Code to mark a SQL string before it's passed to Dapper.

ExampleUsage.cs

public static List<T> Query<T>(this DataContext db, string sql, object param = null, int? commandTimeout = null, IDbTransaction transaction = null, [CallerFilePath]string fromFile = null, [CallerLineNumber]int onLine = 0, string comment = null)
{
    using (db.Connection.EnsureOpen())
    {
        try
        {
            return db.Connection.Query<T>(MarkSqlString(sql, fromFile, onLine, comment), param, transaction ?? db.Transaction, true, commandTimeout).AsDapperList();
        }
        catch (SqlException ex) when (ex.Is(SqlErrorCode.DatabaseReadOnly_3906))
        {
            HandleReadOnlyException(db, ex);
            return EmptyDapperList<T>();
        }
    }
}

MarkSqlString.cs

private static readonly ConcurrentDictionary<MarkedSqlStringKey, string> MarkedSql = new ConcurrentDictionary<MarkedSqlStringKey, string>();

private struct MarkedSqlStringKey : IEquatable<MarkedSqlStringKey>
{
    public string Sql { get; }
    public string Path { get; }
    public int LineNumber { get; }
    public string Comment { get; }
    public MarkedSqlStringKey(string sql, string path, int lineNumber, string comment)
    {
        Sql = sql;
        Path = path;
        LineNumber = lineNumber;
        Comment = comment;
    }
    public bool Equals(MarkedSqlStringKey other)
    {
        return 
            string.Equals(Sql, other.Sql) && 
            string.Equals(Path, other.Path) && 
            LineNumber == other.LineNumber && 
            string.Equals(Comment, other.Comment);
    }
    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        return obj is MarkedSqlStringKey && Equals((MarkedSqlStringKey) obj);
    }
    public override int GetHashCode()
    {
        unchecked
        {
            var hashCode = Sql?.GetHashCode() ?? 0;
            hashCode = (hashCode*397) ^ (Path?.GetHashCode() ?? 0);
            hashCode = (hashCode*397) ^ LineNumber;
            hashCode = (hashCode*397) ^ (Comment?.GetHashCode() ?? 0);
            return hashCode;
        }
    }
}

/// <summary>
/// Takes a SQL query, and inserts the path and line in as a comment.
/// </summary>
private static string MarkSqlString(string sql, string path, int lineNumber, string comment)
{
    if (path.IsNullOrEmpty() || lineNumber == 0) return sql;
    var key = new MarkedSqlStringKey(sql, path, lineNumber, comment);
    // Have we seen this before???
    string output;
    if (MarkedSql.TryGetValue(key, out output)) return output;
    // nope
    var commentWrap = " ";
    var i = sql.IndexOf(Environment.NewLine);
    // if we didn't find \n, or it was the very end, go to the first space method
    if (i < 0 || i == sql.Length - 1)
    {
        i = sql.IndexOf(' ');
        commentWrap = Environment.NewLine;
    }
    if (i < 0) return sql;
    // Grab one directory and the file name worth of the path
    //   this dodges problems with the build server using temp dirs
    //   but also gives us enough info to uniquely identify a queries location
    var split = path.LastIndexOf('\\') - 1;
    if (split < 0) return sql;
    split = path.LastIndexOf('\\', split);
    if (split < 0) return sql;
    split++; // just for Craver
    var sqlComment = " /* " + path.Substring(split) + "@" + lineNumber.ToString() + (comment.HasValue() ? " - " + comment : "") + " */" + commentWrap;
    var ret = sql.Substring(0, i) + sqlComment + sql.Substring(i);
    // Cache, don't allocate all this pass again
    MarkedSql[key] = ret;
    return ret;
}