if (exists (select * from sys.objects where name = 'Query_Login'))
    drop proc Query_Login
go
create proc Query_Login
(
    @name nvarchar(20),
    @password nvarchar(20),
    @isAvailable varchar(5) out,
    @num int out
)
as
    select @num=COUNT(*) from Users u,[login] l
    where u.name = @name and l.[passWord] = @password and
    u.id=l.userID;
    select @isAvailable = IsAvailable from [login] 
    where userID = (select id from Users where name = @name)
    --print @name+'  '+@password
go

public int DeleteMX(string X1,string X2,out string X3, out string strMsg)
        {
            SqlParameter[] parameters = {
                    new SqlParameter("@name", SqlDbType.nvarchar,20),
                    new SqlParameter("@password", SqlDbType.nvarchar,20),
                    new SqlParameter("@isAvailable", SqlDbType.varchar,5),
                    new SqlParameter("@num", SqlDbType.Int)
                };
            parameters[0].Value = X1;
            parameters[1].Value = X2;
            parameters[2].Direction = ParameterDirection.Output;
            parameters[3].Direction = ParameterDirection.Output;

            int flag;
            flag = DbHelperSQL.RunProcedureTran("Query_Login", parameters);//调用存储过程

            X3 = Convert.ToString(parameters[2].Value);//获取返回的参数isAvailable

            strMsg = Convert.ToString(parameters[3].Value);//获取返回的参数num

            return flag;
        }

------解决方案--------------------
4楼已经给你写了例子了。
返回来的不再是sql语句是数据
strMsg = Convert.ToString(parameters[3].Value);//获取返回的参数num

------解决方案--------------------
if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[存储过程名]') and OBJECTPROPERTY(id, N'IsProcedure') = 1)
这个才是判断存储过程是否存在，你那是个bug


out 是地址引用， string X3 , string Msg的会随着处理改变，你传入的就是要输出的。

------解决方案--------------------
就是判断当前登录用户是否有效，返回是否有效值，以及条数