Help me..calling a scalar function in sql server within c#?

Question

i have a scalar function "[finance_dep].[sum_view1]" in sql server :

ALTER function [finance_dep].[sum_view1] (@dt1 datetime, @dt2 datetime )
returns decimal
as begin
Declare @d decimal
set @d=(select sum([amount_dnr]) from [finance_dep].[view1] where [wsl_date] between @dt1 and @dt2)
return @d
end

when i calling it from c#:

public static decimal sum_v(DateTime @dt1, DateTime @dt2)
{
return exscalarfunction("select cast(finance_dep.sum_view1(@dt1,@dt2) as decimal) ", CommandType.Text,
CreateParam("@dt1", SqlDbType.DateTime, @dt1),
CreateParam("@dt2", SqlDbType.DateTime, @dt2));
}
protected static decimal exscalarfunction(string querystoredprocedure, CommandType type, params SqlParameter[] param)
{
cn.Open();
//*******************
decimal f;
cmd = new SqlCommand(querystoredprocedure, cn);
cmd.CommandType = type;
cmd.Parameters.AddRange(param);
//********************
f = (decimal) cmd.ExecuteScalar();// here appear error with message ()
//***********************************
cn.Close();
return f;
}