在C＃中将Class初始化为变量

Question

I couldn't understand this code. Why i need to initialize a class as variable Like "private InvoiceMaster _invoiceMaster" And "InvoiceMaster im = new InvoiceMaster()". Please Help me in details. I need to very clear understand.

  namespace AHS.Invoice.UI
 {
  public partial class ReRouteDetail : BasePage
  {
    #region Declaration
    private InvoiceMaster _invoiceMaster;
    DataSet _ds = new DataSet();
    InvoiceMasterCollection _invoiceMasterCollection = new InvoiceMasterCollection();

    #endregion

    #region Page Events
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        if (!IsPostBack)
        {

        }
        this.LoadColumns();
        this.LoadGridData();
    }
    #endregion  

    #region Methods


    private void LoadGridData()
    {

        if (base.CurrentScreen.ID == 3780)
        {
            _ds = new InvoiceMasterCollection().LoadReRouteData();
            gc.GridDataSource = _ds;
            gc.GridDataBind();
        }
        else if (base.CurrentScreen.ID == 3781)
        {
            _ds = new InvoiceMasterCollection().LoadReRouteFromServiceApproverData();
            gc.GridDataSource = _ds;
            gc.GridDataBind();
        }
        else if (base.CurrentScreen.ID == 3782)
        {
            _ds = new InvoiceMasterCollection().LoadReRouteFromServiceConfirmationData();
            gc.GridDataSource = _ds;
            gc.GridDataBind();
        }
    }

    #endregion


    #region Events
    protected void gc_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {

                DropDownList ddlStatus = e.Row.Cells[7].FindControl("ddlStatus") as DropDownList;
                ddlStatus.CssClass = "ReRouteddlStatus";
                if (base.CurrentScreen.ID == 3780)
                {
                    DataSet reRouteDataSet = new InvoiceMasterCollection().LoadStatus(Convert.ToInt32(e.Row.Cells[4].Text));
                    ddlStatus.DataTextField = "Description";
                    ddlStatus.DataValueField = "ID";
                    ddlStatus.DataSource = reRouteDataSet;
                    ddlStatus.DataBind();
                }

                if (base.CurrentScreen.ID == 3781 || base.CurrentScreen.ID == 3782)
                {
                    ddlStatus.Enabled = false;                    
                }
            System.Web.UI.WebControls.Button btnReRoute = e.Row.Cells[8].FindControl("btnReRoute") as System.Web.UI.WebControls.Button;
            btnReRoute.CssClass = "btnBackToReRoute";
            //Button btnReRoute = e.Row.Cells[8].FindControl("btnReRoute") as Button;
            btnReRoute.CommandName = "ReRoute";
            btnReRoute.CommandArgument = e.Row.Cells[0].Text;
            e.Row.Cells[8].Controls.Add(btnReRoute);

        }
    }

    protected void gc_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "ReRoute")
        {
              int masterID = Convert.ToInt32(e.CommandArgument);

              Button button = null;
              foreach (GridViewRow rows in gc.GridRows)
              {
                  InvoiceMaster im = new InvoiceMaster();
                  im.ID = masterID;
                 // this.LoadGridData();

                  _invoiceMaster = new InvoiceMaster();
                  _invoiceMaster = im.GetData();

                  int id = Convert.ToInt32(rows.Cells[0].Text);
                  if (id == masterID)
                  {
                      button = rows.FindControl("btnReRoute") as Button;

   DropDownList ddlStatus(DropDownList)rows.Cells[6].FindControl("ddlStatus");
            if (base.CurrentScreen.ID == 3780)
                      {
                          _invoiceMaster.StatusID = Convert.ToInt32(ddlStatus.SelectedItem.Value);
                      }
                      if (base.CurrentScreen.ID == 3781)
                      {
                          _invoiceMaster.StatusID = 13;
                      }
                      if (base.CurrentScreen.ID == 3782)
                      {
                          _invoiceMaster.StatusID = 11;
                      }
                      _invoiceMaster.Save();

                      break;
                  }                     
              }

            LoadColumns();
            LoadGridData();
            base.ShowClientMessage("Invoice Backed Successfully.");
        }
    }

    #endregion
}

}

Answer 1

When you write this line:

private InvoiceMaster _invoiceMaster;

you are just defining a private member of the class of type InvoiceMaster . At this point however the reference is pointing to nothing (the "value" is null ).

In this line:

InvoiceMaster im = new InvoiceMaster();

you are also creating a private member of the class (the default in c# is private ) and this time you are assigning to that reference a new object that you are creating.

The following lines will not compile and must be in a scope of a function:

im.ID = masterID;

_invoiceMaster = new InvoiceMaster();
_invoiceMaster = im.GetData();

I recommend that you go through one of the many tutorial out there to better understand about data types, variables and scopes