using System.Collections;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Resources;
using System.Globalization;
using System.Linq;
using System.Reflection;

namespace System.ComponentModel.DataAnnotations {
    /// <summary>
    /// Helper class to validate objects, properties and other values using their associated <see cref="ValidationAttribute"/>
    /// custom attributes.
    /// </summary>
    public static class Validator {
        private static ValidationAttributeStore _store = ValidationAttributeStore.Instance;

        /// <summary>
        /// Tests whether the given property value is valid.
        /// </summary>
        /// <remarks>
        /// This method will test each <see cref="ValidationAttribute"/> associated with the property
        /// identified by <paramref name="validationContext"/>.  If <paramref name="validationResults"/> is non-null,
        /// this method will add a <see cref="ValidationResult"/> to it for each validation failure.
        /// <para>
        /// If there is a <see cref="RequiredAttribute"/> found on the property, it will be evaluated before all other
        /// validation attributes.  If the required validator fails then validation will abort, adding that single
        /// failure into the <paramref name="validationResults"/> when applicable, returning a value of <c>false</c>.
        /// </para>
        /// <para>
        /// If <paramref name="validationResults"/> is null and there isn't a <see cref="RequiredAttribute"/> failure,
        /// then all validators will be evaluated.
        /// </para>
        /// </remarks>
        /// <param name="value">The value to test.</param>
        /// <param name="validationContext">Describes the property member to validate and provides services and context for the validators.</param>
        /// <param name="validationResults">Optional collection to receive <see cref="ValidationResult"/>s for the failures.</param>
        /// <returns><c>true</c> if the value is valid, <c>false</c> if any validation errors are encountered.</returns>
        /// <exception cref="ArgumentException">
        /// When the <see cref="ValidationContext.MemberName"/> of <paramref name="validationContext"/> is not a valid property.
        /// </exception>
        public static bool TryValidateProperty(object value, ValidationContext validationContext, ICollection<ValidationResult> validationResults) {
            // Throw if value cannot be assigned to this property.  That is not a validation exception.
            Type propertyType = _store.GetPropertyType(validationContext);
            string propertyName = validationContext.MemberName;
            EnsureValidPropertyType(propertyName, propertyType, value);

            bool result = true;
            bool breakOnFirstError = (validationResults == null);

            IEnumerable<ValidationAttribute> attributes = _store.GetPropertyValidationAttributes(validationContext);

            foreach (ValidationError err in GetValidationErrors(value, validationContext, attributes, breakOnFirstError)) {
                result = false;

                if (validationResults != null) {
                    validationResults.Add(err.ValidationResult);
                }
            }

            return result;
        }

        /// <summary>
        /// Tests whether the given object instance is valid.
        /// </summary>
        /// <remarks>
        /// This method evaluates all <see cref="ValidationAttribute"/>s attached to the object instance's type.  It also
        /// checks to ensure all properties marked with <see cref="RequiredAttribute"/> are set.  It does not validate the
        /// property values of the object.
        /// <para>
        /// If <paramref name="validationResults"/> is null, then execution will abort upon the first validation
        /// failure.  If <paramref name="validationResults"/> is non-null, then all validation attributes will be
        /// evaluated.
        /// </para>
        /// </remarks>
        /// <param name="instance">The object instance to test.  It cannot be <c>null</c>.</param>
        /// <param name="validationContext">Describes the object to validate and provides services and context for the validators.</param>
        /// <param name="validationResults">Optional collection to receive <see cref="ValidationResult"/>s for the failures.</param>
        /// <returns><c>true</c> if the object is valid, <c>false</c> if any validation errors are encountered.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="instance"/> is null.</exception>
        /// <exception cref="ArgumentException">When <paramref name="instance"/> doesn't match the
        /// <see cref="ValidationContext.ObjectInstance"/>on <paramref name="validationContext"/>.</exception>
        public static bool TryValidateObject(object instance, ValidationContext validationContext, ICollection<ValidationResult> validationResults) {
            return TryValidateObject(instance, validationContext, validationResults, false /*validateAllProperties*/);
        }

        /// <summary>
        /// Tests whether the given object instance is valid.
        /// </summary>
        /// <remarks>
        /// This method evaluates all <see cref="ValidationAttribute"/>s attached to the object instance's type.  It also
        /// checks to ensure all properties marked with <see cref="RequiredAttribute"/> are set.  If <paramref name="validateAllProperties"/>
        /// is <c>true</c>, this method will also evaluate the <see cref="ValidationAttribute"/>s for all the immediate properties
        /// of this object.  This process is not recursive.
        /// <para>
        /// If <paramref name="validationResults"/> is null, then execution will abort upon the first validation
        /// failure.  If <paramref name="validationResults"/> is non-null, then all validation attributes will be
        /// evaluated.
        /// </para>
        /// <para>
        /// For any given property, if it has a <see cref="RequiredAttribute"/> that fails validation, no other validators
        /// will be evaluated for that property.
        /// </para>
        /// </remarks>
        /// <param name="instance">The object instance to test.  It cannot be null.</param>
        /// <param name="validationContext">Describes the object to validate and provides services and context for the validators.</param>
        /// <param name="validationResults">Optional collection to receive <see cref="ValidationResult"/>s for the failures.</param>
        /// <param name="validateAllProperties">If <c>true</c>, also evaluates all properties of the object (this process is not
        /// recursive over properties of the properties).</param>
        /// <returns><c>true</c> if the object is valid, <c>false</c> if any validation errors are encountered.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="instance"/> is null.</exception>
        /// <exception cref="ArgumentException">When <paramref name="instance"/> doesn't match the
        /// <see cref="ValidationContext.ObjectInstance"/>on <paramref name="validationContext"/>.</exception>
        public static bool TryValidateObject(object instance, ValidationContext validationContext, ICollection<ValidationResult> validationResults, bool validateAllProperties) {
            if (instance == null) {
                throw new ArgumentNullException("instance");
            }

            if (validationContext != null && instance != validationContext.ObjectInstance) {
                throw new ArgumentException(Resources.DataAnnotationsResources.Validator_InstanceMustMatchValidationContextInstance, "instance");
            }

            bool result = true;
            bool breakOnFirstError = (validationResults == null);

            foreach (ValidationError err in GetObjectValidationErrors(instance, validationContext, validateAllProperties, breakOnFirstError)) {
                result = false;

                if (validationResults != null) {
                    validationResults.Add(err.ValidationResult);
                }
            }

            return result;
        }

        /// <summary>
        /// Tests whether the given value is valid against a specified list of <see cref="ValidationAttribute"/>s.
        /// </summary>
        /// <remarks>
        /// This method will test each <see cref="ValidationAttribute"/>s specified .  If
        /// <paramref name="validationResults"/> is non-null, this method will add a <see cref="ValidationResult"/>
        /// to it for each validation failure.
        /// <para>
        /// If there is a <see cref="RequiredAttribute"/> within the <paramref name="validationAttributes"/>, it will
        /// be evaluated before all other validation attributes.  If the required validator fails then validation will
        /// abort, adding that single failure into the <paramref name="validationResults"/> when applicable, returning a
        /// value of <c>false</c>.
        /// </para>
        /// <para>
        /// If <paramref name="validationResults"/> is null and there isn't a <see cref="RequiredAttribute"/> failure,
        /// then all validators will be evaluated.
        /// </para>
        /// </remarks>
        /// <param name="value">The value to test.  It cannot be null.</param>
        /// <param name="validationContext">Describes the object being validated and provides services and context for the validators.</param>
        /// <param name="validationResults">Optional collection to receive <see cref="ValidationResult"/>s for the failures.</param>
        /// <param name="validationAttributes">The list of <see cref="ValidationAttribute"/>s to validate this <paramref name="value"/> against.</param>
        /// <returns><c>true</c> if the object is valid, <c>false</c> if any validation errors are encountered.</returns>
        public static bool TryValidateValue(object value, ValidationContext validationContext, ICollection<ValidationResult> validationResults, IEnumerable<ValidationAttribute> validationAttributes) {
            bool result = true;
            bool breakOnFirstError = validationResults == null;

            foreach (ValidationError err in GetValidationErrors(value, validationContext, validationAttributes, breakOnFirstError)) {
                result = false;

                if (validationResults != null) {
                    validationResults.Add(err.ValidationResult);
                }
            }

            return result;
        }

        /// <summary>
        /// Throws a <see cref="ValidationException"/> if the given property <paramref name="value"/> is not valid.
        /// </summary>
        /// <param name="value">The value to test.</param>
        /// <param name="validationContext">Describes the object being validated and provides services and context for the validators.  It cannot be <c>null</c>.</param>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        /// <exception cref="ValidationException">When <paramref name="value"/> is invalid for this property.</exception>
        public static void ValidateProperty(object value, ValidationContext validationContext) {
            // Throw if value cannot be assigned to this property.  That is not a validation exception.
            Type propertyType = _store.GetPropertyType(validationContext);
            EnsureValidPropertyType(validationContext.MemberName, propertyType, value);

            IEnumerable<ValidationAttribute> attributes = _store.GetPropertyValidationAttributes(validationContext);

            ValidationError err = GetValidationErrors(value, validationContext, attributes, false).FirstOrDefault();
            if (err != null) {
                err.ThrowValidationException();
            }
        }

        /// <summary>
        /// Throws a <see cref="ValidationException"/> if the given <paramref name="instance"/> is not valid.
        /// </summary>
        /// <remarks>
        /// This method evaluates all <see cref="ValidationAttribute"/>s attached to the object's type.
        /// </remarks>
        /// <param name="instance">The object instance to test.  It cannot be null.</param>
        /// <param name="validationContext">Describes the object being validated and provides services and context for the validators.  It cannot be <c>null</c>.</param>
        /// <exception cref="ArgumentNullException">When <paramref name="instance"/> is null.</exception>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        /// <exception cref="ArgumentException">When <paramref name="instance"/> doesn't match the
        /// <see cref="ValidationContext.ObjectInstance"/> on <paramref name="validationContext"/>.</exception>
        /// <exception cref="ValidationException">When <paramref name="instance"/> is found to be invalid.</exception>
        public static void ValidateObject(object instance, ValidationContext validationContext) {
            ValidateObject(instance, validationContext, false /*validateAllProperties*/);
        }

        /// <summary>
        /// Throws a <see cref="ValidationException"/> if the given object instance is not valid.
        /// </summary>
        /// <remarks>
        /// This method evaluates all <see cref="ValidationAttribute"/>s attached to the object's type.
        /// If <paramref name="validateAllProperties"/> is <c>true</c> it also validates all the object's properties.
        /// </remarks>
        /// <param name="instance">The object instance to test.  It cannot be null.</param>
        /// <param name="validationContext">Describes the object being validated and provides services and context for the validators.  It cannot be <c>null</c>.</param>
        /// <param name="validateAllProperties">If <c>true</c>, also validates all the <paramref name="instance"/>'s properties.</param>
        /// <exception cref="ArgumentNullException">When <paramref name="instance"/> is null.</exception>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        /// <exception cref="ArgumentException">When <paramref name="instance"/> doesn't match the
        /// <see cref="ValidationContext.ObjectInstance"/> on <paramref name="validationContext"/>.</exception>
        /// <exception cref="ValidationException">When <paramref name="instance"/> is found to be invalid.</exception>
        public static void ValidateObject(object instance, ValidationContext validationContext, bool validateAllProperties) {
            if (instance == null) {
                throw new ArgumentNullException("instance");
            }
            if (validationContext == null) {
                throw new ArgumentNullException("validationContext");
            }
            if (instance != validationContext.ObjectInstance) {
                throw new ArgumentException(Resources.DataAnnotationsResources.Validator_InstanceMustMatchValidationContextInstance, "instance");
            }

            ValidationError err = GetObjectValidationErrors(instance, validationContext, validateAllProperties, false).FirstOrDefault();
            if (err != null) {
                err.ThrowValidationException();
            }
        }

        /// <summary>
        /// Throw a <see cref="ValidationException"/> if the given value is not valid for the <see cref="ValidationAttribute"/>s.
        /// </summary>
        /// <remarks>
        /// This method evaluates the <see cref="ValidationAttribute"/>s supplied until a validation error occurs,
        /// at which time a <see cref="ValidationException"/> is thrown.
        /// <para>
        /// A <see cref="RequiredAttribute"/> within the <paramref name="validationAttributes"/> will always be evaluated first.
        /// </para>
        /// </remarks>
        /// <param name="value">The value to test.  It cannot be null.</param>
        /// <param name="validationContext">Describes the object being tested.</param>
        /// <param name="validationAttributes">The list of <see cref="ValidationAttribute"/>s to validate against this instance.</param>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        /// <exception cref="ValidationException">When <paramref name="value"/> is found to be invalid.</exception>
        public static void ValidateValue(object value, ValidationContext validationContext, IEnumerable<ValidationAttribute> validationAttributes) {
            if (validationContext == null) {
                throw new ArgumentNullException("validationContext");
            }

            ValidationError err = GetValidationErrors(value, validationContext, validationAttributes, false).FirstOrDefault();
            if (err != null) {
                err.ThrowValidationException();
            }
        }

        /// <summary>
        /// Creates a new <see cref="ValidationContext"/> to use to validate the type or a member of
        /// the given object instance.
        /// </summary>
        /// <param name="instance">The object instance to use for the context.</param>
        /// <param name="validationContext">An parent validation context that supplies an <see cref="IServiceProvider"/>
        /// and <see cref="ValidationContext.Items"/>.</param>
        /// <returns>A new <see cref="ValidationContext"/> for the <paramref name="instance"/> provided.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        internal static ValidationContext CreateValidationContext(object instance, ValidationContext validationContext) {
            if (validationContext == null) {
                throw new ArgumentNullException("validationContext");
            }

            // Create a new context using the existing ValidationContext that acts as an IServiceProvider and contains our existing items.
            ValidationContext context = new ValidationContext(instance, validationContext, validationContext.Items);
            return context;
        }

        /// <summary>
        /// Determine whether the given value can legally be assigned into the specified type.
        /// </summary>
        /// <param name="destinationType">The destination <see cref="Type"/> for the value.</param>
        /// <param name="value">The value to test to see if it can be assigned as the Type indicated by <paramref name="destinationType"/>.</param>
        /// <returns><c>true</c> if the assignment is legal.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="destinationType"/> is null.</exception>
        private static bool CanBeAssigned(Type destinationType, object value) {
            if (destinationType == null) {
                throw new ArgumentNullException("destinationType");
            }

            if (value == null) {
                // Null can be assigned only to reference types or Nullable or Nullable<>
                return !destinationType.IsValueType ||
                        (destinationType.IsGenericType && destinationType.GetGenericTypeDefinition() == typeof(Nullable<>));
            }

            // Not null -- be sure it can be cast to the right type
            return destinationType.IsAssignableFrom(value.GetType());
        }

        /// <summary>
        /// Determines whether the given value can legally be assigned to the given property.
        /// </summary>
        /// <param name="propertyName">The name of the property.</param>
        /// <param name="propertyType">The type of the property.</param>
        /// <param name="value">The value.  Null is permitted only if the property will accept it.</param>
        /// <exception cref="ArgumentException"> is thrown if <paramref name="value"/> is the wrong type for this property.</exception>
        private static void EnsureValidPropertyType(string propertyName, Type propertyType, object value) {
            if (!CanBeAssigned(propertyType, value)) {
                throw new ArgumentException(String.Format(CultureInfo.CurrentCulture, DataAnnotationsResources.Validator_Property_Value_Wrong_Type, propertyName, propertyType), "value");
            }
        }

        /// <summary>
        /// Internal iterator to enumerate all validation errors for the given object instance.
        /// </summary>
        /// <param name="instance">Object instance to test.</param>
        /// <param name="validationContext">Describes the object type.</param>
        /// <param name="validateAllProperties">if <c>true</c> also validates all properties.</param>
        /// <param name="breakOnFirstError">Whether to break on the first error or validate everything.</param>
        /// <returns>A collection of validation errors that result from validating the <paramref name="instance"/> with
        /// the given <paramref name="validationContext"/>.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="instance"/> is null.</exception>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        /// <exception cref="ArgumentException">When <paramref name="instance"/> doesn't match the
        /// <see cref="ValidationContext.ObjectInstance"/> on <paramref name="validationContext"/>.</exception>
        private static IEnumerable<ValidationError> GetObjectValidationErrors(object instance, ValidationContext validationContext, bool validateAllProperties, bool breakOnFirstError) {
            if (instance == null) {
                throw new ArgumentNullException("instance");
            }

            if (validationContext == null) {
                throw new ArgumentNullException("validationContext");
            }

            // Step 1: Validate the object properties' validation attributes
            List<ValidationError> errors = new List<ValidationError>();
            errors.AddRange(GetObjectPropertyValidationErrors(instance, validationContext, validateAllProperties, breakOnFirstError));

            // We only proceed to Step 2 if there are no errors
            if (errors.Any()) {
                return errors;
            }

            // Step 2: Validate the object's validation attributes
            IEnumerable<ValidationAttribute> attributes = _store.GetTypeValidationAttributes(validationContext);
            errors.AddRange(GetValidationErrors(instance, validationContext, attributes, breakOnFirstError));

#if !SILVERLIGHT
            // We only proceed to Step 3 if there are no errors
            if (errors.Any()) {
                return errors;
            }

            // Step 3: Test for IValidatableObject implementation
            IValidatableObject validatable = instance as IValidatableObject;
            if (validatable != null) {
                IEnumerable<ValidationResult> results = validatable.Validate(validationContext);

                foreach (ValidationResult result in results.Where(r => r != ValidationResult.Success)) {
                    errors.Add(new ValidationError(null, instance, result));
                }
            }
#endif

            return errors;
        }

        /// <summary>
        /// Internal iterator to enumerate all the validation errors for all properties of the given object instance.
        /// </summary>
        /// <param name="instance">Object instance to test.</param>
        /// <param name="validationContext">Describes the object type.</param>
        /// <param name="validateAllProperties">If <c>true</c>, evaluates all the properties, otherwise just checks that
        /// ones marked with <see cref="RequiredAttribute"/> are not null.</param>
        /// <param name="breakOnFirstError">Whether to break on the first error or validate everything.</param>
        /// <returns>A list of <see cref="ValidationError"/> instances.</returns>
        private static IEnumerable<ValidationError> GetObjectPropertyValidationErrors(object instance, ValidationContext validationContext, bool validateAllProperties, bool breakOnFirstError) {
            ICollection<KeyValuePair<ValidationContext, object>> properties = GetPropertyValues(instance, validationContext);
            List<ValidationError> errors = new List<ValidationError>();

            foreach (KeyValuePair<ValidationContext, object> property in properties) {
                // get list of all validation attributes for this property
                IEnumerable<ValidationAttribute> attributes = _store.GetPropertyValidationAttributes(property.Key);

                if (validateAllProperties) {
                    // validate all validation attributes on this property
                    errors.AddRange(GetValidationErrors(property.Value, property.Key, attributes, breakOnFirstError));
                } else {
                    // only validate the Required attributes
                    RequiredAttribute reqAttr = attributes.FirstOrDefault(a => a is RequiredAttribute) as RequiredAttribute;
                    if (reqAttr != null) {
                        // Note: we let the [Required] attribute do its own null testing,
                        // since the user may have subclassed it and have a deeper meaning to what 'required' means
                        ValidationResult validationResult = reqAttr.GetValidationResult(property.Value, property.Key);
                        if (validationResult != ValidationResult.Success) {
                            errors.Add(new ValidationError(reqAttr, property.Value, validationResult));
                        }
                    }
                }

                if (breakOnFirstError && errors.Any()) {
                    break;
                }
            }

            return errors;
        }

        /// <summary>
        /// Retrieves the property values for the given instance.
        /// </summary>
        /// <param name="instance">Instance from which to fetch the properties.</param>
        /// <param name="validationContext">Describes the entity being validated.</param>
        /// <returns>A set of key value pairs, where the key is a validation context for the property and the value is its current
        /// value.</returns>
        /// <remarks>Ignores indexed properties.</remarks>
        private static ICollection<KeyValuePair<ValidationContext, object>> GetPropertyValues(object instance, ValidationContext validationContext) {
#if SILVERLIGHT
            IEnumerable<PropertyInfo> properties = from property in instance.GetType().GetProperties()
                                                   where !property.GetIndexParameters().Any()
                                                   select property;

            List<KeyValuePair<ValidationContext, object>> items = new List<KeyValuePair<ValidationContext, object>>(properties.Count());
            foreach (PropertyInfo property in properties) {
                ValidationContext context = CreateValidationContext(instance, validationContext);
                context.MemberName = property.Name;

                if (_store.GetPropertyValidationAttributes(context).Any()) {
                    items.Add(new KeyValuePair<ValidationContext, object>(context, property.GetValue(instance, null)));
                }
            }
#else
            PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(instance);
            List<KeyValuePair<ValidationContext, object>> items = new List<KeyValuePair<ValidationContext, object>>(properties.Count);
            foreach (PropertyDescriptor property in properties) {
                ValidationContext context = CreateValidationContext(instance, validationContext);
                context.MemberName = property.Name;

                if (_store.GetPropertyValidationAttributes(context).Any()) {
                    items.Add(new KeyValuePair<ValidationContext, object>(context, property.GetValue(instance)));
                }
            }
#endif
            return items;
        }


        /// <summary>
        /// Internal iterator to enumerate all validation errors for an value.
        /// </summary>
        /// <remarks>
        /// If a <see cref="RequiredAttribute"/> is found, it will be evaluated first, and if that fails,
        /// validation will abort, regardless of the <paramref name="breakOnFirstError"/> parameter value.
        /// </remarks>
        /// <param name="value">The value to pass to the validation attributes.</param>
        /// <param name="validationContext">Describes the type/member being evaluated.</param>
        /// <param name="attributes">The validation attributes to evaluate.</param>
        /// <param name="breakOnFirstError">Whether or not to break on the first validation failure.  A
        /// <see cref="RequiredAttribute"/> failure will always abort with that sole failure.</param>
        /// <returns>The collection of validation errors.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        private static IEnumerable<ValidationError> GetValidationErrors(object value, ValidationContext validationContext, IEnumerable<ValidationAttribute> attributes, bool breakOnFirstError) {
            if (validationContext == null) {
                throw new ArgumentNullException("validationContext");
            }

            List<ValidationError> errors = new List<ValidationError>();
            ValidationError validationError;

            // Get the required validator if there is one and test it first, aborting on failure
            RequiredAttribute required = attributes.FirstOrDefault(a => a is RequiredAttribute) as RequiredAttribute;
            if (required != null) {
                if (!TryValidate(value, validationContext, required, out validationError)) {
                    errors.Add(validationError);
                    return errors;
                }
            }

            // Iterate through the rest of the validators, skipping the required validator
            foreach (ValidationAttribute attr in attributes) {
                if (attr != required) {
                    if (!TryValidate(value, validationContext, attr, out validationError)) {
                        errors.Add(validationError);

                        if (breakOnFirstError) {
                            break;
                        }
                    }
                }
            }

            return errors;
        }

        /// <summary>
        /// Tests whether a value is valid against a single <see cref="ValidationAttribute"/> using the <see cref="ValidationContext"/>.
        /// </summary>
        /// <param name="value">The value to be tested for validity.</param>
        /// <param name="validationContext">Describes the property member to validate.</param>
        /// <param name="attribute">The validation attribute to test.</param>
        /// <param name="validationError">The validation error that occurs during validation.  Will be <c>null</c> when the return value is <c>true</c>.</param>
        /// <returns><c>true</c> if the value is valid.</returns>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        private static bool TryValidate(object value, ValidationContext validationContext, ValidationAttribute attribute, out ValidationError validationError) {
            if (validationContext == null) {
                throw new ArgumentNullException("validationContext");
            }

            ValidationResult validationResult = attribute.GetValidationResult(value, validationContext);
            if (validationResult != ValidationResult.Success) {
                validationError = new ValidationError(attribute, value, validationResult);
                return false;
            }

            validationError = null;
            return true;
        }

        /// <summary>
        /// Private helper class to encapsulate a ValidationAttribute with the failed value and the user-visible
        /// target name against which it was validated.
        /// </summary>
        private class ValidationError {
            internal ValidationError(ValidationAttribute validationAttribute, object value, ValidationResult validationResult) {
                this.ValidationAttribute = validationAttribute;
                this.ValidationResult = validationResult;
                this.Value = value;
            }

            internal object Value { get; set; }

            internal ValidationAttribute ValidationAttribute { get; set; }

            internal ValidationResult ValidationResult { get; set; }

            internal void ThrowValidationException() {
                throw new ValidationException(this.ValidationResult, this.ValidationAttribute, this.Value);
            }
        }
    }
}