Hi I am trying to do a MVVM wpf project and I receive this error on the MainWindow. xaml view, the error is appearing on the user controls as well. However, I am able to launch the exe. Can someone assist me with this, I am using Visual Studio 2019 community edition, entity framework 6.4.4. I think it is to do with my entity framework, but I really do not what is wrong here. This is my connection string. <add name="DBoneDBContext" connectionString="data source=DE21TOP-123G2C0\SQLEXPRESS;initial catalog=KAL_Local_Reasonsv1;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework" providerName="System.Data.SqlClient" />

My running theory is that in your XAML file or codebehind you are hooking up DataContext or creating an instance of one of your data classes. You are doing that by querying the DB. However your app's configuration files and therefore the DB are not available at design time and therefore it will fail. You should not do anything related to querying data or runtime stuff at design time. Basically that means your XAML code and constructors should not have any code that requires DB access. All this is executed in the designer sandbox and will fail. At design time use the designer data context to associate your data types (not instances) to get Intellisense. If you really need data then use a design-time only data instance.

XAML designer in WPF project throws error - no connection string named xxx could be found.

Accepted answer

Michael Taylor 51,346 Reputation points

2022-08-15T14:50:43.133+00:00

My running theory is that in your XAML file or codebehind you are hooking up DataContext or creating an instance of one of your data classes. You are doing that by querying the DB. However your app's configuration files and therefore the DB are not available at design time and therefore it will fail.

You should not do anything related to querying data or runtime stuff at design time. Basically that means your XAML code and constructors should not have any code that requires DB access. All this is executed in the designer sandbox and will fail. At design time use the designer data context to associate your data types (not instances) to get Intellisense. If you really need data then use a design-time only data instance.
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Kalpana 21 Reputation points

2022-10-17T07:39:51.177+00:00

This makes so much of sense, but how can I avoid this in my application.
For example, let's say I have an ellipse that has to be filled with a colour that is determined by the returned database value. How can I load this into the application but not in the constructor class?
Any advice would be helpful.

So sorry for the late reply.

Michael Taylor 51,346 Reputation points

2022-10-17T13:46:41.227+00:00

You have to defer the loading until runtime. I believe the best place for this is the Loaded event of the main window. When this event is raised then the UI is ready to be rendered but hasn't yet. This is your chance to load any runtime-specific stuff. AFAIK this event isn't raised in the designer.

Kalpana 21 Reputation points

2022-10-18T02:38:45.387+00:00

Hi cooladtx,
Would that break the mvvm pattern if I include a loaded event ?
I have tried the design time method, however, I m having difficulties in getting my main window to render properly during the design time, the user control is rendered fine.
This is my code

MainWindow xaml : -

<Border Background="Black" Grid.Row="3" Grid.Column="0" > <uc:MachineButtonView></uc:MachineButtonView> </Border>

MachineButtonView xaml:-

<UserControl x:Class="Orlofv4.View.MachineButtonView" 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:local="clr-namespace:Orlofv4.View" mc:Ignorable="d" d:DesignHeight="450" d:DesignWidth="800" d:DataContext="{d:DesignInstance local:MachineButtonViewModel}"> <Grid> <Button Style="{StaticResource MachineButton}" > </Button> </Grid> </UserControl>

I think my question is, how can I host a user control with design time data context in the main window? Is this something that I should create a new post or shall I just continue with this post, please do let me know.

Michael Taylor 51,346 Reputation points

2022-10-18T13:33:18.047+00:00

At design time you have no access to resources that you normally would at runtime including network connections, APIs and databases. If you want to show some design time data to help with styling/displaying the control in the designer then you'll need to use the design time context. This is independent of the data context used at runtime and is strictly for setting up sample data in the designer.

To do that you need to add a couple namespaces to the XAML file if they are not there yet.

<Window ... xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d"> ... </Window>

Then you add the data context for design on the control(s) that need it.

<SomeElement d:DesignContext="{d:DesignData /SampleData}"

Refer to this article on the full process and what you can do with the design context. The end result is that at design time you can see data so you know what it might look like at runtime but the data is just sample data. You cannot access dynamic data in the designer, nor would it make sense since it would be inconsistent.

Kalpana 21 Reputation points

2022-10-20T02:03:08.277+00:00

Thanks, yeap, I found those, but the controls goes missing during runtime, I read that that's how it works as it a design time control. I am looking at using the loaded event to load the data context in, perhaps that would work..
Just confused if its against the MVVM rule, but I have seen some articles implementing it at the loaded event in the view's code behind. As in, I call the data context in loaded event handler instead of at the constructor level.

Michael Taylor 51,346 Reputation points

2022-10-20T14:23:28.417+00:00

Yes the d:DesignContext is design time only. But you should also have the DataContext property set and that is what would be used at runtime.

In terms of whether it is MVVM or not I believe so because setting the context is the only way to bind the data object itself. If that has to be done programmatically then the loaded event is approriate. But this is just my opinion.
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XAML designer in WPF project throws error - no connection string named xxx could be found.

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