var cs = new MsSqlConnectionStringBuilder
        { User = "dmitry", Password = "Password!" }
        .Build();

        Console.WriteLine(cs); // Password=Password!;User=dmitry;

        cs = cs.With("Provider", "MSSQL");
        Console.WriteLine(cs); // Provider=MSSQL;Password=Password!;User=dmitry;
        Console.WriteLine(cs["Provider"]); // MSSQL

        cs = cs.Excerpt("Provider"); // Password=Password!;User=dmitry;
        Console.WriteLine(cs);

public class MsSqlConnectionStringBuilder
{
    public string User { get; set; } = "sa";
    public string Password { get; set; }

    public ConnectionString Build() =>
        ConnectionString.Empty
            .With("User", User ?? throw new InvalidOperationException())
            .With("Password", Password ?? throw new InvalidOperationException());
}

public sealed class ConnectionString
{
    public static explicit operator ConnectionString(string text) => Parse(text);
    public static implicit operator string(ConnectionString connectionString) => connectionString.ToString();

    public static readonly ConnectionString Empty = new ConnectionString(ConnectionParameter.None);

    public static ConnectionString Parse(string text, string regex = "(?<name>[^=]+)=(?<value>[^;]);?") =>
        new ConnectionString(
            new Regex(regex ?? throw new ArgumentNullException(nameof(regex)))
                .Matches(text ?? throw new ArgumentNullException(nameof(text)))
                .Select(m => new ConnectionParameter(m.Groups["name"].Value, m.Groups["value"].Value)));

    ConnectionString(IEnumerable<ConnectionParameter> parameters)
    {
        Parameters = parameters.ToArray();
    }

    IEnumerable<ConnectionParameter> Parameters { get; }

    public string this[string name] => Parameters
        .FirstOrDefault(p => p.Name.ToUpper() == name.ToUpper())?.Value;

    public ConnectionString With(string name, object value) =>
        new ConnectionString(Parameters
            .Prepend(new ConnectionParameter(name, $"{value}")));

    public ConnectionString Excerpt(string name) =>
        new ConnectionString(Enumerable.Concat(
            Parameters.TakeWhile(p => p.Name.ToUpper() != name.ToUpper()),
            Parameters.SkipWhile(p => p.Name.ToUpper() != name.ToUpper()).Skip(1)));

    public override string ToString() => ToString("{0}={1};");

    public string ToString(string format) =>
        string.Join("", from p in Parameters
                        select string.Format(format, p.Name, p.Value));
}

sealed class ConnectionParameter
{
    public static readonly ConnectionParameter[] None = new ConnectionParameter[0];

    internal ConnectionParameter(string name, string value)
    {
        Name = name ?? throw new ArgumentNullException(nameof(name));
        Value = value ?? throw new ArgumentNullException(nameof(value));
    }

    public string Name { get; }
    public string Value { get; }
}