Object reference not set to an instance of an object. [duplicate]

Question

I keep getting this error when I run the program.

Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:  Line with error:  Line 156:        if (strSearch == "" || strSearch.Trim().Length == 0) 

What is the correct way it should be written?

Answer 1

The correct way in .NET 4.0 is:

if (String.IsNullOrWhiteSpace(strSearch)) 

The String.IsNullOrWhiteSpace method used above is equivalent to:

if (strSearch == null || strSearch == String.Empty || strSearch.Trim().Length == 0)  // String.Empty is the same as "" 

Reference for IsNullOrWhiteSpace method

http://msdn.microsoft.com/en-us/library/system.string.isnullorwhitespace.aspx

Indicates whether a specified string is Nothing, empty, or consists only of white-space characters.

In earlier versions, you could do something like this:

if (String.IsNullOrEmpty(strSearch) || strSearch.Trim().Length == 0) 

The String.IsNullOrEmpty method used above is equivalent to:

if (strSearch == null || strSearch == String.Empty) 

Which means you still need to check for your "IsWhiteSpace" case with the .Trim().Length == 0 as per the example.

Reference for IsNullOrEmpty method

http://msdn.microsoft.com/en-us/library/system.string.isnullorempty.aspx

Indicates whether the specified string is Nothing or an Empty string.

Explanation:

You need to ensure strSearch (or any variable for that matter) is not null before you dereference it using the dot character (.) - i.e. before you do strSearch.SomeMethod() or strSearch.SomeProperty you need to check that strSearch != null.

In your example you want to make sure your string has a value, which means you want to ensure the string:

• Is not null
• Is not the empty string (String.Empty / "")
• Is not just whitespace

In the cases above, you must put the "Is it null?" case first, so it doesn't go on to check the other cases (and error) when the string is null.

Answer 2

All versions of .Net:

if (String.IsNullOrEmpty(strSearch) || strSearch.Trim().Length == 0) 

.Net 4.0 or later:

if (String.IsNullOrWhitespace(strSearch))