WPF set DataContext to a ViewModel without default constructor

Question

I have a ViewModel that accepts several constructor parameters. As I understand because of this the only way to set View's DataContext is using code behind.

This has its downsides:

Visual Studio will not show ViewModel inteliscence for the view that is being constructed
There is no way to see design time data that is defined in my ViewModel constructor in XAML designer because designer simply breaks

What are my options?

I would like to have a ViewModel that can accept constructor parameters, have design time data and that my Visual Studio inteliscence provide me suggestions about members in my ViewModel so I could have a good design experience.

PS. I'm using MVVM Toolkit / Windows Community Toolkit from Microsoft, but I would appreciate any answer on how to achieve my end goal. Thank you.

score 6 · Accepted Answer · answered Feb 08 '22 at 14:10

6

What are my options?

A common approach is to create a separate design time view model class with a parameterless constructor and set the design time data context to this type:

<Window ... d:DataContext="{d:DesignInstance Type=local:DesignTimeViewModel, IsDesignTimeCreatable=True}"

What you shouldn't do is to design your application and define your classes according to how the designer in Visual Studio works.

answered Feb 08 '22 at 14:10

mm8

163,881
10
57
88

I ended up using your solution and adding additional parameterless constructor to my ViewModel's that didn't had one. Then in those constructors I just check if I'm in design mode and if that's True I just load design time data from my ViewModel. Thank you. – Alias Feb 27 '22 at 12:25

Peregrine · Answer 2 · 2022-02-08T08:17:27.450

The cleanest way to assign a view-model with a paramterised constructor as the data context for a view is using the ViewModelLocator pattern.

public class ViewModelLocator
{
    public ViewModelLocator()
    {
        // define view-model definitions in IoC container.
    }

    public MainViewModel MainViewModel
    {
        get
        {
            // use the appropriate construct for your choice of IoC container
            var result = IoCContainer.GetInstance<MainViewModel>();
            return result;
        }
    }
}

An instance of this class can be created in App.xaml

<Application.Resources>
    <local:ViewModelLocator x:Key="ViewModelLocator">
</Application.Resources>

which becomes an application wide resource, that can be referenced in each view.

Now we can fetch an instance of a specific view-model at any time, including in the designer, and assign it as the DataContext of the view.

<Window
    x:Class="MyApplication.MainView"
    ...
    DataContext="{Binding Source={StaticResource ViewModelLocator}, Path=MainViewModel}" />

For more details on the usage of this ViewModelLocator pattern, including having design-time data in your ViewModel, check out my blog post.

WPF set DataContext to a ViewModel without default constructor

2 Answers2