MVC 3 how do I validate entire ViewModel?

Question

I am trying to validate a Model where the rules are not always the same and depend on the other attributes within the model. What is the best way to go about doing this? Example below:

Hypothetical Example 1

Using MVVM pattern with MVC 3. My (hypothetical) ViewModel looks like this:

public string OrderType { get; set; }
public string Requestor { get; set; }
public int NumberOfPeanuts { get; set; }
public int NumberOfJellyBeans { get; set; }
public int NumberOfAlmonds { get; set; }

My view basically looks like this:

 @Html.EditorFor(model => model.OrderType ) 
 @Html.EditorFor(model => model.Requestor ) 
 @Html.EditorFor(model => model.NumberOfPeanuts ) 
 @Html.EditorFor(model => model.NumberOfJellyBeans ) 
 @Html.EditorFor(model => model.NumberOfAlmonds )

How would I implement validation that would return "Html.ValidationMessageFor" results for the following rules:

If OrderType = "Peanuts" then NumberOfPeanuts must be greater than 0, and NumberOfJellyBeans and NumberOfAlmonds MUST be null or 0, else display "this is a peanut-only order"

If OrderType = "Sample" then NumberOfPeanuts + NumberOfJellyBeans + NumberOfAlmonds must be less than 30, else display validation message "total amount of sample not high enough"

etc... etc...

Answer 1

You can extend ValidationAttribute with your own custom validation attribute.

public class CustomAttribute : ValidationAttribute
{    
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        // validation logic
    }
}

Answer 2

I will expand on Sam's answer to use multiple model properties to validate:

public class PeanutOrderAttribute : ValidationAttribute, IClientValidatable
{
    private readonly string _peanutsProperty;
    private readonly string _orderTypeProperty;

    public PeanutOrderAttribute(string peanutsPropertyName, string orderTypePropertyName)
    {
        _peanutsProperty = peanutsPropertyName;
        _orderTypeProperty = orderTypePropertyName;
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        // get target property and value
        var peanutInfo = validationContext.ObjectType.GetProperty(this._peanutsProperty);
        var peanutValue = peanutInfo.GetValue(validationContext.ObjectInstance, null);

        // get compare property and value
        var ordertypeInfo = validationContext.ObjectType.GetProperty(this._orderTypeProperty);
        var ordertypeValue = ordertypeInfo.GetValue(validationContext.ObjectInstance, null);

        // if validation does not pass
        if (ordertypeValue == "Peanuts" && peanutValue < 1){
             return new ValidationResult("Error Message");
        }

        // else return success
        return ValidationResult.Success;
    }

    public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
    {
        var rule = new ModelClientValidationRule
        {
            ErrorMessage = this.ErrorMessageString,
            ValidationType = "PeanutOrder"
        };

        rule.ValidationParameters["peanuts"] = this._peanutsProperty;
        rule.ValidationParameters["ordertype"] = this._orderTypeProperty;

        yield return rule;
    }
}

Then put the validation tag on the appropriate model property:

[PeanutOrder("Peanuts", "OrderType")]
public int Peanuts{ get; set; }

public string OrderType { get; set; }

Hope this helps!