How to delete all keychain items accessible to an app?

Do it for all classes

Objective-C:

NSArray *secItemClasses = @[(__bridge id)kSecClassGenericPassword,
                       (__bridge id)kSecClassInternetPassword,
                       (__bridge id)kSecClassCertificate,
                       (__bridge id)kSecClassKey,
                       (__bridge id)kSecClassIdentity];
for (id secItemClass in secItemClasses) {
    NSDictionary *spec = @{(__bridge id)kSecClass: secItemClass};
    SecItemDelete((__bridge CFDictionaryRef)spec);
}

Swift:

let secItemClasses = [kSecClassGenericPassword, kSecClassInternetPassword, kSecClassCertificate, kSecClassKey, kSecClassIdentity]
for itemClass in secItemClasses {
    let spec: NSDictionary = [kSecClass: itemClass]
    SecItemDelete(spec)
}

Xamarin iOS version (MonoTouch) of accepted answer on How to delete all keychain items accessible to an app is below:

foreach (var recordKind in new []{
                SecKind.GenericPassword,
                SecKind.Certificate,
                SecKind.Identity,
                SecKind.InternetPassword,
                SecKind.Key,
            })
    {
          SecRecord query = new SecRecord(recordKind);
          SecKeyChain.Remove(query);
    }

If you want to make sure you indeed delete the records, you may during development check number of items in KeyChain of specific kind before and after with this code:

SecStatusCode scc;
var records = SecKeyChain.QueryAsRecord(new SecRecord(SecKind.GenericPassword), 1000, out scc);

I rewrote Daij-Djan's answer in Swift:

let secItemClasses = [kSecClassGenericPassword,
    kSecClassInternetPassword,
    kSecClassCertificate,
    kSecClassKey,
    kSecClassIdentity]
for secItemClass in secItemClasses {
    let dictionary = [kSecClass as String:secItemClass]
    SecItemDelete(dictionary as CFDictionary)
}