Thread: Object reference not set to an instance of an object

This is a common error that .Net developers often face at some time or another while developing their applications. This is to be used as a guide for diagnosing the common causes of this error.

Prior to .Net 2.0, you had to declare every control in your code behind. For example:

.aspx

Code:

<asp:TextBox ID="FirstName" runat="server" />

.aspx.vb

Code:

Public Class MyPage
Inherits System.Web.UI.Page
 
Protected WithEvents FirstName As System.Web.UI.WebControls.TextBox 

Without the bolded line above, you would receive the error mentioned in the title of this thread. This is ONLY in versions of .Net prior to 2.0.

Variable scoping is another common issue. For example, you can't declare a variable in one sub and then expect it to work in another:

Code:

Private Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim myVariable As String = "Test"
End Sub
 
Private Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    Response.Write(myVariable)
End Sub

The above will result in the error because the variable "myVariable" is out of scope in the Button1_Click sub.

Bad inits and constructs are another cause. Take the following example:

Code:

Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) 
     Dim dirInfo As System.IO.DirectoryInfo
     dirInfo.GetDirectory("C:\")
End Sub

Above, we have defined dirInfo as a DirectoryInfo object but haven't actually created it:

Code:

Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim dirInfo As System.IO.DirectoryInfo
    dirInfo = New System.IO.DirectoryInfo("C:\")
End Sub

Another possible cause of this error is if you do not give a user control an ID:

Code:

<YourPrefix:YourControl runat="server" />

The correct way:

Code:

<YourPrefix:YourControl ID="myControlID" runat="server" />

If anyone has any other causes, feel free to add them here.

Another possible way to cause this error is not check for null or Nothing prior to using an object. For example:

Code:

TextBox tb = gvRow.FindControl("nonesuch") as TextBox;
tb.Text;

If "nonsuch" does not exists (as if I coded ID="NoneSuch" in my apsx page)