Make C#

Can somebody help me convert this SQL2000 Function TO C# Function please

CREATE Function dbo.CalculateNextRentDate ( @.Rent_Payment_Date datetime, @.Frequency varchar(50) , @.Number int)

RETURNS DATETIME

AS
begin

DECLARE @.NextPaymentDate Datetime
SET @.Number = @.Number - 1
.
IF @.Frequency = 'month'
IF @.rent_payment_date = dateadd(month, datediff(month, 0+@.Number, @.rent_payment_date) + 1, -1)
BEGIN
SET @.NextPaymentDate = dateadd(month, datediff(month, 0, @.rent_payment_date) + 2, -1)
END
ELSE
BEGIN
SET @.NextPaymentDate = dateadd(month, 1+@.Number, @.rent_payment_date)
END

return @.NextPaymentDate
end

I can't understand why in the T-SQL function it first checks whether the @.Rent_Payment_Date is the last day of the month, so I just remove the check and here is the code:

public DateTime CalculateNextRentDate(DateTime Rent_Payment_Date, string Frequency, int Number)
{
DateTime NextPaymentDate;
NextPaymentDate = Rent_Payment_Date;
Number--;
if (Frequency == "month")
{
NextPaymentDate = Rent_Payment_Date.AddMonths(Number);
}
return NextPaymentDate;
}