CS0854 - no build error with VS2026

[adrian-cockburn-appspace]

Hi, I doubt if this is a moq issue specifically.

Typically when setting up your mocks you would see a build error CS0854 - An expression tree may not contain a call or invocation that uses optional arguments if you missed an optional parameter in your setup. With VS2026 I no longer get these, which is an issue as its not consistent with our ci/cd platform which does throw a build error.

Anybody know how to get the old behaviour back with VS2026?

[GillesTourreau]

@adrian-cockburn-appspace Can you give an example of the code which raise build error CS0854 with Moq ?

[adrian-cockburn-appspace]

    [TestClass]
    public class ReplicateCS0854
    {
        [TestMethod]
        public void TestMethod()
        {
            var mock = new Mock<SampleClass>();
            mock.Setup(m => m.TestMethodWithOptionalParam(It.IsAny<int>())).Verifiable();
        }

        public class SampleClass : ISampleInterface
        {
            public void TestMethodWithOptionalParam(int requiredParam, int optionalParam = 42)
            {
                throw new NotImplementedException();
            }
        }

        public interface ISampleInterface
        {
            void TestMethodWithOptionalParam(int requiredParam, int optionalParam = 42);
        }
    }

this results in a build error in VS2022, adding an extra , It.IsAny<int>() to the setup resolves the build error (which is what I would expect). In VS2026 I don't get this error, which is actually preferable - optional parameters should be optional - but it means these errors go unnoticed until it hits the ci/cd platform.

[GillesTourreau]

This is not related to VS2026, but it is an improvement of .NET 10.0 / C# 14.0 which support optional and named arguments in Expression trees.
It has been implemented and delivered in .NET 10.0 SDK belong this issue: dotnet/csharplang#9246.
So it is not related to Moq issue.

[adrian-cockburn-appspace]

Cool, thanks. Its definitely an improvement.
Kind of strange as the project is still targeting .net8, I'll have to dig a little deeper.

[GillesTourreau]

Check the version of C# you use in your projects (https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version), maybe you target the last version.
When you use VS2026, by default it is use the .NET SDK 10.0 to compile the code if nothing is specified (even you target the .NET 8.0 libraries).
To avoid this issue next time, specify explicitly the .NET SDK version by adding a global.json file at the root of your repository with an explicit version : https://learn.microsoft.com/en-us/dotnet/core/versions/selection and also the version of C# to use in your .csproj.

[adrian-cockburn-appspace]

That's great, thanks. Specifying the langversion gives me what I'm looking for.