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