How can I use the ASP.NET [Range] annotation for IEnumerable elements?

I want to use the ASP.NET [Range] Annotation but for the elements IEnumerables.

I used the existing RangeAttribute like this:

public class RangeEnumerable : RangeAttribute
{
	/// <inheritdoc/>
	public RangeEnumerable(double minimum, double maximum) : base(minimum, maximum)
	{
	}

	/// <inheritdoc/>
	public RangeEnumerable(int minimum, int maximum) : base(minimum, maximum)
	{
	}

	/// <inheritdoc/>
	public RangeEnumerable([DynamicallyAccessedMembers((DynamicallyAccessedMemberTypes)(-1))] Type type, string minimum, string maximum) : base(type, minimum, maximum)
	{
	}

	/// <inheritdoc/>
	public override bool IsValid(object? value)
	{
		if (null == value) { return true; } 

		IEnumerable<object> list = ((IEnumerable)value).Cast<object>();
		
		foreach (object item in list)
		{
			if (!base.IsValid(item))
			{
				return false;
			}                
		}

		return true;
	}
}

and annotated my Parameter like this:

[RangeEnumerable(MINIMUM_ANGLE, MAXIMUM_ANGLE)]
public IEnumerable<Double> PhaseAnglesVoltage { get; set; } = new List<double>();

And wrote the following unit test:

[Test]
public void TestInvalidPhaseAngleVoltageTooLow()
{
	// Arrange       
	Loadpoint loadpoint1 = new Loadpoint();
	loadpoint1.PhaseAnglesVoltage.Append(-1);

	// Act
	var errCount = ValidateObject(loadpoint1);

	// Assert
	Assert.AreEqual(1, errCount);
}

private int ValidateObject(object obj)
{
	var validationContext = new ValidationContext(obj, null, null);
	var validationResults = new List<ValidationResult>();
	Validator.TryValidateObject(obj, validationContext, validationResults, true);
	return validationResults.Count;
}

I expected the loop to iterate over the elements of the List I used the annotation with, but in the IsValid-Function I always get an empty List instead of one with the element appended in the test.

Ok, I've found the error, which was in the unit test. IEnumerable.Append doesn't add the element to the original object like List.Add does (see Difference between a List's Add and Append method?).

Changing the unit test to the following does the trick.

        [Test]
        public void TestInvalidPhaseAngleVoltageTooLow()
        {
            // Arrange       
            Loadpoint loadpoint1 = new Loadpoint();
            loadpoint1.PhaseAnglesVoltage = loadpoint1.PhaseAnglesVoltage.Append(-1);

            // Act
            var errCount = ValidateObject(loadpoint1);
   
            // Assert
            Assert.AreEqual(1, errCount);
        }

I created one console app based on your code.

on line no 8 as you can see and as you said it does not change the source. But after modifying to below line will create another object from that yielded IEnumerable

model.Doubles = model.Doubles.Append(1.1).toList();

I think the change you made is not working as expected. You need to do toList() or assign IEnumerable with new List() and then use Add() method of List which will change the source.