }
+ Debug.Assert(
+  clientOperation == ClientOperation.None,
+ "CosmosLinqQuery Assert!",
+  $"Unexpected client operation. Expected 'None', Received '{clientOperation}'");
 
  ITrace rootTrace;
  using (rootTrace = Trace.GetRootTrace("Aggregate LINQ Operation"))
@@ -250,7 +250,9 @@ internal T ExecuteScalar()
  System.Diagnostics.Debug.Assert(result.Count <= 1, "CosmosLinqQuery Assert!", "At most one result is expected!");
  return result.FirstOrDefault();
 
- // None typically gets returned during invocation of this method when aggregates are called and evaluates to FirstOrDefault.
+ // ExecuteScalar gets called when (sync) aggregates such as Max, Min, Sum are invoked on the IQueryable.
+ // Since query fully supprots these operations, there is no client operation involved.
+ // In these cases we return FirstOrDefault which handles empty/undefined/null result set from the backend.
  case ClientOperation.None:
  return result.FirstOrDefault();
 

Microsoft.Azure.Cosmos/src/Query/v2Query/DocumentQueryExecutionContextBase.cs

@@ -138,7 +138,7 @@ protected SqlQuerySpec QuerySpec
 
  if (linqQuery.ClientOperation != ClientOperation.None)
  {
- throw new InvalidOperationException($"This operation does not support support LINQ expression that contains client operation {linqQuery.ClientOperation}");
+ throw new NotSupportedException($"This operation does not support the supplied LINQ expression since it involves client side operation : {linqQuery.ClientOperation}");
  }
 
  this.querySpec = linqQuery.SqlQuerySpec;

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.EmulatorTests/Linq/LinqCleanupTests.cs

@@ -19,7 +19,7 @@ namespace Microsoft.Azure.Cosmos.Services.Management.Tests.LinqProviderTests
  [TestClass]
  public class LinqCleanupTests
  {
- [Ignore]
+ // [Ignore]
  [TestMethod]
  public async Task CleanupLinqTestDatabases()
  {

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.EmulatorTests/Linq/LinqScalarFunctionBaselineTests.cs

@@ -16,6 +16,7 @@ namespace Microsoft.Azure.Cosmos.Services.Management.Tests.LinqProviderTests
  using Microsoft.VisualStudio.TestTools.UnitTesting;
  using Newtonsoft.Json;
  using Newtonsoft.Json.Linq;
+ using Microsoft.Azure.Cosmos.Linq;
 
  /// <summary>
  /// LINQ tests for Non aggregate scalar functions such as FirstOrDefault