Strange behaviour during RijndaelManaged construction

Question

I've noticed the following strange behaviour in the following code, if I set the Key in an object initializer it generates a random key and doesn't set my key. Is this a glitch?

var algorithm = new RijndaelManaged
{
    Mode = CipherMode.CBC,
    Key = keyBytes,        //if i set the keyBytes here
    KeySize = _keySize,
    IV = Encoding.ASCII.GetBytes(_initVector),
    BlockSize = 128,
    Padding = PaddingMode.Zeros
}; // Set encryption mode to Cipher Block Chaining   

bool wtf= algorithm.Key.AreEqual(keyBytes);

if (!wtf) // <!-- the Key is not the same here
{
    algorithm.Key = keyBytes; // so i end up having to set it again here so that i can decrypt properly
}

Answer 1

Its not bug. Look at source code

This is Key property.

    public virtual byte[] Key {
        get { 
            if (KeyValue == null) GenerateKey();
            return (byte[]) KeyValue.Clone();
        }
        set { 
            if (value == null) throw new ArgumentNullException("value");
            Contract.EndContractBlock();
            if (!ValidKeySize(value.Length * 8))
                throw new CryptographicException(Environment.GetResourceString("Cryptography_InvalidKeySize"));

            // must convert bytes to bits
            KeyValue = (byte[]) value.Clone(); // your byte[] will be set
            KeySizeValue = value.Length * 8;   // key size will be set too
        }
    }

This is KeySize property.

public virtual int KeySize {
    get { return KeySizeValue; }
    set {
        if (!ValidKeySize(value))
            throw new CryptographicException(Environment.GetResourceString("Cryptography_InvalidKeySize"));

        KeySizeValue = value;
        KeyValue = null; // here keyvalue becomes null
    }
}

That's because you set KeySize after setting KeyValue therefore the problem you get.

I think you should not setKeySize because it will be set automatically as you can see in source code. if you set KeySize your Key becomes null by implementation for what ever reason.

var algorithm = new RijndaelManaged
        {
            Mode = CipherMode.CBC,
            Key = keyBytes,
            // KeySize = _keySize, // remove this
            IV = Encoding.ASCII.GetBytes(_initVector),
            BlockSize = 128,
            Padding = PaddingMode.Zeros
        };