MVC 5自定义验证：下拉框选特定值后关联控件必填配置咨询

Hey there! Let's tackle your two questions one by one, using your existing MVC code as a starting point.

Question 1: Do I need to add boolean properties for each control to manage validation pass/fail? #

Nope, you don't need extra boolean fields for this. Instead, you can handle conditional validation directly within your model's logic—either by enhancing your custom validation attribute or using MVC's built-in IValidatableObject interface. This keeps your model clean, avoids cluttering it with unnecessary flags, and centralizes all validation logic in one place.

Question 2: How to set individual validation messages for each field when Usage = 2? #

The key here is to associate validation errors with specific fields so their corresponding @Html.ValidationMessageFor() helpers display the right message. Here are two solid approaches:

Option 1: Enhance your custom CheckDropDown validation attribute #

Modify the IsValid method to check each field separately and return errors tied to specific member names:

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    var model = (PartNumberViewModel)validationContext.ObjectInstance;
    var validationErrors = new List<ValidationResult>();

    // Handle Usage = 1 case
    if (model.Usage == 1)
    {
        if (!string.IsNullOrEmpty(model.RequiredProject))
        {
            validationErrors.Add(new ValidationResult(
                "If there are 'No Immediate Requirements' for this part number, then a 'Required Project' is not necessary.",
                new[] { nameof(model.RequiredProject) }));
        }
    }
    // Handle Usage = 2 case
    else if (model.Usage == 2)
    {
        // Check RequiredProject
        if (string.IsNullOrEmpty(model.RequiredProject))
        {
            validationErrors.Add(new ValidationResult(
                "Required Project is required when Usage is 'Procure for Projects listed'.",
                new[] { nameof(model.RequiredProject) }));
        }
        // Check Quantity (since it's an int, 0 is the default—adjust if you need to allow 0 in other scenarios)
        if (model.Quantity <= 0)
        {
            validationErrors.Add(new ValidationResult(
                "Quantity must be greater than 0 when Usage is 'Procure for Projects listed'.",
                new[] { nameof(model.Quantity) }));
        }
        // Check Units
        if (!model.Units.HasValue)
        {
            validationErrors.Add(new ValidationResult(
                "Units is required when Usage is 'Procure for Projects listed'.",
                new[] { nameof(model.Units) }));
        }
    }

    // Return the first error, or combine if needed (though IValidatableObject is better for multiple errors)
    return validationErrors.Any() ? validationErrors.First() : ValidationResult.Success;
}

Option 2: Use IValidatableObject (Recommended for multiple conditional validations) #

Implement the IValidatableObject interface on your view model—this lets you return multiple validation errors at once, each linked to its respective field:

public class PartNumberViewModel : IValidatableObject
{
    [Required]
    public int Usage { get; set; }

    [Display(Name = "Required Project(s)")]
    public string RequiredProject { get; set; }

    [Range(0, Int32.MaxValue)]
    public int Quantity { get; set; }

    public int? Units { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        var errors = new List<ValidationResult>();

        if (Usage == 1)
        {
            if (!string.IsNullOrEmpty(RequiredProject))
            {
                errors.Add(new ValidationResult(
                    "If there are 'No Immediate Requirements' for this part number, then a 'Required Project' is not necessary.",
                    new[] { nameof(RequiredProject) }));
            }
        }
        else if (Usage == 2)
        {
            if (string.IsNullOrEmpty(RequiredProject))
            {
                errors.Add(new ValidationResult(
                    "Required Project is required when Usage is 'Procure for Projects listed'.",
                    new[] { nameof(RequiredProject) }));
            }
            if (Quantity <= 0)
            {
                errors.Add(new ValidationResult(
                    "Quantity must be greater than 0 when Usage is 'Procure for Projects listed'.",
                    new[] { nameof(Quantity) }));
            }
            if (!Units.HasValue)
            {
                errors.Add(new ValidationResult(
                    "Units is required when Usage is 'Procure for Projects listed'.",
                    new[] { nameof(Units) }));
            }
        }

        return errors;
    }
}

Why this works: #

• By passing new[] { nameof(FieldName) } to the ValidationResult, you tell MVC to display the error message in the corresponding @Html.ValidationMessageFor(x => x.FieldName) element.
• Your existing view is already set up correctly with these validation message helpers, so no changes are needed there—messages will automatically appear next to the right inputs.

内容的提问来源于stack exchange，提问作者Bigbear