Having error getting a Varchar from a DuckDBValue

[lucap-irion]

Hi Giorgi,

I’m experimenting with your .NET project, and I’m trying to add some CAPI bindings to your solution, including NativeMethods for ProfilingInfo and other related pieces.

With the CAPI profiling info, I can extract profile metrics like CPU_TIME as a DuckdbValue, which needs to be converted to Varchar. When I tried to convert the value to Varchar, I got a memory error.

Looking at the DuckDBGetVarchar definition, I noticed that the return type is not IntPtr as aspected from the

DUCKDB_C_API char *duckdb_get_varchar(duckdb_value value);

but string:

DuckDB.NET/DuckDB.NET.Bindings/NativeMethods/NativeMethods.Value.cs

Line 216 in 96e4851

public static partial string DuckDBGetVarchar(DuckDBValue value);

I changed the return type to IntPtr and updated the GetValue<T> to use your ToManagedString() on the pointer. It worked fine as expected. This is my first work with CAPI and interop, so I don’t know all the implications of this change, but after reading some blogs, I saw that it’s safer than telling the CLR to implicitly convert the pointer to string.

Do you have any experience or knowledge about this approach? Do you think it’s safe to adopt the IntPtr approach instead of the current string conversion in the library?

Thanks!

[Giorgi]

What error did you get when reading it as string?

[lucap-irion]

I’ve investigated this a bit more. My situation is that I’m not using the native duckdb.dll, but a debug-compiled one, because I’m trying to use it on top of a custom extension that extends profiling. Only when I use this custom DLL do I get the error: “Critical error detected c0000374” (a heap corruption / memory corruption error typically caused by mismatched C runtime libraries, double-free, buffer overruns, or calling free() on memory allocated by a different CRT). When I use the native duckdb.dll shipped with your project, it works without any changes. I also saw your README.md, which says to check out the native code for debugging, but it’s already checked out. Have you ever encountered issues when using a debug-compiled DuckDB?

…

________________________________ From: Giorgi Dalakishvili ***@***.***> Sent: Thursday, 2 April 2026 17:45 To: Giorgi/DuckDB.NET ***@***.***> Cc: Luca Prezzi ***@***.***>; Author ***@***.***> Subject: Re: [Giorgi/DuckDB.NET] Having error getting a Varchar from a DuckDBValue (Discussion #321) What error did you get when reading it as string? — Reply to this email directly, view it on GitHub<#321?email_source=notifications&email_token=ABLVRYTKOHEAIEWJ455YVJT4T2DLHA5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNRUGI2TEMJRUZZGKYLTN5XKMYLVORUG64VFMV3GK3TUVRTG633UMVZF6Y3MNFRWW#discussioncomment-16425211>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ABLVRYVIW5KAV77Y4CAZVN34T2DLHAVCNFSM6AAAAACXKX7YIKVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNBSGUZDCMI>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[Giorgi]

I have used DuckDB debug builds but I haven't made such changes. DuckDBGetVarchar has [return: MarshalUsing(typeof(DuckDBCallerOwnedStringMarshaller))] attirbute which calls duckdb_free on the returned pointer. I guess you are also freeing it on the DuckDB side causing double-free.

[lucap-irion]

I’ll double-check this. I also see that I’m maybe using an old version of the repo, because this attribute is missing. I need to update first, and I’ll keep you updated!

I saw the benchmark of your last build; it’s awesome :D

[lucap-irion]

Updating with your last version it worked also with my debug dll!
Thanks for your time!

And love your work!

[Giorgi]

Great! Consider sponsoring the project or sending a PR for profiling support.

[lucap-irion]

If I can make it work why not 😄