CustomValidator¶

The CustomValidator component provides custom validation logic using a server-side function. It emulates the ASP.NET Web Forms asp:CustomValidator control, allowing you to define arbitrary validation rules.

Original Web Forms documentation: https://docs.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.customvalidator?view=netframework-4.8

Features Supported in Blazor¶

ServerValidate - Func<string, bool> parameter that executes custom validation logic
ValidateEmptyText - Controls whether empty values are validated
ControlRef - Reference to the input control being validated
Text / ErrorMessage - Inline and summary error messages
ClientValidationFunction - Migration stub (accepted but not executed in Blazor)
IsValid - Indicates whether validation passes

Web Forms Features NOT Supported¶

ClientValidationFunction execution - Blazor does not execute client-side JavaScript validation functions. The property is accepted for migration compatibility.
ValidateOnLoad - Not supported

Web Forms Declarative Syntax¶

<asp:CustomValidator
    ID="CustomValidator1"
    runat="server"
    ControlToValidate="TextBox1"
    ClientValidationFunction="validateLength"
    ErrorMessage="Value must be at least 5 characters."
    OnServerValidate="CustomValidator1_ServerValidate" />

Blazor Syntax¶

@using BlazorWebFormsComponents.Validations

<EditForm Model="@model">
    <InputText @ref="Input.Current" @bind-Value="model.Value" />
    <CustomValidator ServerValidate="@ValidateLength"
                     ControlRef="@Input"
                     Text="Must be at least 5 characters."
                     ErrorMessage="Value too short!" />
</EditForm>

@code {
    ForwardRef<InputBase<string>> Input = new();

    private bool ValidateLength(string value)
    {
        return !string.IsNullOrEmpty(value) && value.Length >= 5;
    }
}

Examples¶

Basic Custom Validation¶

@using BlazorWebFormsComponents.Validations

<EditForm Model="@model" OnValidSubmit="HandleSubmit">
    <label>Enter a value starting with 'A':</label>
    <InputText @ref="NameRef.Current" @bind-Value="model.Name" />
    <CustomValidator ServerValidate="@StartsWithA"
                     ControlRef="@NameRef"
                     Text="Must start with 'A'."
                     ErrorMessage="Value must start with 'A'!" />
    <button type="submit">Submit</button>
</EditForm>

@code {
    ForwardRef<InputBase<string>> NameRef = new();
    private MyModel model = new();

    private bool StartsWithA(string value)
        => value.StartsWith("A", StringComparison.OrdinalIgnoreCase);

    private void HandleSubmit() { }
    public class MyModel { public string Name { get; set; } }
}

With ClientValidationFunction (Migration Stub)¶

@* ClientValidationFunction is accepted for migration but not executed in Blazor *@
<CustomValidator ServerValidate="@AlwaysValid"
                 ControlRef="@InputRef"
                 ClientValidationFunction="validateOnClient"
                 IsValid="true"
                 Text="Validation failed." />

@code {
    private bool AlwaysValid(string value) => true;
}

ValidateEmptyText¶

@* By default, empty values pass validation. Set ValidateEmptyText to validate them. *@
<CustomValidator ServerValidate="@RequireValue"
                 ControlRef="@InputRef"
                 ValidateEmptyText="true"
                 Text="A value is required." />

@code {
    private bool RequireValue(string value) => !string.IsNullOrWhiteSpace(value);
}

Key Properties¶

Property	Type	Default	Description
`ServerValidate`	`Func<string, bool>`	null	Custom validation function
`ValidateEmptyText`	`bool`	false	Whether to validate empty/null values
`ClientValidationFunction`	`string`	null	Migration stub — name of client-side function (not executed)
`IsValid`	`bool`	true	Whether the control currently passes validation
`ControlRef`	`ForwardRef<InputBase<T>>`	null	Reference to the input control being validated
`Text`	`string`	null	Error text displayed inline
`ErrorMessage`	`string`	null	Error text shown in ValidationSummary

Migration Notes¶

Remove asp: prefix and runat="server"
Replace OnServerValidate="Handler" with ServerValidate="@Handler" — change from event to Func<string, bool>
Replace ControlToValidate="TextBox1" with ControlRef="@ref" using ForwardRef
ClientValidationFunction is accepted but has no effect in Blazor
IsValid defaults to true — use it to programmatically indicate validation state