AES file encryption with .Net Framework and decrypt it with IOS

Most likely the problem is in the deriving actual key from the password (12345678 cannot be the AES key directly - it is only 8 bytes).

Technically this should work though I've never tested it, both methods uses the same ad-hoc format.

Encrypt using my authenticated encryption example.

//use your secret data you want to encrypt instead.
String secretMessage = "Message";

var rnCryptorHeader = new Byte[]{
                            2, //RNCryptor Format version 2
                            0  //RNCryptor Uses password
                        };

//encryptedString is base64 encoded
var encryptedString = AESThenHMAC.SimpleEncryptWithPassword(secretMessage, 
                                                            password:"1234567891011",      
                                                            nonSecretPayload:rnCryptorHeader);

Then Decrypt using RNCryptor and NSData+Base64 for IOS

//This is the encrypted data passed from .net
NSString *encryptedString = @"AgE8C9E7gsfyOAmSotIOgyLQ0O6mdcuMXXjN/iZa3azym4KVWZAkfykIP6mqMt/qkpfftdB3XQhMkoxtQEM+rA0iHxOvZiNlmA2KJtg6BOnmlg==";

NSData *encryptedData = [NSData dataFromBase64String: encryptedString];
NSError *error;
NSData *decryptedData = [RNDecryptor decryptData:encryptedData
                                    withPassword:@"1234567891011"
                                           error:&error];
NSString *secretMessage = [[[NSString alloc] initWithData:decryptedData
                                                 encoding:NSUTF8StringEncoding] autorelease];

Since you aren't dealing with strings and are dealing with bytes directly, just remove the Base64 and utf8 encoding/decoding from this objective-c example and the linked c# example, once you are sure this is working.