create table SomeTable
(
    id uniqueidentifier default newsequentialid(),
    foo text
)

insert into SomeTable (foo)
output inserted.id
values ('bar')

using (var conn = new SqlConnection(@"..."))
using (var cmd = conn.CreateCommand())
{
    cmd.CommandText = "insert into SomeTable (foo) output inserted.id values ('bar')";
    conn.Open();
    var guid = (Guid)cmd.ExecuteScalar();
    Console.WriteLine(guid);
}