Weird System.DBNull behaviour on nullable columns

[mhkolk]

When dealing with nullable columns in Postgres database and those columns actually holding null values (for instance, a nullable numeric column) when reading data with NpgsqlDataReader, the values will be shown as an object (of type System.DBNull) in debugger with a value depicted as {} in Watch window.

This empty JSON string of {} will eventually and surprisingly end up downstream in calling methods when a data row is being returned as a result or if datarow is turned into a Dictionary<string, object?>.

I would expect System.DBNull to be turned into null immediately or when casting to nullable object but it doesn't happen.

We ran into this because in the end we ended with empty JSON string{}in responses from our API in places where a null should be present in that JSON.

Is the issue in any way related to this? dotnet/runtime#418 (there is no reference in my sample project - see below - to System.Text.Json)
I find this hard to believe since when at ADO level serializers shouldn't be involved, or should they?

[I published a repo with tests using TestContainers which will spin up in Docker and you can inspect/debug it.]
(https://github.com/mhkolk/NpgsqlTests)

This is the test you might want to inspect
https://github.com/mhkolk/NpgsqlTests/blob/main/NpgsqlTestProject/PostgresContainerTests.cs#L48

This is using .Net8, EF8, Npgsql 8..

[RobertGiesecke]

I am not a maintainer, but this is perfectly normal ADO.Net behavior. If they behaved differently then they would be breaking 25 year old conventions and a lot of code. In ADO.Net, you are meant to check a reader column with IsDbNull and only retrieve it when that’s false. DBNull was added so that you can distinguish between actually passing or getting null to/from the database vs accidentally leaving the default value null.

…

On Mon 15. Dec 2025 at 13:07 mhkolk ***@***.***> wrote: *mhkolk* created an issue (npgsql/npgsql#6379) <#6379> When dealing with nullable columns in Postgres database and those columns actually holding null values (for instance, a nullable numeric column) when reading data with NpgsqlDataReader, the values will be shown as an object (of type System.DBNull) in debugger with a value depicted as {} in Watch window. This empty JSON string of {} will eventually and surprisingly end up downstream in calling methods when a data row is being returned as a result or if datarow is turned into a Dictionary<string, object?>. I would expect System.DBNull to be turned into null immediately or when casting to nullable object but it doesn't happen. We ran into this because in the end we ended with empty JSON string{}in responses from our API in places where a null should be present in that JSON. Is this in any way related to this? I find this hard to believe since when at ADO level serializers shouldn't be involved, or should they? [I published a repo with tests using TestContainers which will spin up in Docker and you can inspect/debug it.] (https://github.com/mhkolk/NpgsqlTests) This is using .Net8, EF8, Npgsql 8.. — Reply to this email directly, view it on GitHub <#6379>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAF65UYXX4OMRZBXCUGX7YL4B2P67AVCNFSM6AAAAACPCEBQICVHI2DSMVQWIX3LMV43ASLTON2WKOZTG4ZTAMJSGAZDIMA> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[mhkolk]

The issue is not the presence of DBNull (that's perfectly fine), the issue is why the value would be retrieved as an empty JSON string by default.

[roji]

@mhkolk nothing is being retrieved as an empty JSON string: Npgsql returns DBNull.Value to represent null values, as is standard in .NET database drivers. Now, if you visualize DBNull in a debugger (or call ToString()) on it, then you'll indeed see an empty string; this is because you've specifically asked the value to be converted to a string, and that's the DBNull convert-to-string behavior.

As written above, you can easily know whether NULL is returned from the database either by calling NpgsqlDataReader.IsDBNull(), or simply by checking if the return value is DBNull, before converting it to a string; you should not be calling ToString() on arbitrary objects returned from e.g. NpgsqlDataReader.GetValue() - that's generally quite brittle.

In any case, Npgsql seems to be behaving as expected here. In .NET database drivers (ADO.NET/System.Data), the standard representation of database NULL is DBNull and not null - the latter is reserved for cases where no value existed at all (e.g. no rows matched the query).

The question of how DBNull converts to string has nothing to do with Npgsql - DBNull is part of .NET itself, and Npgsql is merely returning it. You could argue that DBNull.ToString() should have returned null rather than an empty string, but ToString() is generally expected to return a non-null string representation - even if ToString() is annotated to possibly return null in the general case - since that's simply more useful.

I'll go ahead and close this as I don't see any issue with Npgsql, but if I've misunderstood or you have additional arguments, feel free to post back here.

[mhkolk]

Empty string sure but not empty JSON string.. it's in the sample repo but no one cares to check it out

[roji]

I did look, but I can't reproduce {} appearing anywhere. Calling ToString() on DBNull returns an empty string, as clearly shown below:

var dbnull = (DBNull)rowData["age"]!;

var asString = dbnull.ToString();
Console.WriteLine(asString == ""); // prints true

So I'm not sure exactly what behavior you're referring to here. I've converted your repro into a simple console program below, feel free to tweak that and post back a modification that shows the exact problem you're trying to report.

Attempted repro as

await using var dataSource = NpgsqlDataSource.Create("Host=localhost;Username=test;Password=test");
await using var connection = await dataSource.OpenConnectionAsync();

await using (var createCommand = connection.CreateCommand())
{
    createCommand.CommandText = @"
        CREATE TABLE IF NOT EXISTS test_users (
            id SERIAL PRIMARY KEY,
            name VARCHAR(100) NOT NULL,
            email VARCHAR(100) NOT NULL,
            age NUMERIC,
            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
        )";
    await createCommand.ExecuteNonQueryAsync();
}

await using (var insertCommand = connection.CreateCommand())
{
    insertCommand.CommandText = @"
        INSERT INTO test_users (name, email)
        VALUES (@name, @email)";
    insertCommand.Parameters.AddWithValue("name", "John Doe");
    insertCommand.Parameters.AddWithValue("email", "john@example.com");
    await insertCommand.ExecuteNonQueryAsync();
}

var rowData = new Dictionary<string, object?>();

await using (var selectCmd = connection.CreateCommand())
{
    selectCmd.CommandText = "SELECT * FROM test_users";
    using (var reader = await selectCmd.ExecuteReaderAsync())
    {
        while (await reader.ReadAsync())
        {
            for (int i = 0; i < reader.FieldCount; i++)
            {
                rowData[reader.GetName(i)] = reader.GetValue(i);
            }
        }
    }
}

var dbnull = (DBNull)rowData["age"]!;

var asString = dbnull.ToString();
Console.WriteLine(asString == ""); // prints true