无论我在c＃中尝试什么都是NullException [重复]

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What is a NullReferenceException, and how do I fix it? 31 answers

什么是NullReferenceException,我该如何解决? 31个答案

I have a datatable session

我有一个数据表会话

Session["Items"] = dt;

I am trying to display the count from the session in Site.Master at the top, kind of like on ecommerece websites where they have the Cart at the top.

我试图在Site.Master顶部的会话中显示计数,有点像在ecommerece网站上,他们在顶部有购物车。

I have this, but keep getting null exception error at int count

我有这个,但在int计数时继续得到null异常错误

DataTable dt = (DataTable)Session["Items"];
            int count = dt.Rows.Count; //error

            if (Session["Items"] == null)
            {
                if (dt.Rows.Count == 0)
                {
                    lblItems.Text = "Items (0)";
                }
                else
                {
                    lblItems.Text = "Items" + count;
                }
            }

3 个解决方案

#1

You should check immediately if Session["Items"] is NOT null, you do it too late in your code. Try this:

你应该立即检查Session [“Items”]是否为null,你在代码中做得太晚了。试试这个:

        DataTable dt = (DataTable)Session["Items"];

        if (dt != null)
        {
            int count = dt.Rows.Count;

            if (dt.Rows.Count == 0)
            {
                lblItems.Text = "Items (0)";
            }
            else
            {
                lblItems.Text = "Items" + count;
            }
        }
        else
            lblItems.Text = "Items (0)";

#2

There are multiple things that can go wrong and only you can start the debugger and check if:

有很多事情可能出错,只有你可以启动调试器并检查是否:

1) Session["Items"] is null.

1)Session [“Items”]为空。

2) Session["Items"] is not a DataTable. The result of casting it to the DataTable is null in this case.

2)Session [“Items”]不是DataTable。在这种情况下,将其转换为DataTable的结果为null。

3) Session["Items"] is a DataTable, but it's Rows property is null

3)Session [“Items”]是一个DataTable,但它的Rows属性为null

#3

This should do it for you, since its checking every possible null pointer possibility:

这应该为你做,因为它检查每个可能的空指针的可能性:

DataTable dt = Session["Items"] as DataTable;
dt = dt ?? new DataTable();
int count = dt.Rows == null? 0 : dt.Rows.Count; 
lblItems.Text = String.Format("Items ({0})", count);

#1