ConnectionString Not Found in App.Config but is Definitely There

Question

I am getting an issue that I have seen posted before but the answers seem only applicable to ASP.NET MVC, whereas I am using WPF (with Entity Framework).

My XAML designer in VS currently looks like this:

This is on a UserControl where the code is very simple:

<UserControl x:Class="door_system.Views.HomeView"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
                 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
                 xmlns:viewModels="clr-namespace:door_system.ViewModels"
                 mc:Ignorable="d" 
                 d:DesignHeight="300" d:DesignWidth="300">

    <UserControl.DataContext>
        <viewModels:HomeViewModel/>
    </UserControl.DataContext>

    <Grid>
        <DataGrid AutoGenerateColumns="False">
            <DataGridTextColumn Width="*"/>
            <DataGridTextColumn Width="*"/>
            <DataGridTextColumn Width="*"/>
        </DataGrid>
    </Grid>
</UserControl>

So my project setup looks like so:

There is only one app.config file that definitely contains the connection string:

<connectionStrings>
  <add name="MicrotrakContext" connectionString="metadata=res://*/Models.MicrotrakModel.csdl|res://*/Models.MicrotrakModel.ssdl|res://*/Models.MicrotrakModel.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=*******;initial catalog=MicrotrakEvolution;user id=******;password=********;multipleactiveresultsets=True;application name=EntityFramework&quot;" providerName="System.Data.EntityClient" />
</connectionStrings>

When I build the project there are no errors. When I run the project it performs without error too. Is this a bug I have discovered?

Aside from name changes I have two other projects that are setup in almost exactly the same way, both build and run without issue.

The Task calling the Context in HomeViewModel:

private Task LoadTransactionsTask()
{
    return Task.Run(() =>
    {
        using (var context = new MicrotrakContext())
        {
            AllTransactions = new ObservableCollection<Transaction>(
                (from record in context.Transactions
                    select record)
                .Where(x => x.CreationTime >= DateTime.Today)
                .ToList());
        }
    });
}

Answer 1

Why the VS Designer throws exception

Some of your application code (e.g. HomeViewModel) is executed by the XAML designer process. Your code (e.g. new MicrotrakContext()) looks for configuration file of the current process, not your app.config.

How to fix it

You should prevent your application to access database, file system or any other I/O operations and provide some design time data instead.

I've described some techniques here: Daniel Turan's answer to: Does mmvm stop the ability for Visual Studio designer to show XAML

Basically, you can check in your code:

if (DesignerProperties.GetIsInDesignMode(someDepenencyObject))
{
    LoadDesignTimeData();
}
else
{
    LoadRealData();
}

EDIT:
my prefered way is DesignTime viewmodel:

public class HomeViewModelDesignTime : HomeViewModel
{
    protected override Task LoadTransactionsTask()
    {
       //load some hard coded design time data
    }
}

tell the designer to use this class:

<UserControl x:Class="door_system.Views.HomeView"
    d:DataContext="{d:DesignInstance Type=local:HomeViewModelDesignTime, IsDesignTimeCreatable=true}"

you can than instatiate you real ViewModel in codebehind for example.

TIP: create your own Project Item Template in Visual Studio, that will create UserControl with ViewModel and DesignTimeViewModel for you, so you don't have do it manually each time.

Answer 2

Is you code compiling properly at this point: using (var context = new MicrotrakContext()) <-- Does it know what MicrotrakContext() is?

You can do a quick search for: : base("name= the expectation would be MicrotrakContext will be the value following name=.

Answer 3

I had the same problem. This happens because you are trying to do some database access in design time. To solve this problem:

1. Remove all the code from the User Control constructor (just let there the InicializeComponent method); The removed code, should be inserted in the userControl_Load method;

2. Just verify if it is in design mode before executing the methods that have queries.

   if (!DesignMode)
   {
     // Functions that have any database access should be here (eg. queries, inserts, etc)
   }