C# – What’s the best way to expose a Model object in a ViewModel

cdesign-patternsentity-frameworkmvvmwpf

In a WPF MVVM application, I exposed my model object into my viewModel by creating an instance of Model class (which cause dependency) into ViewModel. Instead of creating separate VM properties, I wrap the Model properties inside my ViewModel Property.

My model is just an entity framework generated proxy class:

public partial class TblProduct
{
    public TblProduct()
    {
        this.TblPurchaseDetails = new HashSet<TblPurchaseDetail>();
        this.TblPurchaseOrderDetails = new HashSet<TblPurchaseOrderDetail>();
        this.TblSalesInvoiceDetails = new HashSet<TblSalesInvoiceDetail>();
        this.TblSalesOrderDetails = new HashSet<TblSalesOrderDetail>();
    }

    public int ProductId { get; set; }
    public string ProductCode { get; set; }
    public string ProductName { get; set; }
    public int CategoryId { get; set; }
    public string Color { get; set; }
    public Nullable<decimal> PurchaseRate { get; set; }
    public Nullable<decimal> SalesRate { get; set; }
    public string ImagePath { get; set; }
    public bool IsActive { get; set; }

    public virtual TblCompany TblCompany { get; set; }
    public virtual TblProductCategory TblProductCategory { get; set; }
    public virtual TblUser TblUser { get; set; }
    public virtual ICollection<TblPurchaseDetail> TblPurchaseDetails { get; set; }
    public virtual ICollection<TblPurchaseOrderDetail> TblPurchaseOrderDetails { get; set; }
    public virtual ICollection<TblSalesInvoiceDetail> TblSalesInvoiceDetails { get; set; }
    public virtual ICollection<TblSalesOrderDetail> TblSalesOrderDetails { get; set; }
}

Here is my ViewModel:

public class ProductViewModel : WorkspaceViewModel
{
    #region Constructor
    public ProductViewModel()
    {
        StartApp();
    }

    #endregion //Constructor

    #region Properties

    private IProductDataService _dataService;
    public IProductDataService DataService
    {
        get
        {
            if (_dataService == null)
            {
                if (IsInDesignMode)
                {
                    _dataService = new ProductDataServiceMock();
                }
                else
                {
                    _dataService = new ProductDataService();
                }
            }
            return _dataService;
        }

    }

    //Get and set Model object
    private TblProduct _product;
    public TblProduct Product
    {
        get
        {
            return _product ?? (_product = new TblProduct());
        }
        set
        {
            _product = value;
        }
    }

    #region Public Properties

    public int ProductId
    {
        get
        {
            return Product.ProductId;
        }
        set
        {
            if (Product.ProductId == value)
            {
                return;
            }

            Product.ProductId = value;
            RaisePropertyChanged("ProductId");
        }
    }

    public string ProductName
    {
        get
        {
            return Product.ProductName;
        }
        set
        {
            if (Product.ProductName == value)
            {
                return;
            }

            Product.ProductName = value;
            RaisePropertyChanged(() => ProductName);
        }
    }

    private ObservableCollection<TblProduct> _productRecords;
    public ObservableCollection<TblProduct> ProductRecords
    {
        get { return _productRecords; }
        set
        {
            _productRecords = value;
            RaisePropertyChanged("ProductRecords");
        }
    }

    //Selected Product
    private TblProduct _selectedProduct;
    public TblProduct SelectedProduct
    {
        get
        {
            return _selectedProduct;
        }
        set
        {
            _selectedProduct = value;
            if (_selectedProduct != null)
            {
                this.ProductId = _selectedProduct.ProductId;
                this.ProductCode = _selectedProduct.ProductCode;
            }
            RaisePropertyChanged("SelectedProduct");
        }
    }

    #endregion  //Public Properties

    #endregion  // Properties

    #region Commands

    private ICommand _newCommand;
    public ICommand NewCommand
    {
        get
        {
            if (_newCommand == null)
            {
                _newCommand = new RelayCommand(() => ResetAll());
            }
            return _newCommand;
        }
    }

    private ICommand _saveCommand;
    public ICommand SaveCommand
    {
        get
        {
            if (_saveCommand == null)
            {
                _saveCommand = new RelayCommand(() => Save());
            }
            return _saveCommand;
        }
    }

    private ICommand _deleteCommand;
    public ICommand DeleteCommand
    {
        get
        {
            if (_deleteCommand == null)
            {
                _deleteCommand = new RelayCommand(() => Delete());
            }
            return _deleteCommand;
        }
    }

    #endregion //Commands

    #region Methods

    private void StartApp()
    {
        LoadProductCollection();
    }

    private void LoadProductCollection()
    {
        var q = DataService.GetAllProducts();
        this.ProductRecords = new ObservableCollection<TblProduct>(q);
    }


    private void Save()
    {
        if (SelectedOperateMode == OperateModeEnum.OperateMode.New)
        {
              //Pass the Model object into Dataservice for save
              DataService.SaveProduct(this.Product);
        }
        else if (SelectedOperateMode == OperateModeEnum.OperateMode.Edit)
        {
             //Pass the Model object into Dataservice for Update
              DataService.UpdateProduct(this.Product);
        }
        ResetAll();
        LoadProductCollection();
    }


    #endregion  //Methods
}

Here is my Service class:

class ProductDataService:IProductDataService
{
    /// <summary>
    /// Context object of Entity Framework model
    /// </summary>
    private MaizeEntities Context { get; set; }

    public ProductDataService()
    {
        Context = new MaizeEntities();
    }

    public IEnumerable<TblProduct> GetAllProducts()
    {
        using(var context=new R_MaizeEntities())
        {
           var q = from p in context.TblProducts
                   where p.IsDel == false
                   select p;

            return new ObservableCollection<TblProduct>(q);
        }
    }

    public void SaveProduct(TblProduct _product)
    {
        using(var context=new R_MaizeEntities())
        {
            _product.LastModUserId = GlobalObjects.LoggedUserID;
            _product.LastModDttm = DateTime.Now;
            _product.CompanyId = GlobalObjects.CompanyID;
            context.TblProducts.Add(_product);

            context.SaveChanges();
        }
    }

    public void UpdateProduct(TblProduct _product)
    {
        using (var context = new R_MaizeEntities())
        {
            context.TblProducts.Attach(_product);

            context.Entry(_product).State = EntityState.Modified;
            _product.LastModUserId = GlobalObjects.LoggedUserID;
            _product.LastModDttm = DateTime.Now;
            _product.CompanyId = GlobalObjects.CompanyID;
            context.SaveChanges();
        }
    }

    public void DeleteProduct(int _productId)
    {
        using (var context = new R_MaizeEntities())
        {
            var product = (from c in context.TblProducts
                    where c.ProductId == _productId
                    select c).First();

            product.LastModUserId = GlobalObjects.LoggedUserID;
            product.LastModDttm = DateTime.Now;
            product.IsDel = true;
            context.SaveChanges();
        }
    }


}

I exposed my model object in my viewModel by creating an instance of it using new keyword, also I instantiated my DataService class in VM. I know this will cause a strong dependency.

So:

What's the best way to expose a Model object in a ViewModel?
What's the best way to use DataService in VM?

Best Answer

As a general rule, it is not a good idea to expose a Model object in a ViewModel object. The form of the ViewModel class should be determined by the needs of the View that will use it. The form of the Model class should be determined by the concerns of your data model.

In my experience with Entity Framework and the MVVM pattern, I have found it best to transform the EF data as I populate my ViewModel object. The ViewModel object will then contain the data from the data model but in a different form. If you need to allow the user to change data, then you often want to have methods that transform the modified ViewModel data back into Model data so that you can persist it to your data store.

Related Solutions

Entity Framework in MVVM WPF – Best Way to Wire Up Database Context

I've looked into this quite a bit and haven't found a "perfect" solution. The repository pattern works wonderfully for MVC applications where the context is short lived because it exists in a short lived controller, but the problem happens when you try to apply that same structure to a wpf app where the VM can persist for long periods of time.

I have used this solution in the past which is much more simple than many of the repo patterns I have seen that attempt to abstract things out to an extreme amount, resulting in near unreadable amounts of code that are difficult to debug. Here are the steps...

Create a separate project for the EDMX to act as your Data access layer
Create a "Repositories" folder under the same project
Create a base class "BaseRepository" to act as the "Unit of Work". IDisposable will allow you to use this in a using(){} and the partial will allow you do implement other repositories
```
public partial class MyEntityRepository : IDisposable
{
    MyEntities context = new MyEntities();

    public void Dispose()
    {
        context.Dispose();
    }
}
```

Create another file called "MyOtherRepository". create the same partial class but implement methods based on what you want that file to contain

public partial class MyEntityRepository
{
    public void MyOtherMethodSave(EntityObject obj)
    {
        //work with context
        ...

        context.SaveChanges();
    }
}

Now in your VM you can do this...

using(MyEntityRepository repo = new MyEntityRepository())
{
     repo.MyOtherMethodSave(objectToSave);
}

This groups all your repositories under one class so you don't have to deal with separate context. It allows you better manage different repos by grouping the methods into different files and helps prevent code duplication. On top of that, your contexts are as short lived as they were without using this pattern.

The disadvantage is that with larger systems, you may have a lot of methods that get bundled under your repo. One solution in that case would be to implement some basic common commands like "Find" or "Add", and implement specialized ones in their respective repository.

MVVM Pattern – Model and ViewModel for WPF View

This is how I see (and do) it.

In your case (which is rather simple), I would have a ViewModel holding a list of Models and a SelectedModel property. The view would have a form and a list of course. The form would be bound to the currently selected model. The View would be bound to the ViewModel and indirectly to the Model (the data in the list and the form). Your ViewModel would also take care of validating and saving the data to the database by calling a method on your repository (or the Context, which is basically a repository, if you're using EF).

The Model is your business logic, the Domain Model. The objects here will have all the data relevant to the business and the methods to manipulate that data. The model should be designed to be as simple as possible while accomplishing the business needs. This means normalized relationships, decoupled design and so on. Make it as easy to maintain as possible, regardless of the view. This means the same Model can be reused in different applications (a desktop and a web app can use the same model). In your case this is an object with the Name, Address and Description. This is all you need to accomplish your goal.

The View will not always be this simple. Sometimes you will need to aggregate data from multiple models, or do some other manipulation of the data just to show the data on a view. A report for instance, can have lots of data from lots of models. Other times, the view will really need to simplify a lot of complex things that are going on in the model, to give you a high level overview without too many details. In your case, the view is also a bit more complex than your domain model. You need a list to see all your domain models and a form to edit one at the same time.

This is where the ViewModel comes in. The VM will be between the model and the view. It will wrap a model object (or multiple model objects), introduce new properties that are a combination of other properties in the model and so on. The ViewModel is the one that is designed in a way that it makes presentation easier. This means simple properties your view can bind to and similar things. No matter how complex your Model or your View is, your ViewModel sits in between and makes all the necessary transformations of the data that you want to display. The same thing but in reverse happens when a view is sending data back (form submission or other input). The ViewModel is the one that transforms the view data into something that your Model understands. In your case the ViewModel will be rather simple, holding just the list of Models and a Selected Model, but in more complex examples, it might do some calculations or whatnot to accomplish what the view needs.

Best Answer

Related Solutions

Entity Framework in MVVM WPF – Best Way to Wire Up Database Context

MVVM Pattern – Model and ViewModel for WPF View

Related Topic