CREATE PROCEDURE [dbo].[GetUSDRate]
  @Date smalldatetime,
  @Rate decimal( 18 ,  4 ) output
AS
begin
  set nocount on

  select @Rate =  0 
  select @Rate = RateUSD from RateUsd
  where Data = @Date
end
GO

public decimal wmGetUSDRate(DateTime Date)
{
    SqlCommand Cmd = new SqlCommand();
    Cmd.CommandText =  "GetUSDRate" ;
    Cmd.CommandType = CommandType.StoredProcedure;
    Cmd.Connection = sqlConnection2;
    Cmd.Parameters.Clear();
    Cmd.Parameters.Add( "@Date" , SqlDbType.SmallDateTime);
    Cmd.Parameters.Add( "@Rate" , SqlDbType.Decimal);
    Cmd.Parameters[ "@Rate" ].Direction = ParameterDirection.Output;
    //Cmd.Parameters[ "@Rate" ].Direction = ParameterDirection.InputOutput;//(вместо верхней строки вот эту)
    Cmd.Parameters[ "@Date" ].Value = Date;
    //Cmd.Parameters[ "@Rate" ].Value =  0 . 1111 

    if (this.sqlConnection2.State != ConnectionState.Open)
      this.sqlConnection2.Open();
    Cmd.ExecuteNonQuery();
    this.sqlConnection2.Close();
    return Convert.ToDecimal(Cmd.Parameters[ "@Rate" ].Value);
}