Validation Attributes C# with Example

Validation attributes are used to enforce various validation rules in a declarative fashion on classes or class 
members. All validation attributes derive from the ValidationAttribute base class. 
Example: RequiredAttribute 
When validated through the ValidationAttribute.Validate method, this attribute will return an error if the Name 
property is null or contains only whitespace. 
 

public class ContactModel 
{ 
[Required(ErrorMessage = "Please provide a name.")] 
public string Name { get; set; } 
} 
Example: StringLengthAttribute 
The StringLengthAttribute validates if a string is less than the maximum length of a string. It can optionally 
specify a minimum length. Both values are inclusive. 
public class ContactModel 
{ 
[StringLength(20, MinimumLength = 5, ErrorMessage = "A name must be between five and twenty 
characters.")] 
public string Name { get; set; } 
} 
Example: RangeAttribute 
The RangeAttribute gives the maximum and minimum value for a numeric field. 
public class Model 
{ 
[Range(0.01, 100.00,ErrorMessage = "Price must be between 0.01 and 100.00")] 
public decimal Price { get; set; } 
} 
Example: CustomValidationAttribute 
The CustomValidationAttribute class allows a custom static method to be invoked for validation. The custom 
method must be static ValidationResult [MethodName] (object input). 
public class Model 
{ 
[CustomValidation(typeof(MyCustomValidation), "IsNotAnApple")] 
public string FavoriteFruit { get; set; } 
} 
Method declaration: 
public static class MyCustomValidation 
{ 
public static ValidationResult IsNotAnApple(object input) 
{ 
var result = ValidationResult.Success; 
if (input?.ToString()?.ToUpperInvariant() == "APPLE") 
{ 
result = new ValidationResult("Apples are not allowed."); 
} 
return result; 
} 
}