asp.net主属性字符串，如果字符串为空，如何在内容页面中检入

Question

if i have a master property string for example:

public String SharedInfo
{
    get { return (String)Session["SharedInfo"]; }
    set { Session["SharedInfo"] = value; }
}

and a label in an content page, i check if the string is empty by doing: if(Master.SharedInfo == null)

now my question is: why does if(Master.SharedInfo == "") not work, because the SharedInfo is a string right?

Answer 1

There is a handy method that "catches" both

if (String.IsNullOrEmpty(Master.SharedInfo)) {
    ...
}

null and "" are not equal. null means no string at all. "" is a string of length 0 .

string s = null;
int i = s.Length; // <-- Throws null reference exception

But

string s = "";
int i = s.Length; // OK, i => 0

"" and String.Empty are equivalent. Some people state that you should always use String.Empty instead of "" , but it makes really no difference.

---

UPDATE

Equal string constants are interned by the compiler, ie the compiler stores equal constants only once. You can make a simple test (in response to @BobTodd's comment),

string s = ""; 
Console.WriteLine(Object.ReferenceEquals(s, "")); // --> true
Console.WriteLine(Object.ReferenceEquals(s, String.Empty)); // --> true

---

For the sake of completeness (according to @JoelEtherton's comment). Starting from .NET Framework 4.0 you can test

if (String.IsNullOrWhitespace(Master.SharedInfo)) {
    ...
}

This will catch strings like " " or "\\t" as well.

Answer 2

In c#, an empty string "" is not null. It's an actual string, with length equals to zero.

You can use string.IsNullOrEmpty(string stringToTest) to check both null and empty strings.

Answer 3

String.Empty ("") and null are quite different

It depends wholly on what has been written to Session["SharedInfo"].

If nothing has, then it will be null, otherwise its the value written, which could be an empty string or null.

To be sure use String.IsNullOrEmpty(string @string)