Fix file-existence validation bypassed when --file is provided explicitly

docs/standard/commandline/get-started-tutorial.md

@@ -340,11 +340,10 @@ scl quotes delete --search-terms David "You can do" Antoine "Perfection is achie
 
    :::code language="csharp" source="snippets/get-started-tutorial/csharp/Stage3/Program.cs" id="fileoption" :::
 
-   This code uses <xref:System.CommandLine.Parsing.ArgumentResult> to provide custom parsing, validation, and error handling.
+   This code uses two separate mechanisms:
 
-   Without this code, missing files are reported with an exception and stack trace. With this code just the specified error message is displayed.
-
-   This code also specifies a default value, which is why it sets <xref:System.CommandLine.Option`1.DefaultValueFactory?displayProperty=nameWithType> to custom parsing method.
+   * <xref:System.CommandLine.Option`1.DefaultValueFactory?displayProperty=nameWithType> supplies `sampleQuotes.txt` as the default when you don't provide `--file`. `DefaultValueFactory` doesn't run when you do provide `--file`.
+   * <xref:System.CommandLine.Option`1.CustomParser?displayProperty=nameWithType> runs when you explicitly provide `--file`. It converts the token to a `FileInfo` and validates that the file exists. Without validation, a missing file would cause an unhandled `FileNotFoundException` with a stack trace. With validation, just the specified error message is displayed.
 
 1. After the code that creates `lightModeOption`, add options and arguments for the `add` and `delete` commands:
 

docs/standard/commandline/snippets/get-started-tutorial/csharp/Stage3/Program.cs

@@ -11,24 +11,15 @@ static int Main(string[] args)
         Option<FileInfo> fileOption = new("--file")
         {
             Description = "An option whose argument is parsed as a FileInfo",
-            Required = true,
-            DefaultValueFactory = result =>
+            DefaultValueFactory = _ => new FileInfo("sampleQuotes.txt"),
+            CustomParser = result =>
             {
-                if (result.Tokens.Count == 0)
-                {
-                    return new FileInfo("sampleQuotes.txt");
-
-                }
-                string filePath = result.Tokens.Single().Value;
-                if (!File.Exists(filePath))
+                var file = new FileInfo(result.Tokens.Single().Value);
+                if (!file.Exists)
                 {
                     result.AddError("File does not exist");
-                    return null;
-                }
-                else
-                {
-                    return new FileInfo(filePath);
                 }
+                return file;
             }
         };
         // </fileoption>