Want to change the Validation attribute at runtime ASP.Net MVC

[Richard Deeming]

Not easily, but it can be done. You're going to need to create a custom DataAnnotationsModelValidatorProvider, and modify the rules it returns. validation - Possible to change Data Annotations during Runtime? (ASP.NET MVC's [Range] [Required] [StringLength] etc.) - Stack Overflow[^] Eg:

public class CustomModelValidatorProvider : DataAnnotationsModelValidatorProvider
{
protected override IEnumerable<ModelValidator> GetValidators(ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
{
if (metadata.Model is ServiceFunctionCategoryLKP model && metadata.PropertyName == nameof(model.Code) && model.CodeLength >= 0)
{
var newAttributes = new List<Attribute>(attributes);
var stringLength = newAttributes.OfType<StringLengthAttribute>().FirstOrDefault();
if (stringLength != null)
{
newAttributes.Remove(stringLength);

            if (model.CodeLength != 0)
            {
                newAttributes.Add(new StringLengthAttribute(model.CodeLength)
                {
                    MinimumLength = model.CodeLength, 
                    ErrorMessage = $"The field {{0}} length must be at least {model.CodeLength}."
                });
            }
            
            attributes = newAttributes;
        }
    }
    
    return base.GetValidators(metadata, context, attributes);
}

}

Register in Globals.asax.cs:

ModelValidatorProviders.Providers.Clear();
ModelValidatorProviders.Providers.Add( new CustomValidatorProvider() );

NB: You're effectively allowing the user to override the validation rule by modifying a hidden field on the page. As a result, the validation can't really be trusted. It would be better to create a separate model class for each required code length.

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"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[indian143]

Just checking, is it going to remove all the other Validations that I have put on other classes? Thanks, Abdul Aleem "There is already enough hatred in the world lets spread love, compassion and affection."

[Richard Deeming]

No. It checks that the instance being validated is the specific model class; that the property being validated is the Code property; and it only replaces the StringLength validator on that property.

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"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[indian143]

OK Got it, and you are saying that there is just one risk in it that it can be overridden in the derived classes, understandable, thanks a lot Richard for giving me the information. Thank you very much have a wonderful weekend buddy. Thanks, Abdul Aleem "There is already enough hatred in the world lets spread love, compassion and affection."

[indian143]

Hi Richard, One last thing, I have created the method in the following way:

public class CustomModelValidatorProvider : System.Web.Mvc.DataAnnotationsModelValidatorProvider
{
    protected override IEnumerable GetValidators(System.Web.Mvc.ModelMetadata metadata, ControllerContext context, IEnumerable attributes)
    { 
        if (metadata.Model is LookupTable)
        {
            LookupTable model = metadata.Model as LookupTable;

            if (metadata.PropertyName == GetPropertyName(() => model.CodeLength) && (model.CodeLength > 0))
            {
                var newAttributes = new List(attributes);
                var stringLength = newAttributes.OfType().FirstOrDefault();
                if (stringLength != null)
                {
                    newAttributes.Remove(stringLength);

                    if (model.CodeLength != 0)
                    {
                        newAttributes.Add(new StringLengthAttribute(model.CodeLength)
                        {
                            MinimumLength = model.CodeLength,
                            ErrorMessage = @"The field {{0}} length must be at least {model.CodeLength}."
                        });
                    }

                    attributes = newAttributes;
                }
            }
        }            

        return base.GetValidators(metadata, context, attributes);
    }

    public string GetPropertyName(Expression\> propertyLambda)
    {
        var me = propertyLambda.Body as MemberExpression;

        if (me == null)
        {
            throw new ArgumentException("You must pass a lambda of the form: '() => Class.Property' or '() => object.Property'");
        }

        return me.Member.Name;
    }
}

can you please let me know how can I decorate or use it on my Model class, can you please give me that example, I am asking this question because this value is coming as null: metadata.PropertyName, so I am not able to compare with property name. So any help please? thank you very very much buddy - as I said earlier have a Wonderful weekend with family and friends buddy. Thanks, Abdul Aleem "There is already enough hatred in the world lets spread love, compassion and affection."

[Richard Deeming]

You register the provider in Global.asax.cs:

ModelValidatorProviders.Providers.Clear();
ModelValidatorProviders.Providers.Add( new CustomValidatorProvider() );

I'd expect to see the PropertyName set to null for the class-level validation, but I'd expect the method to be called again for each property on the class. You're currently checking whether the validators are for the CodeLength property; you need to customise the validators for the Code property instead.

if (metadata.PropertyName == GetPropertyName(() => model.Code) && ...

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"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[indian143]

ModelValidatorProviders.Providers.Clear(); ModelValidatorProviders.Providers.Add( new CustomValidatorProvider() ); I did this registration in the Global.asax.cs file, in the Application_Start() event. And even if I changed the Condition to:

if (metadata.PropertyName == GetPropertyName(() => model.Code) && (model.CodeLength > 0))

Still I am getting the metadata.PropertyName null always, can you please suggest me what am I missing? Just asking am I using any wrong namespace in references?, Because it was saying there is an ambiguous class names and those classes present in both the following namespaces using System.Web.ModelBinding; using System.Web.Mvc; Am I using any wrong namespace or something like that kind of error, I am not understanding why is it giving metadata.PropertyName as null in all the cases? Thanks, Abdul Aleem "There is already enough hatred in the world lets spread love, compassion and affection."

[Richard Deeming]

Sorry, I picked the wrong property to test against. Replace metadata.Model with metadata.Container:

if (metadata.Model is ServiceFunctionCategoryLKP model && metadata.PropertyName == nameof(model.Code) && model.CodeLength >= 0)

---

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[indian143]

But still the attributes value in IEnumerable attributes coming as null when I already have one Attribute on that class and when I try to expan the attributes collection object, it shows me "Enumeration yielded no results" in the end.

public class LookupTable
{
    public int Id { get; set; }

    \[Required(ErrorMessage = "Code is required.")\]        
    public string Code { get; set; }

    \[Required(ErrorMessage = "Description is required.")\]
    public string Description { get; set; }
    public int? ForeignKeyId { get; set; }
    public string ForeignKeyValue { get; set; }
    public DateTime? CreatedDate {get;set;} 
    public string CreatedBy {get;set;}       
    public int CodeLength { get; set; }
}

Do I need to add anymore Attributes to the Code Property?, what am I missing here my friend? Even after I changed my function to have Container instead of Model, still I get null at metadata.PropertyName Always metadata.PropertyName is coming as null irrespective of any changes I do at if (metadata.PropertyName == GetPropertyName(() => model.Code) && (model.CodeLength > 0)) in the below function and attributes is also coming as null.

    protected override IEnumerable GetValidators(System.Web.Mvc.ModelMetadata metadata, ControllerContext context, IEnumerable attributes)
    { 
        if (metadata.Model is LookupTable)
        {
            LookupTable model = metadata.Container as LookupTable;                

            if (metadata.PropertyName == GetPropertyName(() => model.Code) && (model.CodeLength > 0))
            {
                var newAttributes = new List(attributes);
                var stringLength = newAttributes.OfType().FirstOrDefault();
                if (stringLength != null)
                {
                    newAttributes.Remove(stringLength);

                    if (model.CodeLength != 0)
                    {
                        newAttributes.Add(new StringLengthAttribute(model.CodeLength)
                        {
                            MinimumLength = model.CodeLength,
                            ErrorMessage = @"The field {{0}} length must be at least {model.CodeLength}."
                        });
                    }

                    attributes = newAttributes;

[Richard Deeming]

I don't know what to suggest. I've tested the code, and it works. :confused: If you're not putting a static StringLength attribute on the property, you'll need to the if (model.CodeLength != 0) block outside of the if (stringLength != null) block:

if (stringLength != null)
{
newAttributes.Remove(stringLength);
}

if (model.CodeLength != 0)
{
newAttributes.Add(new StringLengthAttribute(model.CodeLength)
{
MinimumLength = model.CodeLength,
ErrorMessage = @"The field {{0}} length must be at least {model.CodeLength}."
});
}

attributes = newAttributes;

But if you're never seeing a value in metadata.PropertyName, then there's something else going on. I assume you have actually enabled unobtrusive validation?

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"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

[indian143]

No I have not done this: unobtrusive validation, can you please let me know how to do it only on one particular class and property that I want. Thanks, Abdul Aleem "There is already enough hatred in the world lets spread love, compassion and affection."

[Richard Deeming]

You enable unobtrusive validation for the site in the web.config file:

<configuration>
<appSettings>
<add key="ClientValidationEnabled" value="true"/>
<add key="UnobtrusiveJavaScriptEnabled" value="true"/>

This should be included by default, but it depends on the template you started with. Creating a MVC 3 Application with Razor and Unobtrusive JavaScript ⇒ Enabling Client-Side Validation[^] I've created a minimal sample project in VS2017 which works as expected; you can download it from OneDrive[^].

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"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer