How to use FormattableString[] in TestCaseSource

Question

I have an NUnit test that needs to test a bunch of different FormattableString cases and I'm passing the cases using TestCaseSource:

static IEnumerable<TestCaseData> TestCaseSourceData()
{
    FormattableString data1 = FormattableStringFactory.Create("test{0}", 1);
    yield return new TestCaseData(new []{ data1 });
}

[TestCaseSource("TestCaseSourceData")]
public void FormattableStringTest(FormattableString[] coreMessages)
{
    //...
}

But this give me an error:

System.ArgumentException : Object of type 'System.Runtime.CompilerServices.FormattableStringFactory+ConcreteFormattableString' cannot be converted to type 'System.FormattableString[]'.

System.ArgumentException : Object of type 'System.Runtime.CompilerServices.FormattableStringFactory+ConcreteFormattableString' cannot be converted to type 'System.FormattableString[]'.
   at System.RuntimeType.TryChangeType(Object value, Binder binder, CultureInfo culture, Boolean needsSpecialCast)
   at System.RuntimeType.CheckValue(Object value, Binder binder, CultureInfo culture, BindingFlags invokeAttr)
   at System.Reflection.MethodBase.CheckArguments(StackAllocedArguments& stackArgs, ReadOnlySpan`1 parameters, Binder binder, BindingFlags invokeAttr, CultureInfo culture, Signature sig)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at NUnit.Framework.Internal.Reflect.InvokeMethod(MethodInfo method, Object fixture, Object[] args)

Now if I use this TestCaseSource:

static IEnumerable<TestCaseData> TestCaseSourceData()
{
    FormattableString data1 = FormattableStringFactory.Create("test{0}", 1);
    yield return new TestCaseData(new []{ data1, data1 });
}

I get a different error:

Too many arguments provided, provide at most 1 arguments.

What's wrong with my code? I can send any other type but not FormattableString.

Answer 1

You are trying to cast FormattableString to FormattableString[] - this is what error message is saying.

You have two options:

1. Use single object:

    [TestFixture]
    public class TestClass
    {
        public static IEnumerable<TestCaseData> TestCaseSourceData()
        {
            FormattableString data1 = FormattableStringFactory.Create("test{0}", 1);
            yield return new TestCaseData(new[] { data1 });
        }
   
        [TestCaseSource("TestCaseSourceData")]
        public void FormattableStringTest(FormattableString coreMessages)
        {
            Assert.True(true);
        }
    }
   

2. Use array of objects:

    [TestFixture]
    public class TestClass
    {
        public static IEnumerable<TestCaseData> TestCaseSourceData()
        {
            FormattableString data1 = FormattableStringFactory.Create("test{0}", 1);
            yield return new TestCaseData(new[] { new FormattableString[] { data1 } });
        }
   
        [TestCaseSource("TestCaseSourceData")]
        public void FormattableStringTest(FormattableString[] coreMessages)
        {
            Assert.AreEqual(1, coreMessages.Length);
        }
    }