DataSet 这里似乎杀伤力过大；我会用一个简单的 MySqlDataReader . (另外，请使用参数来防止sql注入。）

string nom = null, prenom = null;
using (var cmd = connection.CreateCommand())
{
    // create the command and bind the parameter; this prevents SQL injection
    cmd.CommandText = "SELECT nom, prenom from liste_personnels where mail = @mail";
    cmd.Parameters.AddWithValue("@mail", mailtest);
    using (var reader = cmd.ExecuteReader())
    {
        // check if a row was found in the DB; you may need to handle if it's false
        if (reader.Read())
        {
            // read the data
            nom = reader.GetString(0);
            prenom = reader.GetString(1);
        }
    }
}

最后，如果您可以向项目中添加另一个库，我将使用dapper简化上述代码：

var result = connection.Query(
        "SELECT nom, prenom from liste_personnels where mail = @mailtest",
        new { mailtest })
    .SingleOrDefault();
var nom = (string) result?.nom;
var prenom = (string) result?.prenom;