“对象引用未设置为对象的实例。”错误

Question

public override bool Equals(object obj)
{
    var par = obj as Parameter;
    if (ParameterName == par.ParameterName )  /* <=throws error at this code */
        return true;
    else
        return false;

}

运行该代码时出现此错误：Object reference not set to an instance of an object

如何删除c＃中的错误？

Answer 1

原则 ONE 防御性编程 - 始终检查NULL！

public override bool Equals(object obj)
{
    // this statement CAN FAIL if your "obj" is *NOT* of type "Parameter"
    var par = obj as Parameter;

    // so therefore you MUST be ready to deal with a NULL value here!
    if (par != null && ParameterName == par.ParameterName)  
    {
        return true;
    }
    else
    {
        return false;
    }

}

Answer 2

在访问par之前，您可以检查null是否不是ParameterName

public override bool Equals(object obj)
{
    var par = obj as Parameter;
    if (par!= null && ParameterName == par.ParameterName )  /* <=throws error at this code */
        return true;
    else
        return false;    
}

Answer 3

如何先检查它是否为空？

public override bool Equals(object obj)
{
    var par = obj as Parameter;
    if (par != null && ParameterName == par.ParameterName )  /* <=throws error at this code */
        return true;
    else
        return false;

}

Answer 4

在转换为Parametre时出现问题，您必须检查是否为null

public override bool Equals(object obj)
{
   var par = obj as Parameter;
   if (par  != null && ParameterName == par.ParameterName ) 
      return true;
   else
      return false;

}

Answer 5

用它来检查par != null null

public override bool Equals(object obj)
{
   var par = obj as Parameter;
   if (par  != null && ParameterName == par.ParameterName ) 
      return true;
   else
      return false;

}