Custom DataAnnotation IsValid Not Called

Question

Background

Hello all,

Basically, I am writing a custom data annotation in a .NET Core Class Library that will validate that an IEnumerable has at least a certain number of elements.

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The Problem

For some reason, when running the validation, the IsValid is never called. I have already found a few other SO questions regarding this issue, but they all have a different problem than I do (basically, they weren't actually validating their objects). I am, however, validating my object (calling Validator.TryValidateObject(...)) and yet, the IsValid is never called.

If I use any of the out-of-the-box validation attribute (e.g. Required), it operates as expected.

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The Code

MinElementsAttribute.cs

public class MinElementsAttribute : ValidationAttribute
{
    readonly int minElements;

    public MinElementsAttribute(int minElements) : base($"Collection must have a size of at least {minElements}")
    {
        this.minElements = minElements;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var list = value as IEnumerable<object>;

        if(list == null || list.Count() < this.minElements)
        {
            return new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName));
        }

        return ValidationResult.Success;
    }
}

MinElementsTests.cs

[Fact]
public void TestValidation()
{
    var validationResults = new List<ValidationResult>();
    var testObject = new TestObject();

    // Should be false since I have not added anything to the list
    var isValid = Validator.TryValidateObject(testObject, new ValidationContext(testObject), validationResults);

    // Fails since isValid comes back as true because IsValid on MinElementsAttribute is never called
    Assert.False(isValid);
    Assert.NotEmpty(validationResults);
}

internal class TestObject
{
    public TestObject()
    {
        this.StringList = new List<string>();
    }

    [MinElements(3)]
    public List<string> StringList { get; set; }
}

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Edit: The Solution

Please see the accepted answer for the solution. I wanted to add this to also note that by changing the inheritance to be from RequiredAttribute rather than ValidationAttribute, you can enforce the validation of all object properties implicitly.

Answer 1

Validator.TryValidateObject class has a few overload methods. You want to use this one:

public static bool TryValidateObject(object instance, ValidationContext validationContext, ICollection<ValidationResult> validationResults, bool validateAllProperties);

From the MSDN article:

validateAllProperties
Type: System.Boolean
true to validate all properties; if false, only required attributes are validated.

You must use this method, because your attribute does not derive from the RequiredAttribute class.

My Sample

var validationResults = new List<ValidationResult>();            
var testObject = new TestObject();
ValidationContext contexts = new ValidationContext(testObject, null, null);            
var isValid = Validator.TryValidateObject(testObject, contexts, validationResults, true);