Add ResolveFile extension method for flexible file path resolution

.gitignore

@@ -191,4 +191,4 @@ Tests/FluentAssertions.Specs/FluentAssertions.Specs.xml
 Tests/Benchmarks/BenchmarkDotNet.Artifacts/
 
 # Documentation spell check
-node_modules/
+node_modules/

Pathy.ApiVerificationTests/ApprovedApi/pathy.net47.verified.txt

@@ -45,6 +45,7 @@ namespace Pathy
         public static void DeleteFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> paths) { }
         public static void MoveFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> sourcePaths, Pathy.ChainablePath destinationDirectory) { }
         public static void MoveFileOrDirectory(this Pathy.ChainablePath sourcePath, Pathy.ChainablePath destinationDirectory, string newName = null) { }
+        public static Pathy.ChainablePath ResolveFile(this Pathy.ChainablePath path, string fileName) { }
     }
     public static class StringExtensions
     {

Pathy.ApiVerificationTests/ApprovedApi/pathy.net8.0.verified.txt

@@ -46,6 +46,7 @@ namespace Pathy
         public static void DeleteFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> paths) { }
         public static void MoveFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> sourcePaths, Pathy.ChainablePath destinationDirectory) { }
         public static void MoveFileOrDirectory(this Pathy.ChainablePath sourcePath, Pathy.ChainablePath destinationDirectory, string newName = null) { }
+        public static Pathy.ChainablePath ResolveFile(this Pathy.ChainablePath path, string fileName) { }
     }
     public static class StringExtensions
     {

Pathy.ApiVerificationTests/ApprovedApi/pathy.netstandard2.0.verified.txt

@@ -45,6 +45,7 @@ namespace Pathy
         public static void DeleteFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> paths) { }
         public static void MoveFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> sourcePaths, Pathy.ChainablePath destinationDirectory) { }
         public static void MoveFileOrDirectory(this Pathy.ChainablePath sourcePath, Pathy.ChainablePath destinationDirectory, string newName = null) { }
+        public static Pathy.ChainablePath ResolveFile(this Pathy.ChainablePath path, string fileName) { }
     }
     public static class StringExtensions
     {

Pathy.ApiVerificationTests/ApprovedApi/pathy.netstandard2.1.verified.txt

@@ -46,6 +46,7 @@ namespace Pathy
         public static void DeleteFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> paths) { }
         public static void MoveFileOrDirectory(this System.Collections.Generic.IEnumerable<Pathy.ChainablePath> sourcePaths, Pathy.ChainablePath destinationDirectory) { }
         public static void MoveFileOrDirectory(this Pathy.ChainablePath sourcePath, Pathy.ChainablePath destinationDirectory, string newName = null) { }
+        public static Pathy.ChainablePath ResolveFile(this Pathy.ChainablePath path, string fileName) { }
     }
     public static class StringExtensions
     {

Pathy.Specs/ChainablePathSpecs.cs

@@ -918,6 +918,99 @@ public void Cannot_find_the_first_existing_path_from_an_empty_array()
             .WithParameterName("paths");
     }
 
+    [Fact]
+    public void Can_resolve_a_file_when_path_is_the_file_itself()
+    {
+        // Arrange
+        var file = testFolder / "test.txt";
+        File.WriteAllText(file, "content");
+
+        // Act
+        var result = file.ResolveFile("test.txt");
+
+        // Assert
+        result.ToString().Should().Be(file.ToString());
+        result.FileExists.Should().BeTrue();
+    }
+
+    [Fact]
+    public void Can_resolve_a_file_when_path_is_a_directory_containing_the_file()
+    {
+        // Arrange
+        var file = testFolder / "test.txt";
+        File.WriteAllText(file, "content");
+
+        // Act
+        var result = testFolder.ResolveFile("test.txt");
+
+        // Assert
+        result.ToString().Should().Be(file.ToString());
+        result.FileExists.Should().BeTrue();
+    }
+
+    [Fact]
+    public void ResolveFile_is_case_insensitive_when_path_is_the_file()
+    {
+        // Arrange
+        var file = testFolder / "Test.txt";
+        File.WriteAllText(file, "content");
+
+        // Act
+        var result = file.ResolveFile("test.TXT");
+
+        // Assert
+        result.ToString().Should().Be(file.ToString());
+        result.FileExists.Should().BeTrue();
+    }
+
+    [Fact]
+    public void ResolveFile_returns_empty_when_file_does_not_exist_in_directory()
+    {
+        // Act
+        var result = testFolder.ResolveFile("nonexistent.txt");
+
+        // Assert
+        result.Should().Be(ChainablePath.Empty);
+    }
+
+    [Fact]
+    public void ResolveFile_returns_empty_when_path_is_a_file_with_different_name()
+    {
+        // Arrange
+        var file = testFolder / "actual.txt";
+        File.WriteAllText(file, "content");
+
+        // Act
+        var result = file.ResolveFile("different.txt");
+
+        // Assert
+        result.Should().Be(ChainablePath.Empty);
+    }
+
+    [Fact]
+    public void ResolveFile_throws_when_fileName_is_null()
+    {
+        // Act
+        var act = () => testFolder.ResolveFile(null);
+
+        // Assert
+        act.Should().Throw<ArgumentException>()
+            .WithMessage("*File name cannot be null or empty*")
+            .WithParameterName("fileName");
+    }
+
+    [Fact]
+    public void ResolveFile_throws_when_fileName_is_empty()
+    {
+        // Act
+        var act = () => testFolder.ResolveFile(string.Empty);
+
+        // Assert
+        act.Should().Throw<ArgumentException>()
+            .WithMessage("*File name cannot be null or empty*")
+            .WithParameterName("fileName");
+    }
+
     [Fact]
     public void Can_get_last_write_time_utc_for_file()
     {

Pathy/ChainablePathExtensions.cs

@@ -87,4 +87,42 @@ public static void MoveFileOrDirectory(this System.Collections.Generic.IEnumerab
             sourcePath.MoveFileOrDirectory(destinationDirectory);
         }
     }
+
+    /// <summary>
+    /// Resolves a file name within the current path.
+    /// If the path represents a file with the specified name and that file exists, returns the path.
+    /// If the path is a directory that contains a file with the specified name, returns the path to that file.
+    /// Otherwise, returns <see cref="ChainablePath.Empty"/>.
+    /// </summary>
+    /// <param name="path">The base path to resolve from (can be a file or directory).</param>
+    /// <param name="fileName">The file name to resolve.</param>
+    /// <returns>
+    /// A <see cref="ChainablePath"/> representing the resolved file if found; otherwise, <see cref="ChainablePath.Empty"/>.
+    /// </returns>
+    /// <exception cref="ArgumentException">Thrown if <paramref name="fileName"/> is null or empty.</exception>
+    public static ChainablePath ResolveFile(this ChainablePath path, string fileName)
+    {
+        if (string.IsNullOrEmpty(fileName))
+        {
+            throw new ArgumentException("File name cannot be null or empty", nameof(fileName));
+        }
+
+        // Case 1: If the path is a file with the specified name and exists, return it
+        if (path.IsFile && string.Equals(path.Name, fileName, StringComparison.OrdinalIgnoreCase))
+        {
+            return path;
+        }
+
+        // Case 2: If the path is a directory, check if it contains the file
+        if (path.IsDirectory)
+        {
+            ChainablePath filePath = path / fileName;
+            if (filePath.FileExists)
+            {
+                return filePath;
+            }
+        }
+
+        return ChainablePath.Empty;
+    }
 }

README.md

@@ -163,6 +163,28 @@ var filesToMove = (ChainablePath.Current / "source").GlobFiles("*.txt");
 filesToMove.MoveFileOrDirectory(ChainablePath.Current / "destination");
 ```
 
+### Resolving files
+
+If you have a `ChainablePath` that could represent either a file or a directory, and you want to resolve a specific file name, you can use the `ResolveFile` extension method:
+
+```csharp
+// When the path is a directory containing the file
+var directory = ChainablePath.From("c:/projects/myapp");
+var configFile = directory.ResolveFile("appsettings.json");
+// Returns: c:/projects/myapp/appsettings.json (if it exists)
+
+// When the path is already the file itself
+var filePath = ChainablePath.From("c:/projects/myapp/appsettings.json");
+var resolved = filePath.ResolveFile("appsettings.json");
+// Returns: c:/projects/myapp/appsettings.json (if it exists)
+
+// When the file doesn't exist
+var missing = directory.ResolveFile("missing.txt");
+// Returns: ChainablePath.Empty
+```
+
+The method performs case-insensitive file name matching, so `ResolveFile("CONFIG.JSON")` will match `config.json`.
+
 ## Building
 
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