How would you unit test data annotations?

Question

Two of the class properties have the following annotations:

    [Key]
    [Column]
    [Required]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }


    [MaxLength(25)]
    public string Name { get; set; }

I understand that testing Key, Column and Required attributes is no longer a unit test, it's an integration test as it would depend on the underlying database, but how do you go about testing MaxLength(25) attribute?

One of the alternatives that I can think of, is to add a code contract into the property.

Update

As suggested, I wrote the following helper:

    public class AttributeHelper <T> where T : class
    {
        private Type GivenClass 
        { 
            get { return typeof (T); }
        }

        public bool HasAnnotation(Type annotation)
        {
            return GivenClass.GetCustomAttributes(annotation, true).Single() != null;
        }

        public bool MethodHasAttribute(Type attribute, string target)
        {
           return GivenClass.GetMethod(target).GetCustomAttributes(attribute, true).Count() == 1;
        }

        public bool PropertyHasAttribute(Type attribute, string target)
        {
            return GivenClass.GetProperty(target).GetCustomAttributes(attribute, true).Count() == 1;
        }

    }

I have then tested my helper:

    [TestMethod]
    public void ThisMethod_Has_TestMethod_Attribute()
    {
        // Arrange
        var helper = new AttributeHelper<AttributeHelperTests>();

        // Act
        var result = helper.MethodHasAttribute(typeof (TestMethodAttribute), "ThisMethod_Has_TestMethod_Attribute");

        // Assert
        Assert.IsTrue(result);
    }

Everything works fine, apart from the fact that methods and properties must be public in order for me to use reflection. I can't think of any cases where I had to add attributes to the private properties/methods.

And then testing the EF annotations:

        public void IdProperty_Has_KeyAttribute()
        {
            // Arrange
            var helper = new AttributeHelper<Player>();

            // Act
            var result = helper.PropertyHasAttribute(typeof (KeyAttribute), "Id");

            // Assert
            Assert.IsTrue(result);
        }

Answer 1

I understand that testing Key, Column and Required attributes is no longer a unit test, it's an integration test as it would depend on the underlying database

How is that so? You can test whether Id property is marked with all those attributes just fine. And it falls into unit-test category.

[Test]
public void Id_IsMarkedWithKeyAttribute()
{
    var propertyInfo = typeof(MyClass).GetProperty("Id");

    var attribute = propertyInfo.GetCustomAttributes(typeof(KeyAttribute), true)
        .Cast<KeyAttribute>()
        .FirstOrDefault();

    Assert.That(attribute, Is.Not.Null);
}

This way you can assure your properties are marked with any attribute you can think of. Sure, this involves some reflection work but that's how you test attribute marking.