In Postgres type casting is performed via value::type. When calling ExecuteAsync or QueryAsync with parameters Dapper (sometimes) interprets the : as parameter key and as a consequence throws an System.ArgumentNullException.

This is not always the case and guessing from the type name this only occurs when the type name (the casting type) is a partial match with parameters passed in to the method.

Here is a snippet of the code which is throwing an exception.

var sql = @"
    INSERT INTO
        public.table (id, title, status)
    VALUES
        (
            @Id,
            @TitleText,
            (@StatusText)::ad_item_status
        )";

using (var connection = new NpgsqlConnection())
{
    await connection.ExecuteAsync(new CommandDefinition(sql, item, cancellationToken: token));
}

The item does contain a property with the name 'item.Status' which may be related to the problem. As far as I can see there is not an option to disable to ; and ? parameter prefixes from being interpreted.

Stacktrace

at System.Reflection.Emit.DynamicILGenerator.Emit(OpCode opcode, MethodInfo meth)
at Dapper.SqlMapper.CreateParamInfoGenerator(Identity identity, Boolean checkForDuplicates, Boolean removeUnused, IList`1 literals) in C:\projects\dapper\Dapper\SqlMapper.cs:line 2575
at Dapper.SqlMapper.GetCacheInfo(Identity identity, Object exampleParameters, Boolean addToCache) in C:\projects\dapper\Dapper\SqlMapper.cs:line 1707
at Dapper.SqlMapper.ExecuteMultiImplAsync(IDbConnection cnn, CommandDefinition command, IEnumerable multiExec) in C:\projects\dapper\Dapper\SqlMapper.Async.cs:line 646

The workaround is straightforward as Postgres also supports the CAST ('value' AS type) syntax, but this is much harder to read. FWIW: The casting does work since I cast regular to JSON via value::jsonb without any issues.

PS. I could not find any similar issues with NpgSQL in Dapper.