Fluent Validation - Multiple messages for one rule

You could try this below code:

let's say I have two condition first will check the null date and second will check the grater than today's day

using FluentValidation;

namespace BlazorApp2
{
    public class EmployeeValidator : AbstractValidator<Employee>

    {
        public EmployeeValidator()
        {
            RuleFor(p => p.StartDate).Custom(ValidateStartDate);
        }
        private void ValidateStartDate(DateTime? startDate, ValidationContext<Employee> context)
        {
            var employee = context.InstanceToValidate;

            // First Validation: Start Date mustn't be in future.
            if (startDate != null)
            {
                context.AddFailure(nameof(employee.StartDate), "Start Date mustn't be in future.");
                
            }

            // Second Validation: Employee must be registered on this date.
            if (startDate > DateTime.Now.Date)
            {
                context.AddFailure(nameof(employee.StartDate), "Employee must be registered on this date.");
            }
        }

    }
}

OR

One condition will check future date selection and second will show past selection:

// First Validation: Start Date mustn't be in future.
 if (startDate != null && startDate > DateTime.Now.Date)
 {
     context.AddFailure(nameof(employee.StartDate), "Start Date mustn't be in future.");
 }
 // Second Validation: Employee must be registered on this date.
 if (startDate != null && startDate < DateTime.Now.AddYears(-1).Date)
 {
     context.AddFailure(nameof(employee.StartDate), "Employee must be registered on this date.");
 }

razor page:

@page "/"
@using FluentValidation
@using MudBlazor
@inject IValidator<Employee> EmployeeValidator

<EditForm Model="@employee" OnValidSubmit="HandleValidSubmit">
    <DataAnnotationsValidator />
    <ValidationSummary />
    <MudForm @ref="form" Model="@employee">
        <MudTextField Label="Name" @bind-Value="employee.Name" />
        <MudDatePicker Label="Start Date" @bind-Date="employee.StartDate" Mask="@(new DateMask("MM/dd/yyyy"))"
                       DateFormat="MM/dd/yyyy" ShowToolbar="false"
                       Variant="Variant.Outlined" Margin="Margin.Dense" Editable="true">
        </MudDatePicker>
        <MudButton OnClick="ValidateForm" Variant="Variant.Filled" Color="Color.Primary">Submit</MudButton>
    </MudForm>
</EditForm>

@code {
    private Employee employee = new Employee();
    private MudForm form;

    private async Task ValidateForm()
    {
        var result = await EmployeeValidator.ValidateAsync(employee);
        if (result.IsValid)
        {
            Console.WriteLine("Form Submitted Successfully!");
        }
        else
        {
            // Log all validation errors
            foreach (var failure in result.Errors)
            {
                Console.WriteLine(failure.ErrorMessage);
            }
        }
    }

    private void HandleValidSubmit()
    {
        // Handle submission logic after the form is valid
        Console.WriteLine("Form submitted successfully.");
    }
}

Let me know if you have any concern regarding the provided information or my understanding is different

update:

EmployeeValidator.cs:

using FluentValidation;
using FluentValidation.Results;

namespace BlazorApp2
{
    public class EmployeeValidator : AbstractValidator<Employee>

    {
        public EmployeeValidator()
        {
            RuleFor(p => p.StartDate).Custom(ValidateStartDate);
        }
        private void ValidateStartDate(DateTime? startDate, ValidationContext<Employee> context)
        {
            var employee = context.InstanceToValidate;
            var failures = new List<ValidationFailure>();
            // Condition 1: Start Date is required
            if (startDate == null)
            {
                failures.Add(new ValidationFailure(nameof(employee.StartDate), "Start Date is required."));
            }
            else
            {
                var publicHolidays = new List<DateTime>
                {
                    new DateTime(DateTime.Now.Year, 12, 25), // Christmas
                    new DateTime(DateTime.Now.Year, 1, 1)   // New Year
                };
                // Condition 2: Start Date must not be in the future
                if (startDate > DateTime.Now.Date)
                {
                    failures.Add(new ValidationFailure(nameof(employee.StartDate), "Start Date mustn't be in future."));
                }
                // Condition 3: Employee must be registered at least 1 year ago
                if (startDate < DateTime.Now.AddYears(-1).Date)
                {
                    failures.Add(new ValidationFailure(nameof(employee.StartDate), "Employee must be registered on this date."));
                }
                // Condition 4: Start Date should not be on weekends
                if (startDate.Value.DayOfWeek == DayOfWeek.Saturday || startDate.Value.DayOfWeek == DayOfWeek.Sunday)
                {
                    failures.Add(new ValidationFailure(nameof(employee.StartDate), "Start Date cannot be on a weekend."));
                }
                // Condition 5: Start Date should not be on a public holiday
                if (publicHolidays.Contains(startDate.Value))
                {
                    failures.Add(new ValidationFailure(nameof(employee.StartDate), "Start Date cannot be on a public holiday."));
                }
            }
            // Add all failures at once so they appear together
            foreach (var failure in failures)
            {
                context.AddFailure(failure);
            }
        }

    }
}

Home.razor:

@page "/"
@using FluentValidation
@using MudBlazor
@inject IValidator<Employee> EmployeeValidator
<EditForm Model="@employee" OnValidSubmit="HandleValidSubmit">
    <DataAnnotationsValidator />
    <MudForm @ref="form" Model="@employee">
        <ValidationSummary /> <!-- This will display all validation messages -->

        <MudTextField Label="Name" @bind-Value="employee.Name" />
        <MudDatePicker Label="Start Date" @bind-Date="employee.StartDate" Mask="@(new DateMask("MM/dd/yyyy"))"
                       DateFormat="MM/dd/yyyy" ShowToolbar="false"
                       Variant="Variant.Outlined" Margin="Margin.Dense" Editable="true">
        </MudDatePicker>

        <!-- Display all validation messages dynamically below the field -->
        <ul>
            @foreach (var error in validationMessages)
            {
                <li style="color: red;">@error</li>
            }
        </ul>

        <MudButton OnClick="ValidateForm" Variant="Variant.Filled" Color="Color.Primary">Submit</MudButton>
    </MudForm>
</EditForm>
@code {
    private Employee employee = new Employee();
    private MudForm form;
    private List<string> validationMessages = new List<string>();
    private async Task ValidateForm()
    {
        validationMessages.Clear();  // Clear previous messages
        var result = await EmployeeValidator.ValidateAsync(employee);
        if (result.IsValid)
        {
            Console.WriteLine("Form Submitted Successfully!");
        }
        else
        {
            foreach (var failure in result.Errors)
            {
                validationMessages.Add(failure.ErrorMessage);  // Store all errors
            }
        }
    }
    private void HandleValidSubmit()
    {
        Console.WriteLine("Form submitted successfully.");
    }
}

Result: