How to check if String is null

问题:

I am wondering if there is a special method/trick to check if a String object is null. I know about the String.IsNullOrEmpty method but I want to differentiate a null String from an empty String (="").

Should I simply use:

if (s == null) {
    // blah blah...
}

...or is there another way?

回答1:

An object can't be null - the value of an expression can be null. It's worth making the difference clear in your mind. The value of s isn't an object - it's a reference, which is either null or refers to an object.

And yes, you should just use

if (s == null)

Note that this will still use the overloaded == operator defined in string, but that will do the right thing.

回答2:

To sure, you should use function to check is null and empty as below:

string str = ...
if (!String.IsNullOrEmpty(str))
{
...
}

回答3:

You can use the null coalescing double question marks to test for nulls in a string or other nullable value type:

textBox1.Text = s ?? "Is null";

The operator '??' asks if the value of 's' is null and if not it returns 's'; if it is null it returns the value on the right of the operator.

More info here: https://msdn.microsoft.com/en-us/library/ms173224.aspx

And also worth noting there's a null-conditional operator ?. and ?[ introduced in C# 6.0 (and VB) in VS2015

textBox1.Text = customer?.orders?[0].description ?? "n/a";

This returns "n/a" if description is null, or if the order is null, or if the customer is null, else it returns the value of description.

More info here: https://msdn.microsoft.com/en-us/library/dn986595.aspx

回答4:

You can check with null or Number.

First, add a reference to Microsoft.VisualBasic in your application.

Then, use the following code:

bool b = Microsoft.VisualBasic.Information.IsNumeric("null");
bool c = Microsoft.VisualBasic.Information.IsNumeric("abc");

In the above, b and c should both be false.