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by Jiří {x2} Činčura

Using string interpolation for ADO.NET parameters

16 Aug 2017 3 mins C#, SQL

I was reading the announcement about Entity Framework Core 2 the other morning and saw “String interpolation in raw SQL methods” paragraph. This was already in preview(s) and I kind of took it as granted, not thinking about how it’s done or what it is doing. Not this time. Somehow my curiosity kicked in and I decided to create same feature and maybe do something with it.

Of course, I could peek into sources, but I decided to use the idea, but build it myself from scratch. I like the idea of using already existing feature from C# and then using for different, but reasonable, purpose. String interpolation and SQL parameters are exactly this.

The string interpolation feature works on top of FormattableString class. Basically whenever there’s a $"whatever" it’s an instance of FormattableString class. For my testing let’s create a method with this signature IEnumerable<object[]> Query(FormattableString sql, DbConnection connection, DbTransaction transaction = null). So I can pass any string, with parameters as interpolated string, connection and optionally transaction and I want this to be safely executed with proper parameters.

public static IEnumerable<object[]> Query(FormattableString sql, DbConnection connection, DbTransaction transaction = null)
	using (var cmd = connection.CreateCommand())
		if (transaction != null)
			cmd.Transaction = transaction;
		var parameters = sql.GetArguments().Select((e, i) =>
			var p = cmd.CreateParameter();
			p.ParameterName = $"@p{i}";
			p.Value = e;
			return p;
		cmd.CommandText = string.Format(sql.Format, parameters.Select(x => x.ParameterName).ToArray());
		using (var reader = cmd.ExecuteReader())
			while (reader.Read())
				var values = new object[reader.FieldCount];
				yield return values;

There’s some classic ADO.NET orchestration around and will skip this focusing only on FormattableString. I’m using GetArguments to get all the values passed for the interpolated string. These will become the parameters’ values. From these I construct regular DbParameters with name @pN. Remaining piece is to replace the {N} in the string – same as in good old string.Format days one would write manually – with proper values. In this case parameter names. Given the Format property already contains a string ready to be passed to, already mentioned, string.Format, it’s just about passing parameter names and it’s done.

The usage feels very natural to me. It fits the language. And it’s also the reason why I did this small brain training. Have a look yourself.

using (var conn = new FbConnection("database=localhost:test.fdb;user=sysdba;password=masterkey"))
	var id = -1;
	var id2 = -10;
	foreach (var item in Database.Query($"select * from mon$attachments where mon$attachment_id <> {id2} and mon$attachment_id <> {id}", conn))
		Console.WriteLine(string.Join("|", item));

This can be extended to almost make a raw SQL a first-ish class citizen in C# I think. While I was thinking about pursuing it a little bit more, maybe like a super thin wrapper-type ORM, I stopped for a while and did some research, because I’m surely not the first one to have this idea. And I’ve found FormattableSql project, that looks 95%-like I imagined where I’d like to end up.

Profile Picture Jiří Činčura is .NET, C# and Firebird expert. He focuses on data and business layers, language constructs, parallelism, databases and performance. For almost two decades he contributes to open-source, i.e. FirebirdClient. He works as a senior software engineer for Microsoft. Frequent speaker and blogger at