c# - 当我尝试在 if() 中使用变量时，它说我正在使用未分配的变量

Question

string x1;                  
Dispatcher.Invoke(new Action (() => x1 = lbl1.Content.ToString()));

(我这样做是因为我正在使用线程) (然后当我尝试在 if 中使用它时)

if(x1 == "X"){}

(我收到一条错误消息，说我正在使用未分配的变量)

有人能告诉我为什么会这样吗？

Answer 1

参见:

  string x1; // <- Just declared, not assigned

  // x1 is assigned, but in the different thread
  Dispatcher.Invoke(new Action (() => x1 = lbl1.Content.ToString()));

  // it may occure, that the diffrent thread hasn't finished yet, and 
  // x1 is still unassigned; that's why the compiler shows the warning
  if(x1 == "X"){}

然而，在某些情况下，编译器不能跟踪赋值，例如

  String x1;

  Action f = 
    () => { x1 = "X"; };

  f(); // <- x1 will be assigned here

  // Compiler erroneously warns here that x1 is unassigned,
  // but x1 is assigned  
  if (x1 == "X")