未将对象引用设置到对象的实例

Public Shared Function getDataTable(ByVal sql As String) As DataTable
Try
Dim sqlAdapter1 As SqlDataAdapter = New SqlDataAdapter(sql, cnn)
Dim dataSet As DataSet = New DataSet

sqlAdapter1.Fill(dataSet)
sqlAdapter1.Dispose()

Return dataSet.Tables(0)
Catch ex As Exception
errors.Log(ex.ToString)
Return New DataTable

End Try
End Function
为什么会出现这样情况 (有时会出现有时不会出现)
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未将对象引用设置到对象的实例。
at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream)
at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.SqlClient.SqlCommand.System.Data.IDbCommand.ExecuteReader(CommandBehavior behavior)
at System.Data.Common.DbDataAdapter.FillFromCommand(Object data, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior)
at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet)
at fashions.DBoperate.getDataTable(String sql)


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