How can I use the Data Validation Attributes in C# in a non-ASP.net context?

TryValidateProperty is just badly written - you have to jump through hoops to get it to work outside a Controller, and even then, if you use it twice, it will end up quietly setting ModelState to valid/invalid and stop alterring that state, and stop returning accurate results from then on.

I gave up on it and just wrote my own validator. This'll loop over any set of objects in any context and tell you if they're valid:

bool isValid = true;
var invalidFields = new List<string>();

foreach (var o in viewModels)
{
    var properties = o.GetType()
        .GetProperties(BindingFlags.Public | BindingFlags.Instance);
    foreach(var prop in properties)
    {
        var attrs = prop.GetCustomAttributes(true);
        if (attrs != null)
        {
            var val = prop.GetValue(o);
            ValidationAttribute[] validatorAttrs = attrs
                .Where(a => a is ValidationAttribute)
                .Select(a => (ValidationAttribute)a).ToArray();

            foreach(var attr in validatorAttrs)
            {                       
                bool thisFieldIsValid = attr.IsValid(val);
                if (!thisFieldIsValid)
                    invalidFields.Add(prop.Name);

                isValid = isValid && thisFieldIsValid;
            }
        }
    }
}