在将数据填充到Datagridview之前将DBNull转换为字符串

Question

我需要将数据从数据库填充到datagridview时将DBNull转换为字符串的方式。以下是我的代码

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允许空条目，因此它们很可能是空值的条目。

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（可选）回退值，你必须明确指定类型：

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