如何查看Blazor是否设置了属性？

Question

My biggest issue here is when I have a property that can be nullable, but I want to check if it have been set, even if the value is null

Inside MyComponent I what I want to differ is this two cases

Passing Foo property

<MyComponent Foo="@foo" />

@code {
    public Foo foo { get; set; }
}

Not passing Foo property

<MyComponent  />

But inside MyComponent , in both cases, Foo will be null .

How can I know that Foo property is being passed, even if the value is the default value?

Answer 1

You can override the SetParametersAsync method to inspect the parameters that are set or not. Even if the value is null. If the parameter is not specified it will not be present in the collection.

Eg

@code {

    [Parameter]
    public Foo Foo { get; set; }

    public override async Task SetParametersAsync(ParameterView parameters)
    {
        foreach(var prm in parameters)
        {
            System.Diagnostics.Debug.WriteLine($"Name: {prm.Name}");
            System.Diagnostics.Debug.WriteLine($"Value: {prm.Value?.ToString()}");
        }

        await base.SetParametersAsync(parameters);

    }
}

Documentation: https://learn.microsoft.com/en-us/do.net/api/microsoft.as.netcore.components.componentbase.setparametersasync?view=as.netcore-3.1

Answer 2

<h3>Component</h3>

@code {
    private bool _setFromOutside;
    private Foo _foo = new Foo(); //values set from inside of your component

    [Parameter]
    public Foo Foo
    {
        get => _foo;
        set
        {
            _setFromOutside = true;
            _foo = value;
        }
    }
}