Enhancing Blazor Validation with ValueExpression

Introduction

Blazor’s form validation system provides a robust way to enforce data integrity using EditForm, EditContext, and validation attributes. However, when using third-party components like Syncfusion’s SfDropDownList, validation can sometimes fail to display errors correctly. One common fix is to utilize ValueExpression, which helps Blazor correctly associate the validation context with your dropdown.

In this guide, we'll explore:

• Why ValueExpression is needed for validation in Blazor components
• How to integrate ValueExpression with SfDropDownList
• Step-by-step implementation in a Blazor application

Understanding ValueExpression in Blazor Validation

Blazor’s validation system relies on expressions to correctly associate fields with validation messages. Standard input components, like <InputText> and <InputSelect>, automatically generate a binding expression, ensuring validation messages display correctly.

However, third-party components like Syncfusion’s SfDropDownList don’t automatically generate these expressions, which can result in missing validation messages.

Using ValueExpression, we explicitly link the dropdown’s Value property to Blazor’s validation system, ensuring errors are properly displayed.

Implementing ValueExpression with SfDropDownList

Step 1: Modify the Wrapper Component

If your SfDropDownList is wrapped inside a custom Blazor component, you need to pass ValueExpression down to ensure proper validation.

Inside your wrapper component (CustomDropdown.razor), define a ValueExpression parameter:

[Parameter] public Expression<Func<int?>> ValueExpression { get; set; }

Step 2: Pass ValueExpression to SfDropDownList

Modify the <SfDropDownList> to receive and use the ValueExpression parameter, while also ensuring the ValueChanged event updates the bound model property:

<SfDropDownList TValue="int?"
                Value="@SelectedValue"
                ValueChanged="@((int? newValue) => SelectedValue = newValue)"
                ValueExpression="@ValueExpression"
                AllowFiltering="true"
                Placeholder="@Placeholder">
</SfDropDownList>
<ValidationMessage For="@ValueExpression" />

Step 3: Pass ValueExpression from Parent Component

In your EditForm, when using the wrapped dropdown component, make sure to pass the ValueExpression explicitly and bind the value change event properly:

<CustomDropdown Value="@Model.SelectedOption"
                ValueChanged="@((int? newValue) => Model.SelectedOption = newValue)"
                ValueExpression="@(() => Model.SelectedOption)" />

Why This Works

✅ Ensures validation binding is correctly associated
✅ Allows ValidationMessage to display errors for SfDropDownList
✅ Works seamlessly with Blazor’s built-in validation system
✅ Prevents missing error messages on form submission
✅ Ensures data binding updates the model correctly

Wrapping Up

If your SfDropDownList isn’t displaying validation errors, implementing ValueExpression is a simple yet powerful fix. It explicitly links the dropdown’s value with the validation system, ensuring error messages appear correctly.

Additionally, passing ValueChanged ensures the model correctly reflects dropdown selections.

With this technique, you’ll have fully functional Blazor validation, even when using third-party components like Syncfusion’s SfDropDownList. 🚀