```
    public string Generate()
    {
        RandomNumberGenerator rdng = new RNGCryptoServiceProvider();
        byte[] bytes = new byte[40];

        rdng.GetBytes(bytes);

        string a = Convert.ToBase64String(bytes); 

        a.Replace("=", "r");
        a.Replace("+", "t");

        return a;
    }

a=43TvtRvrTrt54g5gtbrTTBR45iu+zqbB03gXej== ;

```

after `a.Replace()` again:

```
a=43TvtRvrTrt54g5gtbrTTBR45iu+zqbB03gXej== ;

```

I want to generate unique value in my `C#` project.
Everything is great.
But When I want to call `string.Replace()`, it doesn't work.
Values after and before this method are the same.
Why ?