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

namespace System.ComponentModel.DataAnnotations
{
    /// <summary>
    /// Base class for all validation attributes.
    /// <para>Override <see cref="IsValid(object, ValidationContext)"/> to implement validation logic.</para>
    /// </summary>
    /// <remarks>
    /// The properties <see cref="ErrorMessageResourceType"/> and <see cref="ErrorMessageResourceName"/> are used to provide
    /// a localized error message, but they cannot be set if <see cref="ErrorMessage"/> is also used to provide a non-localized
    /// error message.
    /// </remarks>
    public abstract class ValidationAttribute : Attribute
    {
        #region Member Fields

        private string _errorMessage;
        private Func<string> _errorMessageResourceAccessor;
        private string _errorMessageResourceName;
        private Type _errorMessageResourceType;
        private bool _isCallingOverload;
        private object _syncLock = new object();

        #endregion

        #region All Constructors

        /// <summary>
        /// Default constructor for any validation attribute.
        /// </summary>
        /// <remarks>This constructor chooses a very generic validation error message.
        /// Developers subclassing ValidationAttribute should use other constructors
        /// or supply a better message.
        /// </remarks>
        protected ValidationAttribute()
            : this(() => DataAnnotationsResources.ValidationAttribute_ValidationError)
        {
        }

        /// <summary>
        /// Constructor that accepts a fixed validation error message.
        /// </summary>
        /// <param name="errorMessage">A non-localized error message to use in <see cref="ErrorMessageString"/>.</param>
        protected ValidationAttribute(string errorMessage)
            : this(() => errorMessage)
        {
        }

        /// <summary>
        /// Allows for providing a resource accessor function that will be used by the <see cref="ErrorMessageString"/>
        /// property to retrieve the error message.  An example would be to have something like
        /// CustomAttribute() : base( () =&gt; MyResources.MyErrorMessage ) {}.
        /// </summary>
        /// <param name="errorMessageAccessor">The <see cref="Func{T}"/> that will return an error message.</param>
        protected ValidationAttribute(Func<string> errorMessageAccessor)
        {
            // If null, will later be exposed as lack of error message to be able to construct accessor
            this._errorMessageResourceAccessor = errorMessageAccessor;
        }

        #endregion

        #region Protected Properties

        /// <summary>
        /// Gets the localized error message string, coming either from <see cref="ErrorMessage"/>, or from evaluating the 
        /// <see cref="ErrorMessageResourceType"/> and <see cref="ErrorMessageResourceName"/> pair.
        /// </summary>
        protected string ErrorMessageString
        {
            get
            {
                this.SetupResourceAccessor();
                return this._errorMessageResourceAccessor();
            }
        }

        /// <summary>
        /// A flag indicating whether a developer has customized the attribute's error message by setting any one of 
        /// ErrorMessage, ErrorMessageResourceName, or ErrorMessageResourceType.
        /// </summary>
        internal bool CustomErrorMessageSet
        {
            get;
            private set;
        }

        /// <summary>
        /// A flag indicating that the attribute requires a non-null <see cref=System.ComponentModel.DataAnnotations.ValidationContext /> to perform validation.
        /// Base class returns false. Override in child classes as appropriate.
        /// </summary>
        public virtual bool RequiresValidationContext
        {
            get
            {
                return false;
            }
        }

        #endregion

        #region Public Properties

        /// <summary>
        /// Gets or sets and explicit error message string.
        /// </summary>
        /// <value>
        /// This property is intended to be used for non-localizable error messages.  Use
        /// <see cref="ErrorMessageResourceType"/> and <see cref="ErrorMessageResourceName"/> for localizable error messages.
        /// </value>
        public string ErrorMessage
        {
            get
            {
                return this._errorMessage;
            }
            set
            {
                this._errorMessage = value;
                this._errorMessageResourceAccessor = null;
                this.CustomErrorMessageSet = true;
            }
        }

        /// <summary>
        /// Gets or sets the resource name (property name) to use as the key for lookups on the resource type.
        /// </summary>
        /// <value>
        /// Use this property to set the name of the property within <see cref="ErrorMessageResourceType"/>
        /// that will provide a localized error message.  Use <see cref="ErrorMessage"/> for non-localized error messages.
        /// </value>
        public string ErrorMessageResourceName
        {
            get
            {
                return this._errorMessageResourceName;
            }
            set
            {
                this._errorMessageResourceName = value;
                this._errorMessageResourceAccessor = null;
                this.CustomErrorMessageSet = true;
            }
        }

        /// <summary>
        /// Gets or sets the resource type to use for error message lookups.
        /// </summary>
        /// <value>
        /// Use this property only in conjunction with <see cref="ErrorMessageResourceName"/>.  They are
        /// used together to retrieve localized error messages at runtime.
        /// <para>Use <see cref="ErrorMessage"/> instead of this pair if error messages are not localized.
        /// </para>
        /// </value>
        public Type ErrorMessageResourceType
        {
            get
            {
                return this._errorMessageResourceType;
            }
            set
            {
                this._errorMessageResourceType = value;
                this._errorMessageResourceAccessor = null;
                this.CustomErrorMessageSet = true;
            }
        }

        #endregion

        #region Private Methods

        /// <summary>
        /// Validates the configuration of this attribute and sets up the appropriate error string accessor.
        /// This method bypasses all verification once the ResourceAccessor has been set.
        /// </summary>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        private void SetupResourceAccessor()
        {
            if (this._errorMessageResourceAccessor == null)
            {
                string localErrorMessage = this._errorMessage;
                bool resourceNameSet = !string.IsNullOrEmpty(this._errorMessageResourceName);
                bool errorMessageSet = !string.IsNullOrEmpty(localErrorMessage);
                bool resourceTypeSet = this._errorMessageResourceType != null;

                // Either ErrorMessageResourceName or ErrorMessage may be set, but not both.
                // The following test checks both being set as well as both being not set.
                if (resourceNameSet == errorMessageSet)
                {
                    throw new InvalidOperationException(DataAnnotationsResources.ValidationAttribute_Cannot_Set_ErrorMessage_And_Resource);
                }

                // Must set both or neither of ErrorMessageResourceType and ErrorMessageResourceName
                if (resourceTypeSet != resourceNameSet)
                {
                    throw new InvalidOperationException(DataAnnotationsResources.ValidationAttribute_NeedBothResourceTypeAndResourceName);
                }

                // If set resource type (and we know resource name too), then go setup the accessor
                if (resourceNameSet)
                {
                    this.SetResourceAccessorByPropertyLookup();
                }
                else
                {
                    // Here if not using resource type/name -- the accessor is just the error message string,
                    // which we know is not empty to have gotten this far.
                    this._errorMessageResourceAccessor = delegate
                    {
                        // We captured error message to local in case it changes before accessor runs
                        return localErrorMessage;
                    };
                }
            }
        }

        private void SetResourceAccessorByPropertyLookup()
        {
            if (this._errorMessageResourceType != null && !string.IsNullOrEmpty(this._errorMessageResourceName))
            {
#if SILVERLIGHT
                var property = this._errorMessageResourceType.GetProperty(this._errorMessageResourceName, BindingFlags.Public | BindingFlags.Static);
#else
                var property = this._errorMessageResourceType.GetProperty(this._errorMessageResourceName, BindingFlags.Public | BindingFlags.Static | BindingFlags.NonPublic);
                if (property != null)
                {
                    MethodInfo propertyGetter = property.GetGetMethod(true /*nonPublic*/);
                    // We only support internal and public properties
                    if (propertyGetter == null || (!propertyGetter.IsAssembly && !propertyGetter.IsPublic))
                    {
                        // Set the property to null so the exception is thrown as if the property wasn't found
                        property = null;
                    }
                }
#endif
                if (property == null)
                {
                    throw new InvalidOperationException(
                        String.Format(
                        CultureInfo.CurrentCulture,
                        DataAnnotationsResources.ValidationAttribute_ResourceTypeDoesNotHaveProperty,
                        this._errorMessageResourceType.FullName,
                        this._errorMessageResourceName));
                }
                if (property.PropertyType != typeof(string))
                {
                    throw new InvalidOperationException(
                        String.Format(
                        CultureInfo.CurrentCulture,
                        DataAnnotationsResources.ValidationAttribute_ResourcePropertyNotStringType,
                        property.Name,
                        this._errorMessageResourceType.FullName));
                }

                this._errorMessageResourceAccessor = delegate
                {
                    return (string)property.GetValue(null, null);
                };
            }
            else
            {
                throw new InvalidOperationException(String.Format(CultureInfo.CurrentCulture, DataAnnotationsResources.ValidationAttribute_NeedBothResourceTypeAndResourceName));
            }
        }

        #endregion

        #region Protected & Public Methods

        /// <summary>
        /// Formats the error message to present to the user.
        /// </summary>
        /// <remarks>The error message will be re-evaluated every time this function is called. 
        /// It applies the <paramref name="name"/> (for example, the name of a field) to the formated error message, resulting 
        /// in something like "The field 'name' has an incorrect value".
        /// <para>
        /// Derived classes can override this method to customize how errors are generated.
        /// </para>
        /// <para>
        /// The base class implementation will use <see cref="ErrorMessageString"/> to obtain a localized
        /// error message from properties within the current attribute.  If those have not been set, a generic
        /// error message will be provided.
        /// </para>
        /// </remarks>
        /// <param name="name">The user-visible name to include in the formatted message.</param>
        /// <returns>The localized string describing the validation error</returns>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        public virtual string FormatErrorMessage(string name)
        {
            return String.Format(CultureInfo.CurrentCulture, this.ErrorMessageString, name);
        }

        /// <summary>
        /// Gets the value indicating whether or not the specified <paramref name="value"/> is valid
        /// with respect to the current validation attribute.
        /// <para>
        /// Derived classes should not override this method as it is only available for backwards compatibility.
        /// Instead, implement <see cref="IsValid(object, ValidationContext)"/>.
        /// </para>
        /// </summary>
        /// <remarks>
        /// The preferred public entry point for clients requesting validation is the <see cref="GetValidationResult"/> method.
        /// </remarks>
        /// <param name="value">The value to validate</param>
        /// <returns><c>true</c> if the <paramref name="value"/> is acceptable, <c>false</c> if it is not acceptable</returns>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        /// <exception cref="NotImplementedException"> is thrown when neither overload of IsValid has been implemented
        /// by a derived class.
        /// </exception>
        internal virtual bool IsValid(object value)
        {
            lock (this._syncLock)
            {
                if (this._isCallingOverload)
                {
                    throw new NotImplementedException(DataAnnotationsResources.ValidationAttribute_IsValid_NotImplemented);
                }
                else
                {
                    this._isCallingOverload = true;

                    try
                    {
                        return this.IsValid(value, null) == null;
                    }
                    finally
                    {
                        this._isCallingOverload = false;
                    }
                }
            }
        }

        /// <summary>
        /// Protected virtual method to override and implement validation logic.
        /// </summary>
        /// <param name="value">The value to validate.</param>
        /// <param name="validationContext">A <see cref="ValidationContext"/> instance that provides
        /// context about the validation operation, such as the object and member being validated.</param>
        /// <returns>
        /// When validation is valid, <see cref="ValidationResult.Success"/>.
        /// <para>
        /// When validation is invalid, an instance of <see cref="ValidationResult"/>.
        /// </para>
        /// </returns>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        /// <exception cref="NotImplementedException"> is thrown when <see cref="IsValid(object, ValidationContext)" />
        /// has not been implemented by a derived class.
        /// </exception>
        protected virtual ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            lock (this._syncLock)
            {
                if (this._isCallingOverload)
                {
                    throw new NotImplementedException(DataAnnotationsResources.ValidationAttribute_IsValid_NotImplemented);
                }
                else
                {
                    this._isCallingOverload = true;

                    try
                    {
                        ValidationResult result = ValidationResult.Success;

                        if (!this.IsValid(value))
                        {
                            string[] memberNames = validationContext.MemberName != null ? new string[] { validationContext.MemberName } : null;
                            result = new ValidationResult(this.FormatErrorMessage(validationContext.DisplayName), memberNames);
                        }
                        return result;
                    }
                    finally
                    {
                        this._isCallingOverload = false;
                    }
                }
            }
        }

        /// <summary>
        /// Tests whether the given <paramref name="value"/> is valid with respect to the current
        /// validation attribute without throwing a <see cref="ValidationException"/>
        /// </summary>
        /// <remarks>
        /// If this method returns <see cref="ValidationResult.Success"/>, then validation was successful, otherwise
        /// an instance of <see cref="ValidationResult"/> will be returned with a guaranteed non-null
        /// <see cref="ValidationResult.ErrorMessage"/>.
        /// </remarks>
        /// <param name="value">The value to validate</param>
        /// <param name="validationContext">A <see cref="ValidationContext"/> instance that provides
        /// context about the validation operation, such as the object and member being validated.</param>
        /// <returns>
        /// When validation is valid, <see cref="ValidationResult.Success"/>.
        /// <para>
        /// When validation is invalid, an instance of <see cref="ValidationResult"/>.
        /// </para>
        /// </returns>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        /// <exception cref="ArgumentNullException">When <paramref name="validationContext"/> is null.</exception>
        /// <exception cref="NotImplementedException"> is thrown when <see cref="IsValid(object, ValidationContext)" />
        /// has not been implemented by a derived class.
        /// </exception>
        public ValidationResult GetValidationResult(object value, ValidationContext validationContext)
        {
            if (validationContext == null)
            {
                throw new ArgumentNullException("validationContext");
            }

            ValidationResult result = this.IsValid(value, validationContext);

            // If validation fails, we want to ensure we have a ValidationResult that guarantees it has an ErrorMessage
            if (result != null)
            {
                bool hasErrorMessage = (result != null) ? !string.IsNullOrEmpty(result.ErrorMessage) : false;
                if (!hasErrorMessage)
                {
                    string errorMessage = this.FormatErrorMessage(validationContext.DisplayName);
                    result = new ValidationResult(errorMessage, result.MemberNames);
                }
            }

            return result;
        }

#if !SILVERLIGHT
        /// <summary>
        /// Validates the specified <paramref name="value"/> and throws <see cref="ValidationException"/> if it is not.
        /// <para>
        /// The overloaded <see cref="Validate(object, ValidationContext)"/> is the recommended entry point as it
        /// can provide additional context to the <see cref="ValidationAttribute"/> being validated.
        /// </para>
        /// </summary>
        /// <remarks>This base method invokes the <see cref="IsValid(object)"/> method to determine whether or not the
        /// <paramref name="value"/> is acceptable.  If <see cref="IsValid(object)"/> returns <c>false</c>, this base
        /// method will invoke the <see cref="FormatErrorMessage"/> to obtain a localized message describing
        /// the problem, and it will throw a <see cref="ValidationException"/>
        /// </remarks>
        /// <param name="value">The value to validate</param>
        /// <param name="name">The string to be included in the validation error message if <paramref name="value"/> is not valid</param>
        /// <exception cref="ValidationException"> is thrown if <see cref="IsValid(object)"/> returns <c>false</c>.
        /// </exception>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        public void Validate(object value, string name)
        {
            if (!this.IsValid(value))
            {
                throw new ValidationException(this.FormatErrorMessage(name), this, value);
            }
        }
#endif

        /// <summary>
        /// Validates the specified <paramref name="value"/> and throws <see cref="ValidationException"/> if it is not.
        /// </summary>
        /// <remarks>This method invokes the <see cref="IsValid(object, ValidationContext)"/> method 
        /// to determine whether or not the <paramref name="value"/> is acceptable given the <paramref name="validationContext"/>.
        /// If that method doesn't return <see cref="ValidationResult.Success"/>, this base method will throw
        /// a <see cref="ValidationException"/> containing the <see cref="ValidationResult"/> describing the problem.
        /// </remarks>
        /// <param name="value">The value to validate</param>
        /// <param name="validationContext">Additional context that may be used for validation.  It cannot be null.</param>
        /// <exception cref="ValidationException"> is thrown if <see cref="IsValid(object, ValidationContext)"/> 
        /// doesn't return <see cref="ValidationResult.Success"/>.
        /// </exception>
        /// <exception cref="InvalidOperationException"> is thrown if the current attribute is malformed.</exception>
        /// <exception cref="NotImplementedException"> is thrown when <see cref="IsValid(object, ValidationContext)" />
        /// has not been implemented by a derived class.
        /// </exception>
        public void Validate(object value, ValidationContext validationContext)
        {
            if (validationContext == null)
            {
                throw new ArgumentNullException("validationContext");
            }

            ValidationResult result = this.GetValidationResult(value, validationContext);

            if (result != null)
            {
                // Convenience -- if implementation did not fill in an error message,
                throw new ValidationException(result, this, value);
            }
        }

        #endregion
    }
}