Records not initialized properly when used as data for data-driven test

[DavidYawCSpeed]

Describe the bug

I have a record which includes some explicit properties that override automatic properties.

public record Size(double Width, double Height)
{
    public double Width { get; } = FilterNegative(Width);
    public double Height { get; } = FilterNegative(Height);

    private static double FilterNegative(double value, [CallerArgumentExpression(nameof(value))] string? paramName = null)
    {
        ArgumentOutOfRangeException.ThrowIfNegative(value, paramName);
        return value;
    }
}

I am using this type in two ways in my unit tests:

• Explicit creation in code:

  [TestMethod]
  public void TestMethod()
  {
    var foo = new Size(1, 2);
  }

• Directly as dynamic data:

  public static IEnumerable<object[]> TestData()
  {
      yield return new object[] { new Size(1, 2) };
  }

Version used

• MSTest version 4.4.1
• Microsoft.NET.Test.Sdk version 18.10.0
• Visual Studio 2026 version 18.10.1

Steps To Reproduce

• When run from the command line dotnet test ..., all tests pass. Explicit properties Width and Height are properly initialized.
• When run from Visual Studio (Test Explorer window, Run All Tests):
  • Records created in the test method are properly initialized.
  • Records created in the dynamic data have their explicit fields at the default value, zero.

At some point in the past, this was not the case. Under MSTest version 3.10.0, .Net 9, and a corresponding version of Visual Studio, running the tests from Visual Studio properly initialized the explicit properties, all the time.

[Evangelink]

Hi David Yaw (@DavidYawCSpeed),

I cannot reproduce the behavior you are mentioning

[DavidYawCSpeed]

I'm still getting zeros.

Here's a slightly larger unit test (part of my actual real tests), which fails for obvious reasons.

Expand my class under test a bit. I have a default constructor. I didn't mention it before because I thought it wasn't relevant, but now it appears it is:

public record Size(double Width, double Height)
{
    public double Width { get; } = FilterNegative(Width);
    public double Height { get; } = FilterNegative(Height);

    private static double FilterNegative(double value, [CallerArgumentExpression(nameof(value))] string? paramName = null)
    {
        ArgumentOutOfRangeException.ThrowIfNegative(value, paramName);
        return value;
    }

    public Size() : this(0, 0) { }
}

Out of curiosity, I tried using other values: public Size() : this(86, 99) { }. With that change made, those values are the ones coming through in my test.

So, somehow my new Size(1,2) is getting replaced with new Size().

[Evangelink]

Alright, I am now able to reproduce the issue.

When working with VSTest, MSTest uses serialization to forward the data to Test Explorer. When the custom type has no serializable ctor then the test is folded (in TE you don't see the children for all the cases):

When you add the default ctor, the test is now considered to be serializable and so unfolded, and when run under VS, this ctor is used and causes the values to be invalid.

This behavior can be worked around by either making your custom types serializable or by setting UnfoldingStrategy = TestDataSourceUnfoldingStrategy.Fold in the TestMethod attribute.

Another alternative I would strongly recommend is for you to migrate your tests from VSTest to MTP (See https://learn.microsoft.com/dotnet/core/testing/migrating-vstest-microsoft-testing-platform)