验证标签内容等于 null 或 string.Empty

Question

我正在尝试检查标签的值是否等于 null, " ", string.Empty，但每次我运行通过我的编码时，我都会收到以下错误：

Object reference not set to an instance of an object.

这是我的编码：

if (lblSupplierEmailAddress.Content.ToString() == "") //Error here
{
    MessageBox.Show("A Supplier was selected with no Email Address. Please update the Supplier's Email Address", "Warning", MessageBoxButton.OK, MessageBoxImage.Warning);
    return;
}

如何检查标签中的字符串值是否为空？我可能遗漏了一些简单的东西，如果是这样请忽略我的无能:P

Answer 1

改变

if (lblSupplierEmailAddress.Content.ToString() == "")

到

if (String.IsNullOrEmpty((string) lblSupplierEmailAddress.Content)

当 lblSupplierEmailAddress.Content 实际上是 null 时，您当然不能对其调用 ToString，因为它会导致 NullReferenceException。但是，如果 Content 是 null.

，则静态 IsNullOrEmpty 方法对此表示尊重 returns true

Answer 2

在 C#6.0 中这样做

if(lblSupplierEmailAddress?.Content?.ToString() == "")

否则如果 lblSupplierEmailAddress 总是存在，你可以简单地做：

if(lblSupplierEmailAddress.Content?.ToString() == "")

等效代码为：

if(lblSupplierEmailAddress.Content != null)
    if (lblSupplierEmailAddress.Content.ToString() == ""){
        //do something
    }

Answer 3

if( null != lblSupplierEmailAddress.Content 
    && string.IsNullOrEmpty(lblSupplierEmailAddress.Content.ToString() )

验证标签内容等于 null 或 string.Empty

Validating label content equal to null or string.Empty

c#

wpf

label