Mapping of the sbyte C# type is invalid for Npgsql / Postgres

If you use a sbyte type in a model, it is mapped to DbType.SByte.
Please check https://github.com/DapperLib/Dapper/blob/b272cc664d933b4b65703d26a79272d549576dff/Dapper/SqlMapper.cs#L173
However, Npgsql for Postgres doesn't seem to support DbType.SByte (An error is thrown that indicates the issue), but DbType.Int16 only. I'm not sure if there has been a recent change in the behaviour in Dapper or if it's a bug of Npgsql though.

The quick hack is to set the dynamic parameters in your query to cast to short like this (Just dummy code, Position is the sbyte property):

await using var connection = await this.GetDatabaseConnection().ConfigureAwait(false);

var sqlParams = new DynamicParameters(new
{
    position = (short)node.Position,
    name = node.Name
});

await connection.ExecuteAsync("UPDATE nodes SET ...", sqlParams).ConfigureAwait(false);

Can you give me information if this is a bug in Dapper or in Npgsql, please?

Version information:

• Dapper: Latest, 2.0.123.
• Npgsql: Latest, 6.0.2.

Start with SqlMapper.cs at the linked line around 173 and reproduce the sbyte parameter case using Dapper 2.0.123 and Npgsql 6.0.2. Compare the DbType.SByte and DbType.Int16 behavior, then establish whether the incompatibility belongs to Dapper or Npgsql and document the result or required change.