Validation is working on Settings -> Indexers again.

NzbDrone.Web/Helpers/Validation/RequiredIfAttribute.cs

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+using System;
+using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
+using System.Linq;
+using System.Web;
+using System.Web.Mvc;
+
+namespace NzbDrone.Web.Helpers.Validation
+{
+    public class RequiredIfAttribute : ValidationAttribute, IClientValidatable
+    {
+        private RequiredAttribute _innerAttribute = new RequiredAttribute();
+
+        public string DependentProperty { get; set; }
+        public object TargetValue { get; set; }
+
+        public RequiredIfAttribute(string dependentProperty, object targetValue)
+        {
+            this.DependentProperty = dependentProperty;
+            this.TargetValue = targetValue;
+        }
+
+        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        {
+            // get a reference to the property this validation depends upon
+            var containerType = validationContext.ObjectInstance.GetType();
+            var field = containerType.GetProperty(this.DependentProperty);
+
+            if (field != null)
+            {
+                // get the value of the dependent property
+                var dependentvalue = field.GetValue(validationContext.ObjectInstance, null);
+
+                // compare the value against the target value
+                if ((dependentvalue == null && this.TargetValue == null) ||
+                    (dependentvalue != null && dependentvalue.Equals(this.TargetValue)))
+                {
+                    // match => means we should try validating this field
+                    if (!_innerAttribute.IsValid(value))
+                        // validation failed - return an error
+                        return new ValidationResult(this.ErrorMessage, new[] { validationContext.MemberName });
+                }
+            }
+
+            return ValidationResult.Success;
+        }
+
+        public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
+        {
+            var rule = new ModelClientValidationRule()
+            {
+                ErrorMessage = FormatErrorMessage(metadata.GetDisplayName()),
+                ValidationType = "requiredif",
+            };
+
+            string depProp = BuildDependentPropertyId(metadata, context as ViewContext);
+
+            // find the value on the control we depend on;
+            // if it's a bool, format it javascript style 
+            // (the default is True or False!)
+            string targetValue = (this.TargetValue ?? "").ToString();
+            if (this.TargetValue.GetType() == typeof(bool))
+                targetValue = targetValue.ToLower();
+
+            rule.ValidationParameters.Add("dependentproperty", depProp);
+            rule.ValidationParameters.Add("targetvalue", targetValue);
+
+            yield return rule;
+        }
+
+        private string BuildDependentPropertyId(ModelMetadata metadata, ViewContext viewContext)
+        {
+            // build the ID of the property
+            string depProp = viewContext.ViewData.TemplateInfo.GetFullHtmlFieldId(this.DependentProperty);
+            // unfortunately this will have the name of the current field appended to the beginning,
+            // because the TemplateInfo's context has had this fieldname appended to it. Instead, we
+            // want to get the context as though it was one level higher (i.e. outside the current property,
+            // which is the containing object (our Person), and hence the same level as the dependent property.
+            var thisField = metadata.PropertyName + "_";
+            if (depProp.StartsWith(thisField))
+                // strip it off again
+                depProp = depProp.Substring(thisField.Length);
+            return depProp;
+        }
+    }
+}