From 12ba476dc6f7219198ffb5f9b2b9e2a3d8ba9e99 Mon Sep 17 00:00:00 2001
From: Daniel Cazzulino <daniel@cazzulino.com>
Date: Sun, 2 Jan 2022 20:21:45 -0300
Subject: [PATCH] Allow passing a null string to TryParse

Fixes #5
---
 src/ISBN/ISBN.cs       | 5 ++++-
 src/Tests/ISBNTests.cs | 3 +++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/ISBN/ISBN.cs b/src/ISBN/ISBN.cs
index 69a836b..df6b5b8 100644
--- a/src/ISBN/ISBN.cs
+++ b/src/ISBN/ISBN.cs
@@ -63,9 +63,12 @@ static ISBN()
     /// <param name="isbn">The ISBN to parse.</param>
     /// <param name="result">The parsed result, if valid.</param>
     /// <returns><see langword="false"/> if the <paramref name="isbn"/> was not a valid ISBN. <see langword="true"/> otherwise.</returns>
-    public static bool TryParse(string isbn, [NotNullWhen(true)] out ISBN? result)
+    public static bool TryParse(string? isbn, [NotNullWhen(true)] out ISBN? result)
     {
         result = default;
+        if (isbn == null)
+            return false;
+
         isbn = new string(isbn.Where(c => c != ' ' && c != '-').ToArray());
         if (isbn.Length == 10)
             isbn += "978";
diff --git a/src/Tests/ISBNTests.cs b/src/Tests/ISBNTests.cs
index 56fd9df..f174826 100644
--- a/src/Tests/ISBNTests.cs
+++ b/src/Tests/ISBNTests.cs
@@ -26,4 +26,7 @@ public void ParseIsbn(string isbn)
             Assert.True(false, "Failed to parse " + isbn);
         }
     }
+
+    [Fact]
+    public void ParseNullIsbnReturnsFalse() => Assert.False(ISBN.TryParse(null, out var _));
 }