Your Blazor component needs data in a certain format, where to Map?

Blazor components can accept parameters of virtually any type.

This makes for a simple option when you fetch data from a backend API/service as you can often use that data “as is” in your component.

For example, you might call an API to fetch product details which returns a list of items of type Product. The chances are you can just use that directly in your Blazor UI.

@foreach(var product in products) {
    <ProductDetails Product="@product"/>
}

In this example the ProductDetails component is specific to this page, and tightly coupled to the structure of the incoming Product data, so this feels like a sound approach.

Another option is to have your component accept primitives like string, or int, and send those in to the component.

<ProductDetails name="@product.Name" price="@product.Price" />

Now we could use ProductDetails anywhere we like, so long as we give it values for its Name and Price.

But, what if we have a truly re-usable component that needs to accept data in a certain format.

For example, let’s say we have a TreeView component which takes in a list of TreeItem objects.

TreeView.razor

[Parameter]
public List<TreeItem> Items { get; set; }

TreeItem.cs

public class TreeItem {

    public string Name { get; set; }

    public List<TreeItem> Children { get; set; }

}

You can imagine this could be useful for showing nav items, or a folder tree, or anything else with a ‘Tree Like’ structure.

But now we’ve got a problem!

Every time we use this TreeView component, we need to map the data we’re trying to use (often from a backend API or service) to a list of TreeItem.

For example, we might pull a list of folders from a server and want to render those as a Tree.

FolderView.razor

<TreeView Items="?"/>

@code {

    protected override void OnInitialized() {
        var folders = _backendAPI.ListFolders();
        // map folders to items of type `TreeItem`
    }

}

So where do we put the mapping code?

Well, one option in this case is to write the mapping code directly in the component (FolderView in this case):

FolderView.razor

<TreeView Items="treeItems"/>

@code {

    List<TreeItem> treeItems = new();

    protected override void OnInitialized() {
        var folders = _backendAPI.ListFolders();

        foreach(var folder in folders){
            treeItems.Add(new TreeItem { Name= folder.Name });
            // possibly map sub folders here as well
        }
    }

}

This is probably fine if the mapping code is simple/minimal (as above).

But if you’re mapping between objects with more properties, and perhaps a bit more logic to ensure the correct data comes out the other side, this can turn into a lot of messy code.

So where else could it go?

Enter Extension Methods - An Alternative#

This is where I quite often turn to extension methods.

You can create a separate method (in a separate class, in a separate file) which has the sole job of mapping data from your backend type to the structure needed for your component.

public static class FolderExtensionMethods
{
    public static IEnumerable<TreeItem> ToTreeItems(this List<Api.Folder> folders)
    {
        List<TreeItem> treeItems = new List<TreeItem>();

        foreach (var folder in folders)
        {
            treeItems.Add(new TreeItem { Name = folder.Name });
            // possibly map sub folders here as well
        }

        return treeItems;
    }
}

Or, for bonus points, you can use yield return here for a more succinct version 🎯

public static class FolderExtensionMethods
{
    public static IEnumerable<TreeItem> ToTreeItems(this List<Api.Folder> folders)
    {
        foreach (var folder in folders)
        {
            // additional mapping goes here
            yield return new TreeItem { Name = folder.Name };
        }
    }
}

Either way, you can use this extension method in your component, and your data is successfully mapped!

FolderView.razor

@using FolderExtensionMethods;
<TreeView Items="treeItems"/>

@code {

     IEnumerable<TreeItem> treeItems;

    protected override void OnInitialized()
    {
        var folders = _backendAPI.ListFolders();
        treeItems = _folders.ToTreeItems();
    }

}

Your component remains easy to read, understand and maintain, plus you’ve encapsulated all that unsightly mapping code into a separate method which you can invoke at will.