please help. I'm trying for days and can not be

Server Error in '/' Application.

The ConnectionString property has not been initialized.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: The ConnectionString property has not been initialized.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[InvalidOperationException: The ConnectionString property has not been initialized.]

System.Data.SqlClient.SqlConnection.PermissionDemand() +5093970

System.Data.SqlClient.SqlConnectionFactory.PermissionDemand(DbConnection outerConnection) +20

System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) +126

System.Data.SqlClient.SqlConnection.Open() +125

YAF.Classes.Data.MsSqlDbConnectionManager.get_OpenDBConnection() +39

YAF.Classes.Data.LegacyDb.system_initialize_fixaccess(Boolean grant) +62

YAF.Core.Services.InstallUpgradeService.UpgradeDatabase(Boolean fullText, Boolean upgradeExtensions) +56

YAF.Install._default.Wizard_NextButtonClick(Object sender, WizardNavigationEventArgs e) +1333

System.Web.UI.WebControls.WizardNavigationEventHandler.Invoke(Object sender, WizardNavigationEventArgs e) +0

System.Web.UI.WebControls.Wizard.OnNextButtonClick(WizardNavigationEventArgs e) +115

System.Web.UI.WebControls.Wizard.OnBubbleEvent(Object source, EventArgs e) +413

System.Web.UI.WebControls.WizardChildTable.OnBubbleEvent(Object source, EventArgs args) +19

System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +37

System.Web.UI.WebControls.Button.OnCommand(CommandEventArgs e) +125

System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +167

System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10

System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13

System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36

System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +5563

<connectionStrings>
  <!-- Connection String for SQL Server 2005/2008 with Integrated Security -->
  <!--<add name="yafnet" connectionString="data source=(local);initial catalog=yafnet;integrated security=SSPI" providerName="System.Data.SqlClient" />-->

  <!-- Connection String for SQL Server 2005/2008 with Standard Security -->
  <!--<add name="yafnet" connectionString="Data Source= WINMSSQL01;Initial Catalog=yafnet;User ID=seliminyeri;Password=your password" providerName="System.Data.SqlClient" />-->
    
  <!-- Connection String for SQL Server 2005/2008 Express -->
  <!--<add name="yafnet" connectionString="Data Source= WINMSSQL01\SQLExpress;Integrated Security=True;User Instance=True;AttachDBFilename=|DataDirectory|Database.mdf;" providerName="System.Data.SqlClient"/>-->
</connectionStrings>

Errors in place that pictures here

Sorry my english not know my problem was resolved mass of the subject Do you?

you need to remove the on the connection string

<connectionStrings>
  <!-- Connection String for SQL Server 2005/2008 with Standard Security -->
  <add name="yafnet" connectionString="Data Source= WINMSSQL01;Initial Catalog=yafnet;User ID=seliminyeri;Password=your password" providerName="System.Data.SqlClient" />
</connectionStrings>