[InvalidOperationException: The ConnectionString property has not been initialized.]
System.Data.SqlClient.SqlConnection.PermissionDemand() +7122241
System.Data.SqlClient.SqlConnectionFactory.PermissionDemand(DbConnection outerConnection) +32
System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions) +315
System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry) +128
System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry) +265
System.Data.SqlClient.SqlConnection.Open() +133
MathExpress.DataAccess.BaiToanDAOTableAdapters.BaiToanCauHoiTableAdapter.TangLuotXem(Nullable`1 IDBaiToan, Nullable`1 SiteID) in D:\2023\HTN\Src\MathExpress\DataAccess\BaiToanDAO.Designer.cs:15266
ME_HoiDapToanHoc_QuestionDetail.TangLuotXem() +256
ME_HoiDapToanHoc_QuestionDetail.Page_Load(Object sender, EventArgs e) +477
System.Web.UI.Control.OnLoad(EventArgs e) +108
System.Web.UI.Control.LoadRecursive() +90
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1533
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