From 170fbbaf8255452795f0ef36fe9fc7aa9bbf9ba0 Mon Sep 17 00:00:00 2001
From: Simon Cropp <simon.cropp@gmail.com>
Date: Tue, 7 Jul 2026 09:16:26 +1000
Subject: [PATCH] Throw for greedy RegisterFileConverter<object>

RegisterFileConverter<object> matches every verified value via the
default `target is T` canConvert, silently hijacking all serialization.
Guard the generic overloads and point users to a concrete type or the
non-generic overload that requires an explicit canConvert.

Fixes #1656
---
 .../RegisterFileConverterTests.cs             | 21 +++++++++++++++++++
 src/Verify/Splitters/Settings_Typed.cs        | 10 +++++++++
 2 files changed, 31 insertions(+)
 create mode 100644 src/StaticSettingsTests/RegisterFileConverterTests.cs

diff --git a/src/StaticSettingsTests/RegisterFileConverterTests.cs b/src/StaticSettingsTests/RegisterFileConverterTests.cs
new file mode 100644
index 0000000000..b5a784bdad
--- /dev/null
+++ b/src/StaticSettingsTests/RegisterFileConverterTests.cs
@@ -0,0 +1,21 @@
+public class RegisterFileConverterTests :
+    BaseTest
+{
+    [Fact]
+    public void ObjectThrows()
+    {
+        var exception = Assert.Throws<Exception>(
+            () => VerifierSettings.RegisterFileConverter<object>(
+                (_, _) => new(null, "txt", "value")));
+        Assert.Contains("too greedy", exception.Message);
+    }
+
+    [Fact]
+    public void ObjectAsyncThrows()
+    {
+        var exception = Assert.Throws<Exception>(
+            () => VerifierSettings.RegisterFileConverter<object>(
+                (_, _) => Task.FromResult<ConversionResult>(new(null, "txt", "value"))));
+        Assert.Contains("too greedy", exception.Message);
+    }
+}
diff --git a/src/Verify/Splitters/Settings_Typed.cs b/src/Verify/Splitters/Settings_Typed.cs
index 4891b4b9c3..848e5d49da 100644
--- a/src/Verify/Splitters/Settings_Typed.cs
+++ b/src/Verify/Splitters/Settings_Typed.cs
@@ -26,6 +26,7 @@ public static void RegisterFileConverter<T>(
         CanConvert<T>? canConvert = null)
     {
         InnerVerifier.ThrowIfVerifyHasBeenRun();
+        ThrowIfObjectConverter<T>();
         RegisterFileConverter(
             (target, context) => Task.FromResult(conversion(target, context)),
             canConvert);
@@ -36,10 +37,19 @@ public static void RegisterFileConverter<T>(
         CanConvert<T>? canConvert = null)
     {
         InnerVerifier.ThrowIfVerifyHasBeenRun();
+        ThrowIfObjectConverter<T>();
         var converter = new TypeConverter((target, context) => conversion((T) target, context), DefaultCanConvert(canConvert));
         typedConverters.Add(converter);
     }
 
+    static void ThrowIfObjectConverter<T>()
+    {
+        if (typeof(T) == typeof(object))
+        {
+            throw new("RegisterFileConverter<object> is too greedy since object matches all verified values. Instead use a more specific type, or use the non-generic RegisterFileConverter overload that takes an explicit `canConvert`.");
+        }
+    }
+
     public static void RegisterFileConverter(
         Conversion conversion,
         CanConvert canConvert)