Add tests.

src/VisualStudio/CSharp/Test/PersistentStorage/AbstractPersistentStorageTests.cs

@@ -368,11 +368,11 @@ private void DoSimultaneousReads(Func<Task<string>> read, string expectedValue)
             Assert.Equal(new List<Exception>(), exceptions);
         }
 
-        private Solution CreateOrOpenSolution()
+        protected Solution CreateOrOpenSolution(bool nullPaths = false)
         {
-            string solutionFile = Path.Combine(_persistentFolder, "Solution1.sln");
-            bool newSolution;
-            if (newSolution = !File.Exists(solutionFile))
+            var solutionFile = Path.Combine(_persistentFolder, "Solution1.sln");
+            var newSolution = !File.Exists(solutionFile);
+            if (newSolution)
             {
                 File.WriteAllText(solutionFile, "");
             }
@@ -386,20 +386,22 @@ private Solution CreateOrOpenSolution()
 
             if (newSolution)
             {
-                string projectFile = Path.Combine(Path.GetDirectoryName(solutionFile), "Project1.csproj");
+                var projectFile = Path.Combine(Path.GetDirectoryName(solutionFile), "Project1.csproj");
                 File.WriteAllText(projectFile, "");
-                solution = solution.AddProject(ProjectInfo.Create(ProjectId.CreateNewId(), VersionStamp.Create(), "Project1", "Project1", LanguageNames.CSharp, projectFile));
+                solution = solution.AddProject(ProjectInfo.Create(ProjectId.CreateNewId(), VersionStamp.Create(), "Project1", "Project1", LanguageNames.CSharp, 
+                    filePath: nullPaths ? null : projectFile));
                 var project = solution.Projects.Single();
 
-                string documentFile = Path.Combine(Path.GetDirectoryName(projectFile), "Document1.cs");
+                var documentFile = Path.Combine(Path.GetDirectoryName(projectFile), "Document1.cs");
                 File.WriteAllText(documentFile, "");
-                solution = solution.AddDocument(DocumentInfo.Create(DocumentId.CreateNewId(project.Id), "Document1", filePath: documentFile));
+                solution = solution.AddDocument(DocumentInfo.Create(DocumentId.CreateNewId(project.Id), "Document1", 
+                    filePath: nullPaths ? null : documentFile));
             }
 
             return solution;
         }
 
-        private IPersistentStorage GetStorage(Solution solution)
+        protected IPersistentStorage GetStorage(Solution solution)
         {
             var storage = GetStorageService().GetStorage(solution);
 
@@ -409,7 +411,7 @@ private IPersistentStorage GetStorage(Solution solution)
 
         protected abstract IPersistentStorageService GetStorageService();
 
-        private Stream EncodeString(string text)
+        protected Stream EncodeString(string text)
         {
             var bytes = _encoding.GetBytes(text);
             var stream = new MemoryStream(bytes);

src/VisualStudio/CSharp/Test/PersistentStorage/SQLitePersistentStorageTests.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System.Linq;
+using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Host;
 using Microsoft.CodeAnalysis.SQLite;
+using Xunit;
 
 namespace Microsoft.CodeAnalysis.UnitTests.WorkspaceServices
 {
@@ -14,5 +17,24 @@ public class SQLitePersistentStorageTests : AbstractPersistentStorageTests
     {
         protected override IPersistentStorageService GetStorageService()
             => new SQLitePersistentStorageService(_persistentEnabledOptionService, testing: true);
+
+        [Fact]
+        public async Task TestNullFilePaths()
+        {
+            var solution = CreateOrOpenSolution(nullPaths: true);
+
+            var streamName = "stream";
+
+            using (var storage = GetStorage(solution))
+            {
+                var project = solution.Projects.First();
+                var document = project.Documents.First();
+                Assert.False(await storage.WriteStreamAsync(project, streamName, EncodeString("")));
+                Assert.False(await storage.WriteStreamAsync(document, streamName, EncodeString("")));
+
+                Assert.Null(await storage.ReadStreamAsync(project, streamName));
+                Assert.Null(await storage.ReadStreamAsync(document, streamName));
+            }
+        }
     }
 }
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