Move exception to setter for UsePollingFileWatcher (dotnet/extensions…

…#2099)

Commit migrated from dotnet/extensions@62536db

src/libraries/Microsoft.Extensions.FileProviders.Physical/src/PhysicalFileProvider.cs

@@ -88,17 +88,22 @@ public bool UsePollingFileWatcher
             {
                 if (_fileWatcher != null)
                 {
-                    throw new InvalidOperationException($"Cannot modify {nameof(UsePollingFileWatcher)} once file watcher has been initialized.");
+                    return false;
                 }
-
                 if (_usePollingFileWatcher == null)
                 {
                     ReadPollingEnvironmentVariables();
                 }
-
-                return _usePollingFileWatcher.Value;
+                return _usePollingFileWatcher ?? false;
+            }
+            set
+            {
+                if (_fileWatcher != null)
+                {
+                    throw new InvalidOperationException($"Cannot modify {nameof(UsePollingFileWatcher)} once file watcher has been initialized.");
+                }
+                _usePollingFileWatcher = value;
             }
-            set => _usePollingFileWatcher = value;
         }
 
         /// <summary>

src/libraries/Microsoft.Extensions.FileProviders.Physical/tests/PhysicalFileProviderTests.cs

@@ -1377,6 +1377,63 @@ public async Task WildCardToken_RaisesEventsWhenFileSystemWatcherDoesNotFire()
             }
         }
 
+        [Fact]
+        public void UsePollingFileWatcher_FileWatcherNull_SetsSuccessfully()
+        {
+            // Arrange
+            using (var root = new DisposableFileSystem())
+            {
+                using (var provider = new PhysicalFileProvider(root.RootPath))
+                {
+                    Assert.False(provider.UsePollingFileWatcher);
+
+                    // Act / Assert
+                    provider.UsePollingFileWatcher = true;
+                    Assert.True(provider.UsePollingFileWatcher);
+                }
+            }
+        }
+
+        [Fact]
+        public void UsePollingFileWatcher_FileWatcherNotNull_SetterThrows()
+        {
+            // Arrange
+            using (var root = new DisposableFileSystem())
+            {
+                using (var fileSystemWatcher = new MockFileSystemWatcher(root.RootPath))
+                {
+                    using (var physicalFilesWatcher = new PhysicalFilesWatcher(root.RootPath + Path.DirectorySeparatorChar, fileSystemWatcher, pollForChanges: false))
+                    {
+                        using (var provider = new PhysicalFileProvider(root.RootPath) { FileWatcher = physicalFilesWatcher })
+                        {
+                            // Act / Assert
+                            Assert.Throws<InvalidOperationException>(() => { provider.UsePollingFileWatcher = true; });
+                        }
+                    }
+                }
+            }
+        }
+
+        [Fact]
+        public void UsePollingFileWatcher_FileWatcherNotNull_ReturnsFalse()
+        {
+            // Arrange
+            using (var root = new DisposableFileSystem())
+            {
+                using (var fileSystemWatcher = new MockFileSystemWatcher(root.RootPath))
+                {
+                    using (var physicalFilesWatcher = new PhysicalFilesWatcher(root.RootPath + Path.DirectorySeparatorChar, fileSystemWatcher, pollForChanges: false))
+                    {
+                        using (var provider = new PhysicalFileProvider(root.RootPath) { FileWatcher = physicalFilesWatcher })
+                        {
+                            // Act / Assert
+                            Assert.False(provider.UsePollingFileWatcher);
+                        }
+                    }
+                }
+            }
+        }
+
         [Fact]
         public void CreateFileWatcher_CreatesWatcherWithPollingAndActiveFlags()
         {