User info dictionary keys
These keys may exist in the user info dictionary.

NSString * const NSLocalizedDescriptionKey;
NSString * const NSErrorFailingURLStringKey;
NSString * const NSFilePathErrorKey;
NSString * const NSStringEncodingErrorKey;
NSString * const NSUnderlyingErrorKey;
NSString * const NSURLErrorKey;
NSString * const NSLocalizedFailureReasonErrorKey;
NSString * const NSLocalizedRecoverySuggestionErrorKey;
NSString * const NSLocalizedRecoveryOptionsErrorKey;
NSString * const NSRecoveryAttempterErrorKey;
NSString * const NSHelpAnchorErrorKey;
NSString * const NSURLErrorFailingURLErrorKey;
NSString * const NSURLErrorFailingURLStringErrorKey;
NSString * const NSURLErrorFailingURLPeerTrustErrorKey;
Constants
NSLocalizedDescriptionKey
The corresponding value is a localized string representation of the error that, if present, will be returned by localizedDescription.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSErrorFailingURLStringKey
The corresponding value is the URL that caused the error. This key is only present in the NSURLErrorDomain. (Deprecated. This constant is deprecated in OS X v10.6, and is superseded by NSURLErrorFailingURLStringErrorKey.)
This constant is deprecated in OS X v10.6, and is superseded by NSURLErrorFailingURLStringErrorKey. Both constants refer to the same value for backward-compatibility, but the new symbol name has a better prefix.
Available in iOS 2.0 and later.
Deprecated in iOS 4.0.
Declared in NSURLError.h.
NSFilePathErrorKey
Contains the file path of the error.
The corresponding value is an NSString object.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSStringEncodingErrorKey
The corresponding value is an NSNumber object containing the NSStringEncoding value.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSUnderlyingErrorKey
The corresponding value is an error that was encountered in an underlying implementation and caused the error that the receiver represents to occur.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSURLErrorKey
The corresponding value is an NSURL object.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSLocalizedFailureReasonErrorKey
The corresponding value is a localized string representation containing the reason for the failure that, if present, will be returned by localizedFailureReason.
This string provides a more detailed explanation of the error than the description.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSLocalizedRecoverySuggestionErrorKey
The corresponding value is a string containing the localized recovery suggestion for the error.
This string is suitable for displaying as the secondary message in an alert panel.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSLocalizedRecoveryOptionsErrorKey
The corresponding value is an array containing the localized titles of buttons appropriate for displaying in an alert panel.
The first string is the title of the right-most and default button, the second the one to the left, and so on. The recovery options should be appropriate for the recovery suggestion returned by localizedRecoverySuggestion.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSRecoveryAttempterErrorKey
The corresponding value is an object that conforms to the NSErrorRecoveryAttempting informal protocol.
The recovery attempter must be an object that can correctly interpret an index into the array returned by recoveryAttempter.
Available in iOS 2.0 and later.
Declared in NSError.h.
NSHelpAnchorErrorKey
The corresponding value is an NSString containing the localized help corresponding to the help button. See helpAnchor for more information.
Available in iOS 4.0 and later.
Declared in NSError.h.
NSURLErrorFailingURLErrorKey
The corresponding value is an NSURL containing the URL which caused a load to fail. This key is only present in the NSURLErrorDomain.
Available in iOS 4.0 and later.
Declared in NSURLError.h.
NSURLErrorFailingURLStringErrorKey
The corresponding value is an NSString object for the URL which caused a load to fail. This key is only present in the NSURLErrorDomain.
This constant supersedes NSErrorFailingURLStringKey, which was deprecated in OS X v10.6. Both constants refer to the same value for backward-compatibility, but this symbol name has a better prefix.
Available in iOS 4.0 and later.
Declared in NSURLError.h.
NSURLErrorFailingURLPeerTrustErrorKey
The corresponding value is the SecTrustRef object representing the state of a failed SSL handshake. This key is only present in the NSURLErrorDomain.
Available in iOS 3.0 and later.
Declared in NSURLError.h.