c# - How to make two-way binding on Blazor component

Question

I want to create custom input, so I created this component:

MyInputComponent.razor:

<div>
  <input type="text" @bind="BindingValue" />
</div>

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

Then the usage:

<EditForm Model="model" OnValidSubmit="Submit">
    <MyInputComponent BindingValue="model.Name" />
</EditForm>

@code {
    User model = new User() { Name = "My Name" };

    private void Submit()
    {
       // here I found model.Name = null;
   }
}

When I debug MyInputComponent, I found the value as I have entered. But when I submit the form, the value is null.

How to do it ?

Answer 1

Quick answer

Quoting Blazor docs:

Component parameters

Binding recognizes component parameters, where @bind-{property} can bind a property value across components.

For your page:

<EditForm Model="model" OnValidSubmit="Submit">
    <MyInputComponent @bind-BindingValue="model.Name" />
</EditForm>

The child component MyInputComponent:

<div>
  <InputText type="text" @bind-Value="@BindingValue" />
</div>

@code {

    private string _value;

    [Parameter]
    public string BindingValue
    {
        get => _value;
        set
        {
            if (_value == value ) return;
            _value = value;
            BindingValueChanged.InvokeAsync(value);
        }
    }

    [Parameter]
    public EventCallback<string> BindingValueChanged { get; set; }   

}

Notice

• You should to raise binding changes from children component through EventCallback<string> BindingValueChanged.
• I chose BindingValue and BindingValueChanged as identifiers, but, you can use just Value and ValueChanged. Then will be: <MyInputComponent @bind-Value="model.Name" />

Try it at BlazorFiddle.

Edited: See Option 2 below for a clean solution:

---

Your control inside an EditForm

If you want to put your component inside an EditForm and deal with validations, or take other actions using the onchange event, you should to raise EditContext.NotifyFieldChanged. You have 2 options to do it.

Option 1: Raising from EditContext

You can get EditContext from CascadeParameter and invoke NotifyFieldChanged by hand:

    [CascadingParameter] EditContext EditContext { get; set; } = default!;
    [Parameter] public Expression<Func<string>>? ValueExpression { get; set; }
    #endregion

    #region bindedValue
    [Parameter] public EventCallback<string> ValueChanged { get; set; }
    private string _value { set; get; } = "";
    [Parameter]
    public string Value
    {
        get => _value;
        set
        {
            if (_value == value) return;
            _value = value;
            ValueChanged.InvokeAsync(value);
            var fieldIdentifier = FieldIdentifier.Create(ValueExpression);
            EditContext.NotifyFieldChanged(fieldIdentifier);

        }
    }

Option 2 (recomended): Through inheritance from InputBase

You can inherit from InputBase<string> and just implement TryParseValueFromString. InputBase will do the work for you,When you inherit from InputBase you have Value, ValueChanged, EditContext, etc.

protected override bool TryParseValueFromString(string? value, out string result, [NotNullWhen(false)] out string? validationErrorMessage)
{
    result = value ?? "";
    validationErrorMessage = null;
    return true;
}