验证标签内容等于null或string.Empty

Question

I'm trying to check if the value of a label is equal to null, " ", string.Empty , but every time I run through my coding, I get the following error:

Object reference not set to an instance of an object.

Here is my coding:

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;
}

How can I check if the string value inside my label is equal to null? I might be missing something simple, if so please ignore my incompetence :P

Answer 1

Change

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

To

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

When lblSupplierEmailAddress.Content is actually null you can of course not call ToString on it as it will cause a NullReferenceException . However the static IsNullOrEmpty -method takes respect on this and returns true if Content is null .

Answer 2

In C#6.0 This will do

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

Else if the lblSupplierEmailAddress always exists, you could simply do:

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

The equivalent code would be:

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

Answer 3

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