Unit testing methods for nested Fluent Validation properties

Helper.cs

// Adapted from http://devio.wordpress.com/2011/01/21/get-name-of-nested-property-as-string-value/

public static string GetPropertyName<T, TValue>(this T model, Expression<Func<T, TValue>> expression) where T : class
{
    var memberExpression = (MemberExpression)expression.Body;
    var memberExpressionOrg = memberExpression;

    var Path = "";

    while (memberExpression != null && memberExpression.Expression.NodeType == ExpressionType.MemberAccess)
    {
        var propInfo = memberExpression.Expression.GetType().GetProperty("Member");
        var propValue = propInfo.GetValue(memberExpression.Expression, null) as PropertyInfo;
        if (propValue != null) Path = propValue.Name + "." + Path;

        memberExpression = memberExpression.Expression as MemberExpression;
    }

    return Path + memberExpressionOrg.Member.Name;
}

Use.cs

target.ShouldHaveNestedValidationErrorFor(a => a.Info.EmailAddress, app);

Validators.cs

public static void ShouldHaveNestedValidationErrorFor<T, TValue>(this IValidator<T> validator, Expression<Func<T, TValue>> expression, T objectToTest) where T : class
{
    var propertyName = objectToTest.GetPropertyName(expression);
    var count = validator.Validate(objectToTest).Errors.Count(x => x.PropertyName == propertyName);

    if (count == 0)
    {
        throw new ValidationTestException(string.Format("Expected a validation error for property {0}", propertyName));
    }
}

public static void ShouldNotHaveNestedValidationErrorFor<T, TValue>(this IValidator<T> validator, Expression<Func<T, TValue>> expression, T objectToTest) where T : class
{
    var propertyName = objectToTest.GetPropertyName(expression);
    var count = validator.Validate(objectToTest).Errors.Count(x => x.PropertyName == propertyName);

    if (count > 0)
    {
        throw new ValidationTestException(string.Format("Expected no validation errors for property {0}", propertyName));
    }
}