diff --git a/Elwig/Helpers/Validator.cs b/Elwig/Helpers/Validator.cs
index a192aff..efb37e9 100644
--- a/Elwig/Helpers/Validator.cs
+++ b/Elwig/Helpers/Validator.cs
@@ -295,5 +295,26 @@ namespace Elwig.Helpers {
             // TODO
             return new(true, "Not implemented yet");
         }
+
+        public static ValidationResult CheckVNr(TextBox input, bool required, AppDbContext ctx, Contract? c) {
+            var res = CheckNumeric(input, required);
+            if (!res.IsValid) {
+                return res;
+            } else if (!required && input.Text.Length == 0) {
+                return new(true, null);
+            }
+
+            int nr = int.Parse(input.Text);
+            if (nr != c?.VNr && Utils.VNrExists(ctx, nr).GetAwaiter().GetResult()) {
+                return new(false, "Vertragsnummer wird bereits verwendet");
+            }
+
+            return new(true, null);
+        }
+
+        public static ValidationResult CheckGstNr(TextBox input, bool required) {
+            // TODO
+            return new(true, "Not implemented yet");
+        }
     }
 }