How to return output parameter from sp and catch/handle in C#?

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ASP.NET

SQL

C#3.5

I have created sp in sql server 2012.Sp have output para of type varchar(15).

create procedure [dbo].[Proc_Authenticate]

    (
           @username int,   
           @password varchar(25),
           @Error varchar(15) output
    )

AS

    BEGIN 

        Declare @count int

        set @count = (select count(*) from loginmaster where userid = @userid and password = @password)

        if(@count = 0)

        begin
            Set @Error = 'Error1'
        end

    END

when I run sp from sql,it is working properly but when I using/calling sp from C#,with the help of executescalar,not working as expected.Executescalar always return null.If the same same thing I do with ExecuteNonquery,it is working properly.Executenonquery is used for insertion/updation.Exeutenonquery return number of row affected.i.e is int value. Int rowaffected = cmd.ExecuteNonquery() Normally I check if rowaffected is greater than 0,take value output para.

if(rowaffected  > 0)
{
  string s = sqlparameterError.value.Tostring()
}

Here How I check,rowaffected is always -1.Is it right way to check it?

What I have tried:

create procedure [dbo].[Proc_Authenticate]

    (
           @username int,   
           @password varchar(25),
           @Error varchar(15) output
    )

AS

    BEGIN 

        Declare @count int

        set @count = (select count(*) from loginmaster where userid = @userid and password = @password)

        if(@count = 0)

        begin
            Set @Error = 'Error1'
        end

    END