MSTest 4.4.1 Fixes the CS0121 Ambiguous Assert Calls That 4.4.0 Broke Below C# 14

If you bumped MSTest to 4.4.0 in a project that targets net8.0 or net9.0, your test project may have stopped compiling on lines that had not changed in years. MSTest 4.4.1, published to NuGet on 2026-09-16, fixes that. Here is what broke, the version matrix I measured, and the two call shapes that 4.4.1 still does not cover.

Span overloads that only C# 14 can rank

MSTest 4.4.0 added Span<T> and ReadOnlySpan<T> overloads next to the existing IEnumerable<T> ones on Assert.Contains and Assert.DoesNotContain. On C# 14, first-class span conversions give the compiler tie-breaking rules for arrays and strings, so it picks an overload. Below C# 14, T[] to Span<T> is only a user-defined op_Implicit, and neither overload is better. Issue #11022 reported this on the day after 4.4.0 shipped:

error CS0121: The call is ambiguous between the following methods or properties:
'Assert.Contains<T>(T, System.Collections.Generic.IEnumerable<T>, string?, string, string)' and
'Assert.Contains<T>(T, System.Span<T>, string?, string, string)'

Since net8.0 defaults to C# 12 and net9.0 to C# 13, those TFMs break out of the box. So does a net10.0 project pinned to an older LangVersion.

The version matrix

I compiled the same file against each version with SDK 10.0.302:

int[] ids = [1, 2, 3];
string name = "Marius";

Assert.Contains(2, ids);
Assert.DoesNotContain(4, ids);
Assert.DoesNotContain('z', name);
Assert.Contains<int>(2, ids);

All four calls fail on 4.4.0, including the explicit Contains<int> and the string case. The only 4.4.0 workaround is raising LangVersion to 14 on an older TFM, which is not a supported combination. Upgrading is the real fix:

<PackageReference Include="MSTest" Version="4.4.1" />

If you use MSTest.Sdk, bump the version in global.json or in the Sdk="MSTest.Sdk/4.4.1" attribute instead.

How 4.4.1 fixes it, and what it leaves out

PR #11038 adds exact T[] overloads to every affected Assert family. An exact match beats both conversions, so inferred and explicit calls on arrays resolve again. It also adds constrained forwarders that keep inferred calls working for other types that convert to a span, such as string, ArraySegment<T>, and your own collections. They forward to the existing IEnumerable<T> code, so behavior matches 4.3.3.

The PR says explicit <T> calls on non-array types are out of scope, and my probe on 4.4.1 with net8.0 agrees:

var seg = new ArraySegment<int>(ids);
Assert.Contains(2, seg);           // OK
Assert.Contains(2, ids.AsSpan());  // OK
Assert.Contains<int>(2, seg);      // CS0121
Assert.Contains<char>('M', name);  // CS0121

Drop the type argument and let inference pick the forwarder, or cast to IEnumerable<T>.

The same release also narrows what the MSTest source generator roots for trimming and Native AOT, and fixes source-generated literals for enums, narrow integral types, NaN, and control characters. If you turned on the generator after MSTest 4.4 graduated it, that is a second reason to take 4.4.1.