Ensure that we generated a CompilationCompletedEvent for compilations with no...

Ensure that we generated a CompilationCompletedEvent for compilations with no source files (example an empty VSIX projects).

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -1709,9 +1709,7 @@ private void CompleteTree(SyntaxTree tree)
                     if (_lazyCompilationUnitCompletedTrees.Count == this.SyntaxTrees.Length)
                     {
                         // if that was the last tree, signal the end of compilation
-                        EventQueue.TryEnqueue(new CompilationCompletedEvent(this));
-                        EventQueue.PromiseNotToEnqueue();
-                        EventQueue.TryComplete();
+                        EnsureCompilationCompleted();
                     }
                 }
             }
@@ -1950,6 +1948,11 @@ internal ImmutableArray<Diagnostic> GetDiagnostics(CompilationStage stage, bool
                 cancellationToken.ThrowIfCancellationRequested();
 
                 builder.AddRange(GetSourceDeclarationDiagnostics(cancellationToken: cancellationToken));
+
+                if (this.EventQueue != null && this.SyntaxTrees.Length == 0)
+                {
+                    EnsureCompilationCompleted();
+                }
             }
 
             cancellationToken.ThrowIfCancellationRequested();

src/Compilers/CSharp/Test/Semantic/Diagnostics/GetDiagnosticsTests.cs

@@ -6,6 +6,7 @@
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Test.Utilities;
 using Roslyn.Test.Utilities;
+using Roslyn.Utilities;
 using Xunit;
 
 namespace Microsoft.CodeAnalysis.CSharp.UnitTests
@@ -287,5 +288,16 @@ private static bool DequeueCompilationEvents(AsyncQueue<CompilationEvent> eventQ
 
             return true;
         }
+
+        [Fact]
+        public void TestEventQueueCompletionForEmptyCompilation()
+        {
+            var compilation = CreateCompilationWithMscorlib45(SpecializedCollections.EmptyEnumerable<SyntaxTree>()).WithEventQueue(new AsyncQueue<CompilationEvent>());
+            
+            // Force complete compilation event queue
+            var unused = compilation.GetDiagnostics();
+
+            Assert.True(compilation.EventQueue.IsCompleted);
+        }
     }
 }

src/Compilers/Core/Portable/Compilation/Compilation.cs

@@ -851,6 +851,22 @@ public INamedTypeSymbol GetTypeByMetadataName(string fullyQualifiedMetadataName)
         /// </summary>
         public abstract ImmutableArray<Diagnostic> GetDiagnostics(CancellationToken cancellationToken = default(CancellationToken));
 
+        internal void EnsureCompilationCompleted()
+        {
+            Debug.Assert(this.EventQueue != null);
+
+            lock (this.EventQueue)
+            {
+                if (!this.EventQueue.IsCompleted)
+                {
+                    // Signal the end of compilation.
+                    EventQueue.TryEnqueue(new CompilationCompletedEvent(this));
+                    EventQueue.PromiseNotToEnqueue();
+                    EventQueue.TryComplete();
+                }
+            }
+        }
+
         internal abstract CommonMessageProvider MessageProvider { get; }
 
         /// <param name="accumulator">Bag to which filtered diagnostics will be added.</param>

src/Compilers/VisualBasic/Portable/Compilation/VisualBasicCompilation.vb

@@ -1644,9 +1644,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
                     If _lazyCompilationUnitCompletedTrees.Count = SyntaxTrees.Length Then
                         ' if that was the last tree, signal the end of compilation
-                        EventQueue.TryEnqueue(New CompilationCompletedEvent(Me))
-                        EventQueue.PromiseNotToEnqueue()
-                        EventQueue.TryComplete()
+                        EnsureCompilationCompleted()
                     End If
                 End If
             End SyncLock
@@ -1925,6 +1923,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                 builder.AddRange(GetBoundReferenceManager().Diagnostics)
                 builder.AddRange(SourceAssembly.GetAllDeclarationErrors(cancellationToken))
                 builder.AddRange(GetClsComplianceDiagnostics(cancellationToken))
+
+                If Me.EventQueue IsNot Nothing AndAlso Me.SyntaxTrees.Length = 0 Then
+                    EnsureCompilationCompleted()
+                End If
             End If
 
             ' Add method body compilation errors.

src/Compilers/VisualBasic/Test/Semantic/Diagnostics/GetDiagnosticsTests.vb

@@ -255,5 +255,15 @@ End Namespace
 
             Return True
         End Function
+
+        <Fact>
+        Public Sub TestEventQueueCompletionForEmptyCompilation()
+            Dim compilation = CreateCompilationWithMscorlib45(SpecializedCollections.EmptyEnumerable(Of SyntaxTree)()).WithEventQueue(New AsyncQueue(Of CompilationEvent)())
+
+            ' Force complete compilation event queue
+            Dim unused = compilation.GetDiagnostics()
+
+            Assert.True(compilation.EventQueue.IsCompleted)
+        End Sub
     End Class
 End Namespace
\ No newline at end of file