//Attributes can be added to properties or classes depending on the attribute.
//They can also be used to keep a sort of static variable hidden to the object on classes, properties, fields, enums, Assemblys, methods, and other objects at verying scopes that can be revied later for different uses
//For example they can mark classes for behaviour or for checks
[Serializable] //<-A class marking attribute marking the your class to do the work of getting ready your class for serlization so you dont have to.
public class SomeAPIClass
{
      //The attribute is always placed above the thing you wish to mark and can , seperated in the [ brackets or stacked in many [ brackets.
    [APIAlwaysRequired(nameof(UserName))]//Property marking attributes
    public string SomeValue { get; set; }
}
//To make a custom attribute class you can look into extending the attribute calls and marking it with attribute for the given object you want to mark
//Here is an example of a attribute to mark properies for a type of check
[System.AttributeUsage(System.AttributeTargets.Property | System.AttributeTargets.Field, Inherited = true)]
public class APIAlwaysRequiredAttribute : System.Attribute
{
    public string ErrorMessage { get; set; } = "Your request is missing or with out a param.";

    public APIAlwaysRequiredAttribute(){}

    public APIAlwaysRequiredAttribute(string ParamName)
    {
        this.ErrorMessage = $"Your request is missing or with out the {ParamName} param."; ;
    }
}
//You then will need to make use of your attribute in some way. Probably through reflection.
//Here is an example of the above attribute in use with a templatable generic check that saves you from writing too many vers your self. Let reflection and attributes take on all the work.
private static List CheckObjectForErrors(T obj, bool SkipPostErrors)
{
    List Errors = new List();
    Type ObjType = typeof(T);
    PropertyInfo[] Properties = ObjType.GetProperties();
    foreach (PropertyInfo P in Properties)
    {
        if (Attribute.IsDefined(P, typeof(APIAlwaysRequiredAttribute)))
        {
            if (P.GetValue(obj) == null)
            {
                APIAlwaysRequiredAttribute ErrorAttribute = (APIAlwaysRequiredAttribute)Attribute.GetCustomAttribute(P, typeof(APIAlwaysRequiredAttribute));
                Errors.Add(ErrorAttribute.ErrorMessage);
            }
        }
    }
    if (Errors.Count() != 0)
        return Errors;
    return null;
}
//There are an an insane amount of ways you can use this like a Regex check that takes the Regex String in a var could be added