如何解决 C# 中 linq 的 lambda 表达式中的对象引用错误？

Question

I am working on WPF application. In which I am trying to fetch records from list as per required condition. But when there is no any record found then it's giving me object reference not found error.

var recordList = _outputVariables.ToList().Where(X => X.symbolName == _symbolName).ToList();
if (recordList != null)
{
       //more coding...
}

so as shown in code when _outputVariables have no any record match as per _symbolName then it's giving error of Object reference not set to an instance of an object.' and X was null. . So how can I handle this issue ? please help for it.

Answer 1

Use the null-conditional operator that was introduced in C#6 (and don't call ToList() more than once):

var recordList = _outputVariables?.Where(X => X?.symbolName == _symbolName).ToList();
if (recordList != null)
{
    //more coding...
}

Answer 2

You can use like below as well

 if (_outputVariables != null && _outputVariables.Count > 0)
            {
               var recordList = _outputVariables.Where(X => X != null && !string.IsNullOrEmpty(X.symbolName) && X.symbolName == _symbolName);

            }

Answer 3

var recordList = _outputVariables.ToList().Where(X => X.symbolName == _symbolName).ToList();

You currently loop through _outputVariables , but if it's null this will give an error, because null does not have a .ToList() . So you should add a ? after your _outputVariables , so it will return null instead of an exception when it's null.

Same goes for X . If X is null and you try to get the property symbolName , you will get an error, because null doesn't have a property called symbolName . So you want to add a ? here too. So it will return null instead of an exception.

Leaving you with: var recordList = _outputVariables?.ToList().Where(X => X?.symbolName == _symbolName).ToList();

Answer 4

You can try this

if(_outputVariables!=null)
{
var recordList = _outputVariables.Where(X => X.symbolName ==_symbolName).ToList();
}
if (recordList != null)
{
       //more coding...
}