Play with C# string interpolation

output.txt

SELECT * FROM blah b WHERE b.Thing = @p0
        p0=plant

SELECT * FROM anotherThing at WHERE at.Value = 'asdf'

Program.cs

using System;

namespace FormattablePlay
{
    class Program
    {
        static void Main(string[] args)
        {
            var stringVar = "plant";
            Append($"SELECT * FROM blah b WHERE b.Thing = {stringVar}");
            var rawSql = new RawSql("'asdf'");
            Append($"SELECT * FROM anotherThing at WHERE at.Value = {rawSql}");
        }

        static void Append(IFormattable query)
        {
            var formatter = new SqlFormatInfo();
            Console.WriteLine(query.ToString(null, formatter));
            foreach (var p in formatter.Parameters)
                Console.WriteLine($"\t{p.Key}={p.Value}");
            Console.WriteLine();
        }
    }
}

RawSql.cs

namespace FormattablePlay
{
    class RawSql
        : ISqlFormattable
    {
        string Sql { get; }
        public RawSql(
            string sql)
        {
            Sql = sql;
        }

        public string ToSql() => Sql;
        public override string ToString() => Sql;
    }

    internal interface ISqlFormattable
    {
        string ToSql();
    }
}

SqlFormatInfo.cs

using System;
using System.Collections.Generic;

namespace FormattablePlay
{
    class SqlFormatInfo
        : ICustomFormatter
        , IFormatProvider
    {
        Dictionary<string, object> parameters { get; } = new Dictionary<string, object>();
        public IReadOnlyDictionary<string, object> Parameters => parameters;
        int LastParameterIndex { get; set; }

        public string Format(string format, object arg, IFormatProvider formatProvider)
        {
            var sqlFormattable = arg as ISqlFormattable;
            if (sqlFormattable != null)
                return sqlFormattable.ToSql();

            var parameterName = $"p{LastParameterIndex++}";
            parameters[parameterName] = arg;
            return $"@{parameterName}";
        }

        public object GetFormat(Type formatType)
            => this;
    }
}