Blazor Forms Validation

Blazor Forms and Validation

EditForm Setup

@rendermode InteractiveServer

<EditForm Model="@_model" FormName="create-item">
    <DataAnnotationsValidator />
    <ValidationSummary />

    <div class="mb-3">
        <label class="form-label">Name</label>
        <InputText @bind-Value="_model.Name" class="form-control" />
        <ValidationMessage For="@(() => _model.Name)" />
    </div>

    <div class="mb-3">
        <label class="form-label">Email</label>
        <InputText @bind-Value="_model.Email" class="form-control" type="email" />
        <ValidationMessage For="@(() => _model.Email)" />
    </div>

    <div class="mb-3">
        <label class="form-label">Category</label>
        <InputSelect @bind-Value="_model.CategoryId" class="form-select">
            <option value="">Select...</option>
            @foreach (var cat in _categories)
            {
                <option value="@cat.Id">@cat.Name</option>
            }
        </InputSelect>
    </div>

    <div class="mb-3">
        <label class="form-label">Accept Terms</label>
        <InputCheckbox @bind-Value="_model.AcceptTerms" />
    </div>

    <button type="submit" class="btn btn-primary" disabled="@_submitting">
        @(_submitting ? "Saving..." : "Submit")
    </button>
</EditForm>

Model with DataAnnotations

public sealed class CreateItemModel
{
    [Required(ErrorMessage = "Name is required")]
    [StringLength(200, MinimumLength = 2)]
    public string Name { get; set; } = "";

    [Required, EmailAddress]
    public string Email { get; set; } = "";

    [Range(1, int.MaxValue, ErrorMessage = "Select a category")]
    public int CategoryId { get; set; }

    [Range(typeof(bool), "true", "true", ErrorMessage = "Must accept terms")]
    public bool AcceptTerms { get; set; }
}

FluentValidation Integration

// Install: Blazored.FluentValidation
public sealed class CreateItemValidator : AbstractValidator<CreateItemModel>
{
    public CreateItemValidator()
    {
        RuleFor(x => x.Name)
            .NotEmpty().WithMessage("Name is required")
            .MaximumLength(200);

        RuleFor(x => x.Email)
            .NotEmpty().EmailAddress();

        RuleFor(x => x.CategoryId)
            .GreaterThan(0).WithMessage("Select a category");
    }
}

@using Blazored.FluentValidation

<EditForm Model="@_model"
    <FluentValidationValidator />
    @* ... inputs ... *@
</EditForm>

SSR Form Handling (.NET 10)

For static SSR pages, use [SupplyParameterFromForm]:

@page "/items/create"

<EditForm Model="@Model" FormName="create-item" method="post">
    <AntiforgeryToken />
    <DataAnnotationsValidator />
    @* inputs *@
</EditForm>

@code {
    [SupplyParameterFromForm]
    private CreateItemModel Model { get; set; } = new();

    private async Task HandleSubmit()
    {
        await ItemService.CreateAsync(Model);
        Navigation.NavigateTo("/items");
    }
}

Input Components

File Upload

<InputFile accept=".pdf,.docx" multiple />

@code {
    private async Task HandleFileSelected(InputFileChangeEventArgs e)
    {
        foreach (var file in e.GetMultipleFiles(maxAllowedFiles: 5))
        {
            if (file.Size > 10 * 1024 * 1024) continue; // 10MB limit

            using var stream = file.OpenReadStream(maxAllowedSize: 10 * 1024 * 1024);
            // Process stream...
        }
    }
}

Data Binding Patterns (from official docs)

Basic @bind

@* Two-way binding - updates on element blur by default *@
<input @bind="inputValue" />
<input @bind="InputValue" />

@code {
    private string? inputValue;
    private string? InputValue { get; set; }
}

@bind:event - Change trigger

@* Update on every keystroke instead of blur *@
<input @bind="searchText" @bind:event="oninput" />

@code {
    private string? searchText;
}

@bind:get / @bind:set - Proper two-way binding with logic

@* CORRECT: Use @bind:get/@bind:set for two-way binding with custom logic *@
<input @bind:get="inputValue" @bind:set="OnInput" />

@code {
    private string? inputValue;

    private void OnInput(string? value)
    {
        var newValue = value ?? string.Empty;
        inputValue = newValue.Length > 4 ? "Long!" : newValue;
    }
}

Important: Do NOT use value="@x" @oninput="handler" for two-way binding - Blazor won't sync the value back. Always use @bind:get/@bind:set.

@bind:after - Run async logic after binding

<input @bind="searchText" @bind:after="PerformSearch" />

@code {
    private string? searchText;

    private async Task PerformSearch()
    {
        // Runs after searchText is updated
        results = await SearchService.SearchAsync(searchText);
    }
}

@bind:format - Date formatting

<input @bind="startDate" @bind:format="yyyy-MM-dd" />

@code {
    private DateTime startDate = new(2020, 1, 1);
}

Child component two-way binding

@* Parent *@
<YearSelector @bind-Year="selectedYear" />

@code {
    private int selectedYear = 2024;
}

@* Child: YearSelector.razor *@
<input @bind:get="Year" @bind:set="YearChanged" />

@code {
    [Parameter] public int Year { get; set; }
    [Parameter] public EventCallback<int> YearChanged { get; set; }
    @* Convention: parameter + "Changed" suffix *@
}

Multiple select binding

<select @bind="SelectedCities" multiple>
    <option value="bal">Baltimore</option>
    <option value="la">Los Angeles</option>
    <option value="sea">Seattle</option>
</select>

@code {
    public string[] SelectedCities { get; set; } = Array.Empty<string>();
}