Child content element uses the same parameter name ('context')
Environment
| Product | UI for Blazor |
Description
The Blazor app produces exceptions when using RenderFragment templates in Telerik Blazor components.
Error Message
The exception message can vary by component name, but the common wording will be similar to:
The child content element ChildContent of component GridCommandColumn uses the same parameter name (context) as enclosing child content element ChildContent of component EditForm. Specify the parameter name like: <ChildContent Context="another_name"> to resolve the ambiguity.
or
The child content element DetailTemplate of component TelerikGrid uses the same parameter name (context) as enclosing child content element DetailTemplate of component TelerikGrid. Specify the parameter name like: <DetailTemplate Context="another_name"> to resolve the ambiguity.
or
RZ9999: The child content element Template of component GridColumn uses the same parameter name (context) as enclosing child content element ... of component .... Specify the parameter name like <Template Context="another_name"> to resolve the ambiguity.
You may also see the following error:
Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type.
Possible Cause
When nesting components, you will usually do that under tags of type RenderFragment. For example, the DetailTemplate or the GridCommandColumn are such examples. Blazor provides an internal variable called context for the delegate of the RenderFragment. However, if there are nested RenderFragments, this will create multiple context variables with the same name in the same programming context. This conflict triggers the exception.
Solution
Use named context variables, as the exception message suggests. Each RenderFragment should expose a Context parameter that lets you choose a name for the variable that you will use instead of the default context. This lets you nest the same tags inside one another, and it also resolves the type of the context data.
The example below uses three-level Grid hierarchy with different RenderFragments with a Context parameter: DetailTemplate, column Template and GridCommandColumn.
Use nested RenderFragments with Telerik UI for Blazor
<p>CATEGORY Grid with nested <code>RenderFragment</code> templates.</p>
<TelerikGrid Data="@CategoryData">
<GridColumns>
<GridColumn Field="@nameof(Category.Name)" Title="Category">
<Template Context="categoryColumnContext">
@{
var categoryItem = categoryColumnContext as Category;
}
<strong>@categoryItem.Name</strong>
</Template>
</GridColumn>
<GridCommandColumn Context="categoryCommandContext">
@{
var categoryItem = categoryCommandContext as Category;
}
<GridCommandButton>Button for <strong>@categoryItem.Name</strong></GridCommandButton>
</GridCommandColumn>
</GridColumns>
<DetailTemplate Context="categoryDetailContext">
@{
var parentCategoryItem = categoryDetailContext as Category;
}
<p>PRODUCT Grid with nested <code>RenderFragment</code> templates.</p>
<TelerikGrid Data="@ProductData.Where(x => x.CategoryId == parentCategoryItem.Id)">
<GridColumns>
<GridColumn Field="@nameof(Product.Name)" Title="Product">
<Template Context="productColumnContext">
@{
var productItem = productColumnContext as Product;
}
<em>@productItem.Name</em>
</Template>
</GridColumn>
<GridCommandColumn Context="productCommandContext">
@{
var productItem = productCommandContext as Product;
}
<GridCommandButton>Button for <em>@productItem.Name</em></GridCommandButton>
</GridCommandColumn>
</GridColumns>
<DetailTemplate Context="productDetailContext">
@{
var parentProductItem = productDetailContext as Product;
}
<p>PART Grid with nested <code>RenderFragment</code> templates.</p>
<TelerikGrid Data="@PartData.Where(x => x.ProductId == parentProductItem.Id)">
<GridColumns>
<GridColumn Field="@nameof(Part.Name)" Title="Part">
<Template Context="partColumnContext">
@{
var partItem = partColumnContext as Part;
}
@partItem.Name
</Template>
</GridColumn>
<GridCommandColumn Context="partCommandContext">
@{
var partItem = partCommandContext as Part;
}
<GridCommandButton>Button for @partItem.Name</GridCommandButton>
</GridCommandColumn>
</GridColumns>
</TelerikGrid>
</DetailTemplate>
</TelerikGrid>
</DetailTemplate>
</TelerikGrid>
@code {
private List<Category> CategoryData { get; set; }
private List<Product> ProductData { get; set; }
private List<Part> PartData { get; set; }
protected override void OnInitialized()
{
CategoryData = new List<Category>();
ProductData = new List<Product>();
PartData = new List<Part>();
for (int i = 1; i <= 3; i++)
{
CategoryData.Add(new Category()
{
Id = i,
Name = "Category " + i
});
}
for (int i = 1; i <= 6; i++)
{
ProductData.Add(new Product()
{
Id = i,
CategoryId = i % 3 + 1,
Name = "Product " + i
});
}
for (int i = 1; i <= 12; i++)
{
PartData.Add(new Part()
{
Id = i,
ProductId = i % 6 + 1,
Name = "Part " + i
});
}
}
public class Category
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Product
{
public int Id { get; set; }
public int CategoryId { get; set; }
public string Name { get; set; }
}
public class Part
{
public int Id { get; set; }
public int ProductId { get; set; }
public string Name { get; set; }
}
}