Passing a StringBuilder to an interpolated string

[friflo]

Hi,
did not find a similar open or closed issue.

A typical use case is the creation of SQL statements. Code typically looks like this

    var sql = new StringBuilder();
    sql.Append($"INSERT INTO {name} ({col1},{col2},{col3}, ...)\nVALUES ");
    AppendRecords(sql, records);

So you have both string interpolation and appending strings or scalars manual to a StringBuilder in AppendRecords().
The string interpolation creates its own StringBuilder instance internally.
Both instances will create (to resize) char[] instances if required.

The idea is simply to enable passing the StringBuilder instance to the interpolated string.
It could look like this:

    var sql = new StringBuilder();
    $(sql)"INSERT INTO {name} ({col1},{col2},{col3}, ...)\nVALUES ";
    AppendRecords(sql, records);

As a result:

• less code
• less object instantiations.
  • -1 StringBuilder instance
  • less char[] instances

[huoyaoyuan]

A typical use case is the creation of SQL statements.

DO NOT use string builder or raw string manipulation to construct SQL. It's the most common source of SQL injection risk.

The string interpolation creates its own StringBuilder instance internally.

It doesn't. From .NET 6 and C# 10, the interpolated string won't be constructed into a string first. It's passed as raw parts to an interpolated string handler defined by the callee, if there is one. See https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/tutorials/interpolated-string-handler.

[HaloFour]

And StringBuilder does explicitly have an overload of Append which supports an interpolation handler, so:

var sql = new StringBuilder();
sql.Append($"INSERT INTO {name} ({col1},{col2},{col3}, ...)\nVALUES ");

is lowered into:

StringBuilder sql = new StringBuilder();
StringBuilder.AppendInterpolatedStringHandler handler = new StringBuilder.AppendInterpolatedStringHandler(30, 4, sql);
handler.AppendLiteral("INSERT INTO ");
handler.AppendFormatted(value);
handler.AppendLiteral(" (");
handler.AppendFormatted(value2);
handler.AppendLiteral(",");
handler.AppendFormatted(value3);
handler.AppendLiteral(",");
handler.AppendFormatted(value4);
handler.AppendLiteral(", ...)\nVALUES ");
sql.Append(ref handler);

Where the StringBuilder.AppendInterpolatedStringHandler is a struct that just delegates the calls back to the original StringBuilder.

[friflo]

Okay thx, Was not aware of this.
Creating an SQL statement was for illustration - could have been another use-case.

[alex-jitbit]

@HaloFour Wow, this is great and I've actually verified this via SharpLab here: https://sharplab.io/#v2:CYLg1APgAgTAjAWAFBQMwAJboMLoN7LpGYZQAs6AsgBQCU+hxTAbgIYBO6AzgEboC86AHYBTAO6Y4AOgAqIgB4AXKQGVF7AJZCA5gCEArhoA2wEezoBuRkyJbF6VgPRwYqK0hvFeUgIIAHPxEhYGoAEgAiLmAAMyi8VgBfcNp3JgTkBKA===

But which .NET runtime uses this? Looks like .NET 6 and above - correct?

[333fred]

Yup. You can read all about it (in lots of detail) in Stephen Toub's blog on the subject here: https://devblogs.microsoft.com/dotnet/string-interpolation-in-c-10-and-net-6/.