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> provides the default `sampleQuotes.txt` value when the user doesn't provide `--file`, and converts the provided token to a `FileInfo` when they do.
+   * <xref:System.CommandLine.Option`1.Validators?displayProperty=nameWithType> adds a custom validator that runs *after* the value is parsed, regardless of whether the user provided a value or the default was used. Without the validator, a missing file would cause an unhandled `FileNotFoundException` with a stack trace. With the validator, 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

@@ -17,20 +17,18 @@ static int Main(string[] args)
                 if (result.Tokens.Count == 0)
                 {
                     return new FileInfo("sampleQuotes.txt");
-
-                }
-                string filePath = result.Tokens.Single().Value;
-                if (!File.Exists(filePath))
-                {
-                    result.AddError("File does not exist");
-                    return null;
-                }
-                else
-                {
-                    return new FileInfo(filePath);
                 }
+                return new FileInfo(result.Tokens.Single().Value);
             }
         };
+        fileOption.Validators.Add(result =>
+        {
+            var file = result.GetValueOrDefault<FileInfo>();
+            if (file is not null && !file.Exists)
+            {
+                result.AddError("File does not exist");
+            }
+        });
         // </fileoption>
 
         Option<int> delayOption = new("--delay")