连接到 SQL Server 数据库 C# ASP.NET 时的 Null 引用 [重复]-解网

问：

什么是 NullReferenceException，如何修复它？（26 个答案）

5年前关闭。

尝试通过 Visual Studio 2017 连接到 SQL Server 数据库时，我一直收到空引用。下面是引用的位置以及它如何尝试连接以收集数据。请帮忙！谢谢！。

配置文件.aspx

protected void updateProfile(object sender, EventArgs e)
    {
        // Database entry
        string message = "Your information has been updated accoridngly!<br><br>";
        double number;
        if (txtName.Text.Equals("") || ddlDegree.SelectedValue.Equals("") || txtExperience.Text.Equals("") || !Double.TryParse(txtExperience.Text, out number) || txtSalary.Text.Equals("") || txtStreet.Text.Equals("") || txtCity.Text.Equals("") || txtState.Text.Equals("") || txtZipcode.Text.Equals(""))
        {
            message += "However:<br>There was an error found in your entry fields, resulting in a failure to store field information. Make sure that all fields are filled and that the Experience field is a double value.";
        }
        else
        {
        **myDataLayer.updateStaff(Session["userid"].ToString(), txtName.Text, ddlDegree.SelectedValue, txtExperience.Text, txtSalary.Text, txtStreet.Text, txtCity.Text, txtState.Text, txtZipcode.Text);** - **Null Reference**
        }

下面是它需要引用的数据层类：

string _ConString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
    SqlConnection con; 
    SqlDataAdapter sda = new SqlDataAdapter();
    SqlCommand cmd = new SqlCommand();
    SqlDataReader data;

// Handles updating staff info
    public bool updateStaff(string userid, string full_name, string degree, string experience, string salary, string street, string city, string state, string zipcode)
    {
        try
        {
            con = new SqlConnection(_ConString);
            con.Open();
            cmd = new SqlCommand();
            cmd.Connection = con;
            cmd.CommandText = "UPDATE staff SET [full_name] = '" + full_name + "', [degree] = '" + degree + "', [experience] = '" + experience + "', [salary] = '" + salary + "', [street] = '" + street + "', [city] = '" + city + "', [state] = '" + state + "', [zipcode] = '" + zipcode + "' WHERE userid = '" + userid + "'";
            cmd.ExecuteNonQuery();
            con.Close();
            return true;
        }
        catch (OleDbException e)
        {
            System.Diagnostics.Debug.WriteLine(e.ToString());
        }
        finally
        {
            con.Close();
        }

        return false;
    }

C# asp.net SQL-Server NullReferenceException

cmd.Connection = con;
cmd.CommandText = 
@"UPDATE staff SET 
      full_name = @parmFullName,
      degree = @parmDegree,
      experience = @parmExperience,
      salary = @parmSalary,
      street = @parmStreet,
      city = @parmCity,
      state = @parmState,
      zipcode = @parmZIP,
   where
      userid = @parmUserID";

// pulling the parameters from the parameters you pass to your UpdateStaff() call

cmd.Parameters.AddWithValue( "parmFullName", full_name );
cmd.Parameters.AddWithValue( "parmDegree", degree );
cmd.Parameters.AddWithValue( "parmExperience", experience );

cmd.Parameters.AddWithValue( "parmSalary", salary );
cmd.Parameters.AddWithValue( "parmStreet", street);
cmd.Parameters.AddWithValue( "parmCity", city );
cmd.Parameters.AddWithValue( "parmState", state );
cmd.Parameters.AddWithValue( "parmZIP", zipcode );

cmd.Parameters.AddWithValue( "parmUserID", userid );

所以，在大多数情况下，你应该很好。没有 Null 值，没有不匹配的单引号字符串。这不是验证的终极方法，不过，您绝对应该阅读更多关于SQL注入和数据清理的信息。

在我的示例中，我显式地在参数中添加了前缀“parm”，以区分您尝试使用的实际值。

此外，如果数据列是日期、整数、双精度、字符串，则让它们保持预期的类型。参数将正确传递。

连接到 SQL Server 数据库 C# ASP.NET 时的 Null 引用 [重复]

Null Reference when connecting to SQL Server database C# ASP.NET [duplicate]

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