「對象引用不設置到對象的實例。」錯誤

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; 

} 

我收到的時候我正在運行的代碼此錯誤：Object reference not set to an instance of an object

如何刪除在C＃中的錯誤？

Answer 1

原理ONE防禦性編程 - 總是檢查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

您可以檢查是否par不null訪問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

如何檢查，如果它是空首過嗎？

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

ther'es在你皈依一個問題Parametre你要檢查是否爲空

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

} 

Answer 5

使用這種檢查par != null空

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

}