what are attributes used for in c# code example

Example: c# attribute

//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<string> CheckObjectForErrors<T>(T obj, bool SkipPostErrors)
{
	List<string> Errors = new List<string>();
	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