I'm creating a Blazor component and running into nullable type issues

MyComponentTest.razor(3,36): error CS0407: 'int? MyComponentTest.DivideByFour(string?)' has the wrong return type

In MyComponentTest.razor, it's expecting me to pass a Func<string, T> to ToValueFunc, even though in MyComponent.razor ToValueFunc is defined as being Func<string?, T?>.

Three things worth mentioning:

• This is just a minimal example to illustrate the issue and is not the same as my production code
• The .csproj specifies <TargetFramework>net9.0</TargetFramework> and <Nullable>enable</Nullable>
• Type T should be able to be either a reference or a value type

MyComponent.razor:

@typeparam T

<div>@ToValueFunc(Text)</div>

@code {
    [Parameter]
    public string? Text { get; set; }

    [Parameter, EditorRequired]
    public Func<string?, T?> ToValueFunc { get; set; } = default!;
}

MyComponentTest.razor:

@page "/mycomponenttest"

<MyComponent T="int" ToValueFunc="@DivideByFour" Text="@_text" />

@code {
    private string? _text = "4";
    
    private int? DivideByFour(string? text)
        => int.TryParse(text, out int value) ? value / 4 : null;
}

Why is this happening and is there any way to get it to obey the nullability specified in MyComponent.razor?

I'm creating a Blazor component and running into nullable type issues

MyComponentTest.razor(3,36): error CS0407: 'int? MyComponentTest.DivideByFour(string?)' has the wrong return type

In MyComponentTest.razor, it's expecting me to pass a Func<string, T> to ToValueFunc, even though in MyComponent.razor ToValueFunc is defined as being Func<string?, T?>.

Three things worth mentioning:

• This is just a minimal example to illustrate the issue and is not the same as my production code
• The .csproj specifies <TargetFramework>net9.0</TargetFramework> and <Nullable>enable</Nullable>
• Type T should be able to be either a reference or a value type

MyComponent.razor:

@typeparam T

<div>@ToValueFunc(Text)</div>

@code {
    [Parameter]
    public string? Text { get; set; }

    [Parameter, EditorRequired]
    public Func<string?, T?> ToValueFunc { get; set; } = default!;
}

MyComponentTest.razor:

@page "/mycomponenttest"

<MyComponent T="int" ToValueFunc="@DivideByFour" Text="@_text" />

@code {
    private string? _text = "4";
    
    private int? DivideByFour(string? text)
        => int.TryParse(text, out int value) ? value / 4 : null;
}

Why is this happening and is there any way to get it to obey the nullability specified in MyComponent.razor?

• c#
• asp-core
• razor
• blazor
• -9.0

1 Answer 1

Your Func specifies a nullable T?, so your MyComponent declaration should be:

<MyComponent T="int?" ToValueFunc="@DivideByFour" Text="@_text" />

Or let the compiler sort it out with:

<MyComponent ToValueFunc="@DivideByFour" Text="@_text" />

You should also probably null check ToValueFunc like this in MyComponent:

@typeparam T

<div>@(Text is null ?  "Not Set" : ToValueFunc(Text))</div>

@code {
    [Parameter] public string? Text { get; set; }

    [Parameter, EditorRequired] public Func<string?, T?> ToValueFunc { get; set; } = default!;

    protected override void OnInitialized()
    {
        ArgumentNullException.ThrowIfNull(ToValueFunc, nameof(ToValueFunc));
    }
}