more AgainstBadExtension

src/Verify/FilePair.cs

@@ -1,9 +1,18 @@
 namespace VerifyTests;
 
-public readonly struct FilePair(string extension, string receivedPath, string verifiedPath)
+public readonly struct FilePair
 {
-    public string Extension { get; } = extension;
-    public string ReceivedPath { get; } = receivedPath;
-    public string VerifiedPath { get; } = verifiedPath;
-    public bool IsText { get; } = FileExtensions.IsTextExtension(extension);
+    public FilePair(string extension, string receivedPath, string verifiedPath)
+    {
+        Guard.AgainstBadExtension(extension);
+        Extension = extension;
+        ReceivedPath = receivedPath;
+        VerifiedPath = verifiedPath;
+        IsText = FileExtensions.IsTextExtension(extension);
+    }
+
+    public string Extension { get; }
+    public string ReceivedPath { get; }
+    public string VerifiedPath { get; }
+    public bool IsText { get; }
 }

src/Verify/Guard.cs

@@ -144,5 +144,10 @@ public static void AgainstBadExtension(string value, [CallerArgumentExpression("
         {
             throw new ArgumentException("Must not start with a period ('.').", argumentName);
         }
+
+        if (value.Contains('\\') || value.Contains('/'))
+        {
+            throw new ArgumentException("Must not contain a directory separator.", argumentName);
+        }
     }
 }

src/Verify/Verifier/InnerVerifier_Stream.cs

@@ -73,6 +73,7 @@ public Task<VerifyResult> VerifyStream(Stream? stream, object? info) =>
 
     public async Task<VerifyResult> VerifyStream(Stream? stream, string extension, object? info)
     {
+        Guard.AgainstBadExtension(extension);
         if (stream is null)
         {
             if (info is null)