Forcing user to use the right generic method in C#

Question

It could be a silly question, but please help to answer it. Currently I have an interface with 2 generic methods:

ValidationResult Validate<T>(T t);


IList<ValidationResult> ValidateList<T>(IEnumerable<T> entities);

What I want is if you want to validate an object, use Validate method; if you want to validate an array of object, use ValidateList method, pretty clear in mind and interface. But it seems user can also use Validate method for a list of object without any compiler errors (of course!). Any there ways to restrict them to ValidateList method? Thank you so much.

Answer 1

You can restrict a function to specific object types (and its derivatives) by doing something like this in the function declaration:

ValidationResult Validate<T>(T t) where T : ValidateBase, ValidateBaseOther

edit:

so in this case, the function will only take objects that are ValidateBase or its derivatives, or ValidateBaseOther or its derivatives. You can of course just use one if you want

ValidationResult Validate<T>(T t) where T : ValidateBase

Answer 2

In short, you can't solve your problem (prevent IEnumerable<> from being passed to a generic method) with generics. This is because you can't have negative generic constraints (ie., everything except someClass). The only reason that generic constraints exists is to allow the compiler to know what signature to expect on the generic type. It is not to provide the developer with design-time cues.

Aside from any unspecified external requirements, you can define your generic type at the interface, rather than the method. This does not ensure that T cannot implement IEnumerable<>; however, it does ensure that ValidateList must accept a list of whatever is passed to Validate. This means that T can be an array, but that would force ValidateList to accept an array of arrays.

If you leave the generic types at the method level, even though they both have an generic type named T, those types don't actually have anything to do with each other.

Generic type defined at interface.

public interface ISomeInterface<T>
{
    ValidationResult Validate(T t);
    IList<ValidationResult> ValidateList(IEnumerable<T> entities);
}

This would be implemented like:

public class SomeClass<T> : ISomeInterface<T>
{
    ValidationResult Validate(T t)
    {
        // Do something ...
    }
    IList<ValidationResult> ValidateList(IEnumerable<T> entities)
    {
        // Do something ...
    }
}

And used like this:

var myInstance = new SomeClass<int>();

int obj = 5;
int arr = new [] {1,2,3};

myInstance.Validate(obj);
myInstance.ValidateList(arr);

myInstance.Validate(arr); // should throw compiler error

Answer 3

Since no better option was found: Check if the type passed to Validate implements IEnumerable<>. If so, maybe raise an exception....

So, here it goes:

ValidationResult Validate(T t)
{
    Type OutResultNotUsed;
    if (typeof(T).IsOrInherits(typeof(IEnumerable<>), out OutResultNotUsed))
        throw new Exeption("glkjglkjsdg");
}

Extension Method to check if some Type Is or Inherits a generic definition.

    public static bool IsOrInheritsGenericDefinition(this Type ThisType, Type GenericDefinition, out Type DefinitionWithTypedParameters)
    {
        DefinitionWithTypedParameters = ThisType;

        while (DefinitionWithTypedParameters != null)
        {
            if (DefinitionWithTypedParameters.IsGenericType)
            {
                if (DefinitionWithTypedParameters.GetGenericTypeDefinition() == GenericDefinition)
                    return true;
            }

            DefinitionWithTypedParameters = DefinitionWithTypedParameters.BaseType;
        }

        return false;
    }

Answer 4

You could do the following:

public class MultipleValidationResults : ValidationResult, IList<ValidationResult>
{
    // stuff...
}

public ValidationResult Validate<T>(T t)
{
    if (t is IEnumerable)
        // use reflection to call return ValidateList<Y>(t)
        // (T is IEnumerable<Y>)
    // other stuff
}

MultipleValidationResults ValidateList<T>(IEnumerable<T> entities);