Allow generic type constraints

[rkone]

We should be able to add generic constraints to our typeparam. For example:
@typeparam TEntity where TEntity : IEntity

As of preview3 gives the error "Unexpected literal following the 'typeparam' directive".

[stsrki]

I also think this is a much needed feature.

Any comment on this?

[prosser]

Without this feature, developers will be creating a lot of boilerplate components to workaround C# 8.0 #nullable limitations.

for example, take this overly-simplified .razor:

@typeparam TEnum
@typeparam TContainer
@inherits EnumButtonsComponent<TEnum, TContainer>

@if (Visible)
{
    <div class="btn-group-toggle" data-toggle="buttons">
        @foreach (var option in Options)
        {
            <label class="btn btn-primary @(Equals(Value, option.Value) ? "active" : "")">
                <input type="radio" name="@Name" value="@option.Value" checked="@(Equals(Value, option.Value))" onchange="@(() => Value = option.Value)"/>
                @option.Description
            </label>
        }
    </div>
}

And the class it's inheriting from:

public class EnumButtonsComponent<TEnum, TContainer> : EnumPickerComponentBase<TEnum, TContainer>
        where TEnum : struct
{
}

public class EnumPickerComponentBase<TEnum, TContainer> : AutoLabeledControlBase<TContainer>
    where TEnum : struct
{
    private string? stringValue;

    // without the constraint (or adding #nullable disable), TEnum is indeterminate. You cannot
    // mark it TEnum?, because it could be a value type. Not marking it as nullable raises a warning
    // because it could be null.
    private TEnum value = default;
    // implementation...
}

The EnumPickerComponentBase class needs the constraint due to #nullable limitations (have to pick either nullable or not nullable). Without this feature in Blazor, you would need to have a .razor for each type that you wanted an EnumPickerComponentBase-derived template.

[springy76]

If Razor Components would create partial classes (last time I checked this was planned for 3.1.0) then you could define these generics including constraints in your own partial c# class.

[smartprogrammer93]

Isn't this solved now with 3.1 preview 1??? partial classes are published

[springy76]

Isn't this solved now with 3.1 preview 1??? partial classes are published

Someone already tried this?

[stsrki]

I have just tried it and it doesn't work for generic components at all. It works for basic partial class.

This works

public partial class CheckBox
{
    [Parameter] public RenderFragment ChildContent { get; set; }
}

Doesn't work

public partial class CheckBox<TValue>
    where TValue : struct
{
    [Parameter] public TValue Checked { get; set; }

    [Parameter] public EventCallback<TValue> CheckedChanged { get; set; }

    [Parameter] public RenderFragment ChildContent { get; set; }
}

[smartprogrammer93]

This is a very important feature in my opinion. I wish if it can be done by .Net Core 3.1

[SQL-MisterMagoo]

@stsrki Can you explain what doesn't work?

If I add @typeparam TValue to a .razor file and add the same to the code behind partial class, with a constraint it works for me.

Razor file MyClass.razor
@typeparam TValue

Code behind MyClass.razor.cs

public partial class MyClass<TValue> where T:struct

Works as I would expect.

[stsrki]

@SQL-MisterMagoo I was getting an error:

Error RZ9985 Multiple components use the tag 'CheckBox'. Components: BlazorApp10.Shared.CheckBox, BlazorApp10.Shared.CheckBox

You're right. If I add the @typeparam TValue to razor file the error is gone.

[rkone]

Thanks for the solution @SQL-MisterMagoo ! I didn't think about templates, as I just needed properties from the constraints for my control.

[miggat]

I used @SQL-MisterMagoo suggestion, and i have:
Test.razor:
@typeparam TValue

Test.razor.cs
public partial class Test<TValue> where TValue: ISomeInterface

But i get this error on build:
Missing partial modifier on declaration of type 'Test'; another partial declaration of this type exists

Also, notice the generated Test.razor.g.cs constructor
public class Test<TValue> : Microsoft.AspNetCore.Components.ComponentBase

I'm probably doing something wrong since i just landed into Blazor, but i can't find the difference between your solution and mine

[FrancescoLorenzetti84]

I can confirm what @miggat said, I'm experiencing the same issue. This must be a bug.

[SQL-MisterMagoo]

@miggat you need to be on 3.1 for the partial classes to work

[FrancescoLorenzetti84]

@SQL-MisterMagoo I am on 3.1.0-preview4.19579.2 and I have that error

[SQL-MisterMagoo]

@miggat @FrancescoLorenzetti84 Here is a working sample for you

https://github.com/SQL-MisterMagoo/BlazorPartialConstraint

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[mkArtakMSFT]

We've moved this to Backlog as this is also possible to achieve using partial classes.

[joshlang]

Unless something changed since Feb 25, no, it's not.

[poke]

@joshlang You may need to explicitly specify the type argument when you use your generic component. E.g.

<Fancy TItem="ItemType" Item="someItem"></Fancy>

[JayArrowz]

:(

[musictopia2]

Actually I found out I was able to achieve what was needed with partial classes. In that case, maybe no need for changes because a person should use partial classes anyways.

[poke]

In that case, maybe no need for changes because a person should use partial classes anyways.

I wouldn't agree with that since there are plenty examples of components that require a generic constraint but do not really need a code-behind.

It's good that we can achieve what we need right now, so this isn't a pressing issue. The end goal however should be that we can do this without code-behind. I'm totally fine if that comes later with 6.0.

[ghost]

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[javiercn]

This has been implemented in a4a3668 and will be available in 6.0 preview4.