Object reference not set to an instance of an object.

 

This question already has an answer here:

  • What is a NullReferenceException, and how do I fix it? 33 answers
  • What does “Object reference not set to an instance of an object” mean? [duplicate] 8 answers

    • 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 ( . ) - ie 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 .

    All versions of .Net: 

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

    .Net 4.0 or later: 

    if (String.IsNullOrWhitespace(strSearch))

    strSearch in this case is probably null (not simply empty).

    Try using

    String.IsNullOrEmpty(strSearch)

    if you are just trying to determine if the string doesn't have any contents.

    链接地址: http://www.djcxy.com/p/28034.html

    上一篇: 如何解决未将对象引用设置为对象的实例？

    下一篇: 你调用的对象是空的。