在將數據填充到Datagridview之前將DBNull轉換爲字符串

Question

我需要將數據從數據庫填充到datagridview時將DBNull轉換爲字符串的方式。以下是我的代碼

Private Sub FetchData() 

    sql = "SELECT ContributionType,Status,PayMethod,BankName,AccountNo,Amount,FullName,District,Refno,ContributorGrp" 
    sql &= " FROM tblincome WHERE TxnID=@id" 

    command = New OleDbCommand(sql, connection) 
    command.Parameters.Add(New OleDbParameter("@id", OleDbType.VarChar)).Value = lbltxnid.Text 

    adapter = New OleDbDataAdapter(command) 
    dt = New DataTable 
    adapter.Fill(dt) 



    For Each row In dt.Rows 
     PopulateGrid(row(0), row(1), row(2), row(3), row(4), row(5), row(6), row(7), row(8), row(9)) 
    Next 

    dt.Rows.Clear() 
    dt.Dispose() 
End Sub 

Private Sub PopulateGrid(ContributionType As String, Status As String, PayMethod As String, BankName As String _ 
         , AccountNo As String, Amount As Double, FullName As String, District As String _ 
         , Refno As String, ContributorGrp As String) 

    Dim row As String() = New String() {ContributionType, Status, PayMethod, BankName, AccountNo, Amount.ToString("N") _ 
             , FullName, District, Refno, ContributorGrp} 
    dgvcontributions.Rows.Add(row) 
End Sub 

字段BankName和Accountno允許空條目，因此它們很可能是空值的條目。

Answer 1

您可以使用此方法：

Public Shared Function ConvertDbVal(Of T)(obj As Object, Optional nullFallbackVal As T = Nothing) As T 
    If obj Is Nothing OrElse obj.Equals(DBNull.Value) Then 
     Return nullFallbackVal 
    Else 
     Return CType(obj, T) 
    End If 
End Function 

用法：

Dim bankName As String = ConvertDbVal(row(3), "") 
Dim accNumber = ConvertDbVal(row(4), "") 

如果不指定（可選）回退值，你必須明確指定類型：

Dim bankName As String = ConvertDbVal(Of String)(row(3))