Question

我有一个if语句

if (materialtype != "WINDOWVINYLREPL"
    || materialtype != "ROOFING"
    && materialtype != "null"
    && materialtype != null)
{                
    subtype = (from a in UEF.MaterialSubTypes 
               where a.MaterialSubType1.Equals(subtype) && a.Code.Contains(materialtype) 
               select a.Code).FirstOrDefault();
}

即使materialtype等于null，if语句仍然被调用，它不应该是

Answer 1

if (materialtype != "WINDOWVINYLREPL" || ...

变量为null所以当然它不等于“WINDOWVINYLREPL”，因此这个条件为真，它会进入。

Answer 2

您有OR ||条件。应该是这样的：

if (materialtype != "WINDOWVINYLREPL" && materialtype != "ROOFING" && materialtype != "null" && materialtype != null)
    {                
        subtype = (from a in UEF.MaterialSubTypes where a.MaterialSubType1.Equals(subtype) && a.Code.Contains(materialtype) select a.Code).FirstOrDefault();
    }

Answer 3

条件不是用适当的括号写的。 AND，或冲突

if ((materialtype != "WINDOWVINYLREPL") || (materialtype != "ROOFING" && materialtype != "null")) && materialtype != null)
    {                
        subtype = (from a in UEF.MaterialSubTypes where a.MaterialSubType1.Equals(subtype) && a.Code.Contains(materialtype) select a.Code).FirstOrDefault();
    }

Answer 4

您的情况相当于：

if((materialtype != "WINDOWVINYLREPL") || (materialtype != "ROOFING" && materialtype != "null" && materialtype != null))

如果materialtype == null，则为真，因为(materialtype != "WINDOWVINYLREPL")为真。

＆amp;＆amp;运算符的优先级高于||

Answer 5

尝试使用括号分隔逻辑“ OR ”和“和” 因为如果值为null，则测试始终为真

if ((materialtype != "WINDOWVINYLREPL" || materialtype != "ROOFING")
        && materialtype != null)
    {   
      .....
    }

变量具有空值但是仍然会调用语句

5 个答案: