Handle single and empty CompositeFileProvider in Flatten

src/Ramstack.FileProviders.Composition/FileProviderComposer.cs

@@ -8,48 +8,76 @@ namespace Ramstack.FileProviders.Composition;
 public static class FileProviderComposer
 {
     /// <summary>
-    /// Tries to flatten the specified <see cref="IFileProvider"/> into a flat list of file providers.
+    /// Attempts to flatten the specified <see cref="IFileProvider"/> into a flat list of file providers.
     /// </summary>
     /// <remarks>
     /// If the <paramref name="provider"/> is not a <see cref="CompositeFileProvider"/>,
     /// the same instance of the <paramref name="provider"/> is returned.
     /// </remarks>
     /// <param name="provider">The <see cref="IFileProvider"/> to flatten.</param>
     /// <returns>
-    /// A <see cref="IFileProvider"/> that represents the flattened version of the specified <see cref="IFileProvider"/>.
+    /// An <see cref="IFileProvider"/> representing the flattened version from the specified <see cref="IFileProvider"/>.
+    /// </returns>
+    public static IFileProvider Flatten(this IFileProvider provider) =>
+        FlattenProvider(provider);
+
+    /// <summary>
+    /// Attempts to flatten the specified <see cref="IFileProvider"/> into a flat list of file providers.
+    /// </summary>
+    /// <remarks>
+    /// If the <paramref name="provider"/> is not a <see cref="CompositeFileProvider"/>,
+    /// the same instance of the <paramref name="provider"/> is returned.
+    /// </remarks>
+    /// <param name="provider">The <see cref="IFileProvider"/> to flatten.</param>
+    /// <returns>
+    /// An <see cref="IFileProvider"/> representing the flattened version from the specified <see cref="IFileProvider"/>.
     /// </returns>
     public static IFileProvider FlattenProvider(IFileProvider provider)
     {
-        if (provider is CompositeFileProvider composite)
-            foreach (var p in composite.FileProviders)
+        while (provider is CompositeFileProvider composite)
+        {
+            var providers = composite.FileProviders as IFileProvider[] ?? composite.FileProviders.ToArray();
+            if (providers.Length == 0)
+                return new NullFileProvider();
+
+            if (providers.Length == 1)
+            {
+                provider = providers[0];
+                continue;
+            }
+
+            foreach (var p in providers)
                 if (p is CompositeFileProvider or NullFileProvider)
-                    return ComposeProviders(composite.FileProviders);
+                    return ComposeProviders(providers);
+
+            break;
+        }
 
         return provider;
     }
 
     /// <summary>
-    /// Creates a provider from the specified list of <see cref="IFileProvider"/> and flattens it into a flat list of file providers.
+    /// Creates a provider from the specified list of <see cref="IFileProvider"/> instances and flattens it into a flat list of file providers.
     /// </summary>
     /// <remarks>
     /// This method returns a <see cref="CompositeFileProvider"/> if more than one provider remains after flattening.
     /// </remarks>
     /// <param name="providers">The list of <see cref="IFileProvider"/> instances to compose and flatten.</param>
     /// <returns>
-    /// A <see cref="IFileProvider"/> that represents the flattened version of the specified list of providers.
+    /// An <see cref="IFileProvider"/> representing the flattened version from the specified list of providers.
     /// </returns>
     public static IFileProvider ComposeProviders(params IFileProvider[] providers) =>
         ComposeProviders(providers.AsEnumerable());
 
     /// <summary>
-    /// Creates a provider from the specified list of <see cref="IFileProvider"/> and flattens it into a flat list of file providers.
+    /// Creates a provider from the specified list of <see cref="IFileProvider"/> instances and flattens it into a flat list of file providers.
     /// </summary>
     /// <remarks>
     /// This method returns a <see cref="CompositeFileProvider"/> if more than one provider remains after flattening.
     /// </remarks>
     /// <param name="providers">The list of <see cref="IFileProvider"/> instances to compose and flatten.</param>
     /// <returns>
-    /// A <see cref="IFileProvider"/> that represents the flattened version of the specified list of providers.
+    /// An <see cref="IFileProvider"/> representing the flattened version from the specified list of providers.
     /// </returns>
     public static IFileProvider ComposeProviders(IEnumerable<IFileProvider> providers)
     {

tests/Ramstack.FileProviders.Composition.Tests/FileProviderComposerTests.cs

@@ -4,15 +4,15 @@
 public sealed class FileProviderComposerTests
 {
     [Test]
-    public void FlattenFileProvider_ReturnsAsIs_WhenNoComposite()
+    public void Flatten_ReturnsAsIs_WhenNoComposite()
     {
         var provider = new TestFileProvider();
         var result = FileProviderComposer.FlattenProvider(provider);
         Assert.That(result, Is.SameAs(provider));
     }
 
     [Test]
-    public void FlattenFileProvider_ReturnsCompositeProvider_WhenNeedComposite()
+    public void Flatten_ReturnsCompositeProvider_WhenNeedComposite()
     {
         var provider = new CompositeFileProvider(new TestFileProvider(), new TestFileProvider());
 
@@ -21,7 +21,7 @@ public void FlattenFileProvider_ReturnsCompositeProvider_WhenNeedComposite()
     }
 
     [Test]
-    public void FlattenFileProvider_ReturnsAsIs_WhenAlreadyFlat()
+    public void Flatten_ReturnsAsIs_WhenAlreadyFlat()
     {
         var provider = new CompositeFileProvider(new TestFileProvider(), new TestFileProvider());
 
@@ -30,7 +30,7 @@ public void FlattenFileProvider_ReturnsAsIs_WhenAlreadyFlat()
     }
 
     [Test]
-    public void FlattenFileProvider_ReturnsCompositeProvider_Flattened()
+    public void Flatten_ReturnsCompositeProvider_Flattened()
     {
         var provider = new CompositeFileProvider(
             new TestFileProvider(),
@@ -48,7 +48,7 @@ public void FlattenFileProvider_ReturnsCompositeProvider_Flattened()
     }
 
     [Test]
-    public void FlattenFileProvider_RemovesNullFileProvider()
+    public void Flatten_RemovesNullFileProvider()
     {
         var provider = new CompositeFileProvider(
             new TestFileProvider(),
@@ -66,7 +66,7 @@ public void FlattenFileProvider_RemovesNullFileProvider()
     }
 
     [Test]
-    public void FlattenFileProvider_ReturnsNullFileProvider_WhenNothingReturn()
+    public void Flatten_ReturnsNullFileProvider_WhenNothingReturn()
     {
         var provider = new CompositeFileProvider(
             new CompositeFileProvider(
@@ -107,7 +107,7 @@ public void FlattenFileProvider_ReturnsNullFileProvider_WhenNothingReturn()
     }
 
     [Test]
-    public void FlattenFileProvider_ReturnsSingleProvider_WhenRemainOneProvider()
+    public void Flatten_ReturnsSingleProvider_WhenRemainOneProvider()
     {
         var provider = new CompositeFileProvider(
             new CompositeFileProvider(
@@ -148,7 +148,7 @@ public void FlattenFileProvider_ReturnsSingleProvider_WhenRemainOneProvider()
     }
 
     [Test]
-    public void FlattenFileProvider_MaintainOrder_WhenComposite()
+    public void Flatten_MaintainOrder_WhenComposite()
     {
         var p1 = new TestFileProvider();
         var p2 = new TestFileProvider();
@@ -208,6 +208,20 @@ public void FlattenFileProvider_MaintainOrder_WhenComposite()
         Assert.That(composite.FileProviders, Is.EquivalentTo(providers));
     }
 
+    [Test]
+    public void Flatten_ReturnsSingleProvider_WhenCompositeContainsOnlyOne()
+    {
+        var provider = new CompositeFileProvider(new TestFileProvider()).Flatten();
+        Assert.That(provider, Is.InstanceOf<TestFileProvider>());
+    }
+
+    [Test]
+    public void Flatten_EmptyComposite_ReturnsNullProvider()
+    {
+        var provider = new CompositeFileProvider().Flatten();
+        Assert.That(provider, Is.InstanceOf<NullFileProvider>());
+    }
+
     private sealed class TestFileProvider : IFileProvider
     {
         public IFileInfo GetFileInfo(string subpath) =>