An Beginner Immigrant from C sharp to Vb simple issue

Public Function ExecCommand(ByVal sql 开发者_StackOverflow社区As String) As SqlDataReader
    Dim dReader As SqlDataReader
    Dim cmd As New SqlCommand()
    cmd.CommandText = sql
    cmd.Connection = sm
    Dim isOpened = OpenConnection()
    If isOpened Then
      dReader = cmd.ExecuteReader()
      CloseConnection()
    End If
    Return dReader
End Function

It says maybe a null refrence may occur at runtime for the declaring of that datareader. Why is that?

---

That's because the dReader variable might not be initialized if the If condition is not satisfied. You could initialize it to Nothing by default:

Dim dReader As SqlDataReader = Nothing

Obviously the caller of this method need to handle this case.

Also as a C# immigrant you are probably surprised why you are not getting a compile time error but only a warning.

---

if OpenConnection() fails, then isOpened is false. The following block will not get executed and dReader will not have been assigned.

Public Function ExecCommand(ByVal sql As String) As SqlDataReader
    Dim dReader As SqlDataReader       // Not assigned yet
    Dim cmd As New SqlCommand()
    cmd.CommandText = sql
    cmd.Connection = sm
    Dim isOpened = OpenConnection()    // Fails to connect
    If isOpened Then                   // Not executed
      dReader = cmd.ExecuteReader()
      CloseConnection()
    End If
    Return dReader                     // Return value unknown

As a sidenote, CloseConnection() is not executed either so you might have some problems later.

---

add else statemnt in which assign dreader as null.

Public Function ExecCommand(ByVal sql As String) As SqlDataReader 
Dim dReader As SqlDataReader=Nothing
 Dim cmd As New SqlCommand()
 cmd.CommandText = sql  


cmd.Connection = sm  

Dim isOpened = OpenConnection() If isOpened Then dReader = cmd.ExecuteReader() CloseConnection()
End If
Return dReader End Function