[英]NullReferenceException for Public class member
调用GetDepAirport()方法时,以下代码引发“在...中发生了'System.NullReferenceException'类型的未处理异常”。
public WYPT GetDepAirport()
{
Console.WriteLine("Retrieving Airport in GetDepApt()");
Console.WriteLine("Departure Airport is {0}", Dep.Ident);
return Dep;
}
public void SetDepAirport(String ident)
{
Console.WriteLine("Setting Airport with ident {0}, ident");
Dep = FetchDBAirport(ident);
Console.WriteLine("WYPT Dep is set to {0}", Dep.Ident);
}
输出为:
Setting Airport with ident KABQ
WYPT Dep is set to KABQ
Retrieving Airport in GetDepApt()
其次是一些例外。 我无法弄清楚为什么从GetDepAirport()方法调用Dep (声明为public)时会返回null 。 这两种方法都是从同一类的单独方法中调用的。
声明是课堂上的佼佼者:
class FlightPlan
{
//Init Pg.1 data
public WYPT Dep, Dest, Altn;
set的调用来自另一个类:
FlightPlan FPlan = new FlightPlan();
FPlan.SetDepAirport(Dep);
稍后致电Get:
public void GetFPlan()
{
for (int i = 0; i < 14; i++)
{
Waypoint[i] = new WYPT();
}
Waypoint[0] = GetDepAirport();
Waypoint[1] = DISCON;
同一FlightPlan类中的FetchDBAirport方法
private WYPT FetchDBAirport(String airport)
{
WYPT Airport = new WYPT();
String databasepath = "C:\\Users\\Family\\documents\\visual studio 2012\\Projects\\FMST\\FMST\\Database\\NavData.mdf";
SqlConnection myConnection = new SqlConnection("Data Source=(LocalDB)\\v11.0;AttachDbFilename=" + databasepath + ";Integrated Security=True");
try
{
myConnection.Open();
SqlCommand cmd = myConnection.CreateCommand();
cmd.CommandText = "SELECT * FROM Airports WHERE Ident='" + airport + "';";
SqlDataReader rdr = cmd.ExecuteReader();
rdr.Read();
Airport.Ident = (String)rdr.GetValue(0);
Airport.Lat = (decimal)rdr.GetValue(2);
Airport.Lon = (decimal)rdr.GetValue(3);
Airport.Elev = (decimal)rdr.GetValue(4);
myConnection.Close();
return Airport;
}
catch (Exception e)
{
Console.WriteLine(e.ToString() + "Happy Face");
String ErrorMsg = "NOT FOUND";
Airport.Ident = ErrorMsg;
return Airport;
}
可以在GetDepAirport中返回NullReferenceException的代码的唯一部分是Dep.Ident
。 Dep
或Ident
可以为null。 如果为这两个都添加了空检查,则该方法将运行良好。
至于为什么这些都不为空,可能是因为:
SetDepAirport
然后再致电GetDepAirport
FetchDBAirport
返回的是null,或者返回Ident
为null的Dep
实例。
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