Почему read only?

KeyValuePair<string, int>[] a = new KeyValuePair<string, int>[5];
a.Append(new KeyValuePair<string, int>("a", 0 ));
a[0].Value = 0;

ff[vv] = new KeyValuePair<string, int>(ff[vv].Key, ff[vv].Value + 1);

KeyValuePair<string, int>[] ff = Freq.OrderByDescending(x => x.Value).ToArray();

Dictionary<string, int> Freq = new Dictionary<string, int>();

struct Pair
{
    string Key;
    int Value;
}
 
Pair[] p = new Pair[Freq.Count()];
int count = 0;
ForEach (var f in Freq)
{
    p[count].Key = f.Key;
    p[count++].Value = f.Value;
}

(string key, int count)[] a = new (string key, int count)[5];

public class KeyCounPair
{
    public string Key {get;}
    public int Count {get; set;}
    public KeyCounPair(string key)
       => Key = key;
}

KeyCounPair[] a = new KeyCounPair[5];
a.Append(newKeyCounPair("a"));
a[0].Count ++;

KeyPair[] ff = new KeyPair[Freq.Count()];
foreach (var f in Freq)
{
    ff.Append();
}

ff.Append(f.Key);

KeyCounPair[] ff = Freq.Select(pair => new KeyCounPair(pair.Key){Count = pair.Value})
                               .ToArray();

BinaryFormatter fmt = new BinaryFormatter();
using (FileStream fs = new FileStream(String.Format(@"c:\work\words3\ff{0}.dat", tb), FileMode.OpenOrCreate))
{
    fmt.Serialize(fs, ff);
}

BinaryFormatter frm = new BinaryFormatter();
using (FileStream fs = new FileStream(@"c:\work\words3\ff8.dat", FileMode.OpenOrCreate))
{
    ff1 = (KeyPair[])formatter.Deserialize(fs);
}

KeyPair[] ff1 = (KeyPair[])frm.Deserialize(fs);

public class KeyPairArray 
{
    public KeyPair[] KeyPairs {get; set;}
}

BinaryFormatter fmt = new BinaryFormatter();
using (FileStream fs = new File.Create(String.Format(@"c:\work\words3\ff{0}.dat", tb)))
{
    fmt.Serialize(fs, new KeyPairArray(){ KeyPairs = ff});
}

BinaryFormatter frm = new BinaryFormatter();
using (FileStream fs = new FileOpen(@"c:\work\words3\ff8.dat"))
{
    ff1 = ((KeyPairArray)formatter.Deserialize(fs)).KeyPairs;
}

        [Serializable]
        public class KeyPair
        {
            public string Key { get; }
            public int Value { get; set; }
            public KeyPair(string key)
               => Key = key;
        }

    [Serializable]
    public class KeyPair
    {
        public string Key { get; }
        public int Value { get; set; }
        public KeyPair(string key)
           => Key = key;
    }
 
    [Serializable]
    public class KeyPairsArray
    {
        public KeyPair[] KeyPairs { get; set; }
    }

    {
 
        KeyPair[] pairs =
        {
            new KeyPair("Один"){Value=123},
            new KeyPair("Два"){Value=456},
            new KeyPair("Три"){Value=789},
            new KeyPair("Четыре"){Value=102},
            new KeyPair("Пять"){Value=345}
        };
 
        BinaryFormatter formatter = new BinaryFormatter();
 
        using (var file = File.Create("KeyPairs.bin"))
            formatter.Serialize(file, new KeyPairsArray() { KeyPairs = pairs });
 
        using (var file = File.OpenRead("KeyPairs.bin"))
            pairs = ((KeyPairsArray) formatter.Deserialize(file)).KeyPairs;
 
    }