SQLTransaction in .NET

Question

I begin transaction as here

this.cmd.Transaction = (await this.cmd.Connection.BeginTransactionAsync(IsolationLevel.Snapshot)) as SqlTransaction;

Then i tried to Commit transaction using

cmd.CommandText = "COMMIT TRAN";
await cmd.ExecuteNonQueryAsync();

The program throws an exeption "Transaction count after EXECUTE indicates a mismatching number of BEGIN and COMMIT statements. Previous count = 1, current count = 0" What is the problem, and why i can't commit transaction such this?

With

cmd.Transaction.CommitAsync()

it works.