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ModelErrorMessage.md

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ModelErrorMessage

The ModelErrorMessage component displays model state error messages for a specific key, matching the ASP.NET Web Forms <asp:ModelErrorMessage> control. It renders a <span> containing the error text when errors exist for the given key, and nothing when there are no errors.

Original Microsoft documentation for the ASP.NET ModelErrorMessage control is at: https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.modelerrormessage?view=netframework-4.8

Features Supported in Blazor

  • ModelStateKey — The key in the model state / EditContext to display errors for
  • AssociatedControlID — The ID of the associated input control, used with SetFocusOnError
  • SetFocusOnError — When true, focuses the associated control when an error is displayed
  • CssClass — CSS class applied to the rendered <span>
  • Style — Inline style applied to the rendered <span>
  • ToolTip — Title attribute on the rendered <span>
  • Enabled — Enable or disable the component
  • Visible — Show or hide the component

Web Forms Features NOT Supported

  • IsValid / Validate() — ModelErrorMessage is a display-only control; validation logic is handled by the EditContext
  • HeaderText / HeaderStyle — Not part of the original ModelErrorMessage control

Web Forms Declarative Syntax

<asp:ModelErrorMessage
    runat="server"
    ModelStateKey="string"
    AssociatedControlID="string"
    CssClass="string"
    SetFocusOnError="True|False"
    ID="string"
    Visible="True|False"
/>

Blazor Syntax

<ModelErrorMessage
    ModelStateKey="NewPassword"
    AssociatedControlID="password"
    CssClass="text-danger"
    SetFocusOnError="true" />

HTML Output

When errors exist for the specified ModelStateKey, ModelErrorMessage renders:

<span class="text-danger">The error message text</span>

When there are no errors, nothing is rendered.

Usage Notes

ModelErrorMessage requires a cascading EditContext — it must be placed inside an <EditForm>. It listens for validation state changes and automatically updates when errors are added or removed.

Adding Errors via ValidationMessageStore

In Web Forms, model state errors are added in code-behind via ModelState.AddModelError(). In Blazor, the equivalent pattern uses ValidationMessageStore on the EditContext:

@using BlazorWebFormsComponents.Validations

<EditForm Model="@model" OnValidSubmit="HandleSubmit">
    <input id="password" type="password" @bind="model.NewPassword" />

    <ModelErrorMessage
        ModelStateKey="NewPassword"
        AssociatedControlID="password"
        CssClass="text-danger"
        SetFocusOnError="true" />

    <Button Text="Submit" />
</EditForm>

@code {
    private MyModel model = new();
    private EditContext editContext;
    private ValidationMessageStore messageStore;

    protected override void OnInitialized()
    {
        editContext = new EditContext(model);
        messageStore = new ValidationMessageStore(editContext);
    }

    private void HandleSubmit()
    {
        messageStore.Clear();

        if (string.IsNullOrEmpty(model.NewPassword))
        {
            // Equivalent to ModelState.AddModelError("NewPassword", "...")
            messageStore.Add(editContext.Field("NewPassword"),
                "Password is required.");
            editContext.NotifyValidationStateChanged();
        }
    }
}

Multiple Errors

If multiple errors are added for the same key, they are displayed separated by <br> tags within a single <span>.

Migration Notes

Migration from Web Forms to Blazor is straightforward:

  1. Remove the asp: prefix and runat="server" attribute
  2. Wrap the form in an <EditForm> (replacing <form runat="server">)
  3. Replace ModelState.AddModelError() calls with ValidationMessageStore.Add()

Before (Web Forms)

<asp:ModelErrorMessage runat="server"
    ModelStateKey="NewPassword"
    AssociatedControlID="password"
    CssClass="text-danger"
    SetFocusOnError="true" />

After (Blazor)

<ModelErrorMessage
    ModelStateKey="NewPassword"
    AssociatedControlID="password"
    CssClass="text-danger"
    SetFocusOnError="true" />

!!! tip "Code-behind migration" The biggest change is in the code-behind: Web Forms uses ModelState.AddModelError(key, message) while Blazor uses ValidationMessageStore.Add(editContext.Field(key), message) followed by editContext.NotifyValidationStateChanged().

See Also