c# - 无法克服此错误：对象引用未设置为对象的实例

Question

这是我的代码：

    protected void Page_Load(object sender, EventArgs e)
    {
        if (Session[ "DistID" ] != "" )
        {
            DistInfo distInfo = GeneralFunctions.GetGeneralInformation( ( int )Session[ "DistID" ] );
            launchDate.Text = distInfo.AnticipatedLaunchDate.ToShortDateString();
        }
    }

    public static DistInfo GetGeneralInformation ( int ClientID )
    {
        using ( var conn = new SqlConnection( GetConnectionString() ) )
        using ( var cmd = conn.CreateCommand() )
        {
            conn.Open();
            cmd.CommandText =
            @"SELECT i.GoLiveDate, i.FirstBonusRun, i.TechFName, i.TechLName, i.TechEmail, i.TechPhone, i.WebISPFName, i.WebISPLName, 
              i.WebISPEmail, i.WebISPPhone, i.FullFillFName, i.FullFillLName, i.FullFillEmail, i.FullFillPhone, d.FName,
              d.LName, d.HomePhone, d.Email
              FROM NC_Information i
              INNER JOIN Distributor d
                ON d.DistID = i.ClientID
              WHERE clientID = @value";
            cmd.Parameters.AddWithValue( "@value", ClientID );
            using ( var reader = cmd.ExecuteReader() )
            {
                while ( reader.Read() )
                {
                    var distInfo = new DistInfo
                    {
                        AnticipatedLaunchDate = reader.GetDateTime( reader.GetOrdinal( "GoLiveDate" ) )
                    };

                    return distInfo;
                }
            }
            return null;
        }
    }

public class DistInfo
{
    public DateTime AnticipatedLaunchDate { get; set; }
}

这是错误：

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 14:         if (Session[ "DistID" ] != "" )
Line 15:         {
Line 16:             DistInfo distInfo = GeneralFunctions.GetGeneralInformation( ( int )Session[ "DistID" ] );
Line 17:             launchDate.Text = distInfo.AnticipatedLaunchDate.ToShortDateString();
Line 18:         }


Source File: c:\inetpub\wwwroot\Base\ClientAccessPortal\GeneralInformation.aspx.cs    Line: 16 

数据库可以从数据库返回 DateTime、空 DateTime 或 NULL 的 i.GoLiveDate（将是 AnticipatedLaunchDate）的值。我不确定如何克服这个错误。提前感谢您的帮助。

Answer 1

Session["DistID"]正在返回 null，您尝试将其转换为整数。

您检查空字符串，但空值将通过该检查 - 使用IsNullOrEmpty或如果 .Net 4.0IsNullOrWhitespace来检查两者。

string distId = Session[ "DistID" ];
if ( !string.IsNullOrWhiteSpace( distId ) )
{ ... }

既然您知道您有一个非 null 非空字符串，请确保它是一个 int：

string distId = Session[ "DistID" ];    
int distIdValue;

if( !string.IsNullOrWhiteSpace( distId ) && integer.TryParse( distId, out distIdValue ) )
{
    DistInfo distInfo = GeneralFunctions.GetGeneralInformation( distIdValue );
    launchDate.Text = distInfo.AnticipatedLaunchDate.ToShortDateString();
}

Answer 2

在第 16 行，如果 Session[ "DistID" ] 为空，则 (int) 的情况将失败。

因此，您需要将 Session["DistID"] 分配给一个临时变量，并在将其转换为整数之前对其进行非空测试。

    protected void Page_Load(object sender, EventArgs e)
    {
        if (Session[ "DistID" ] != "" )
        {
            var distId = Session[ "DistID" ];
            if (distId != null) {
              DistInfo distInfo = GeneralFunctions.GetGeneralInformation( ( int )Session[ "DistID" ] );
              launchDate.Text = distInfo.AnticipatedLaunchDate.ToShortDateString();
            } else {
              // default display value
            }
        }
    }