1 files changed, 26 insertions, 0 deletions
diff --git a/Validators/ISBNAttribute.cs b/Validators/ISBNAttribute.cs
new file mode 100644
index 0000000..425e9e9
--- /dev/null
+++ b/Validators/ISBNAttribute.cs
@@ -0,0 +1,26 @@
+using System.ComponentModel.DataAnnotations;
+using LibraryAPI.Models;
+
+namespace LibraryAPI.Validators {
+    public class ISBNAttribute : ValidationAttribute {
+        public int ISBNlength { get; }
+
+        public ISBNAttribute(int isbn_length) {
+            ISBNlength = isbn_length;
+        }
+
+        public string GetErrorMessage() {
+          return $"ISBN strings must be {ISBNlength} characters long";
+        }
+
+        protected override ValidationResult? IsValid(object? value, ValidationContext validationContext) {
+            var book = (Book) validationContext.ObjectInstance;
+
+            if(book.ISBN.Length != ISBNlength) {
+                return new ValidationResult(GetErrorMessage());
+            }
+
+            return ValidationResult.Success;
+        }
+    }
+}
+\ No newline at end of file