Difference between String, FormattableString, IFormattable

Question

FormattableString has been Introduced in C# 6.0. As we can use same string formatting using string object why is there need of using FormattableS

Answer 1

As an aside, https://www.meziantou.net/interpolated-strings-advanced-usages.htm covers some examples of what FormattableString allows you to do (e.g. auto-parameterising SQL statements)

e.g.

void ExecuteNonQuery(DbConnection connection, FormattableString formattableString)
{
    using (var command = connection.CreateCommand())
    {
        // Replace values by @p0, @p1, @p2, ....
        var args = Enumerable.Range(0, formattableString.ArgumentCount).Select(i => (object)("@p" + i)).ToArray();

        command.CommandType = System.Data.CommandType.Text;
        command.CommandText = string.Format(formattableString.Format, args);

        // Create parameters
        for (var i = 0; i < formattableString.ArgumentCount; i++)
        {
            var arg = formattableString.GetArgument(i);
            var p = command.CreateParameter();
            p.ParameterName = "@p" + i;
            p.Value = arg;
            command.Parameters.Add(p);
        }

        // Execute the command
        command.ExecuteNonQuery();
    }
}

using (var sqlConnection = new SqlConnection())
{
    sqlConnection.Open();
    ExecuteNonQuery(sqlConnection, $@"UPDATE Customers SET Name = {"Meziantou"} WHERE Id = {1}");
}