Data Annotation Basics C# with Example

Data annotations are a way of adding more contextual information to classes or members of a class. There are 
three main categories of annotations: 
Validation Attributes: add validation criteria to data 
Display Attributes: specify how the data should be displayed to the user 
Modelling Attributes: add information on usage and relationship with other classes 
Usage 
Here is an example where two ValidationAttribute and one DisplayAttribute are used: 
class Kid 
{ 
[Range(0, 18)] // The age cannot be over 18 and cannot be negative 
public int Age { get; set; } 
[StringLength(MaximumLength = 50, MinimumLength = 3)] // The name cannot be under 3 chars or 
more than 50 chars 
public string Name { get; set; } 
[DataType(DataType.Date)] // The birthday will be displayed as a date only (without the time) 
public DateTime Birthday { get; set; } 
} 
Data annotations are mostly used in frameworks such as ASP.NET. For example, in ASP.NET MVC, when a model is 
received by a controller method, ModelState.IsValid() can be used to tell if the received model respects all its 
ValidationAttribute. DisplayAttribute is also used in ASP.NET MVC to determine how to display values on a web 
page.