我该如何改进这个设计？

Question

Let's assume that our system can perform actions, and that an action requires some parameters to do its work. I have defined the following base class for all actions (simplified for your reading pleasure):

public abstract class BaseBusinessAction<TActionParameters> 
    : where TActionParameters : IActionParameters
{
    protected BaseBusinessAction(TActionParameters actionParameters)
    {
        if (actionParameters == null) 
            throw new ArgumentNullException("actionParameters"); 

        this.Parameters = actionParameters;

        if (!ParametersAreValid()) 
            throw new ArgumentException("Valid parameters must be supplied", "actionParameters");
    }

    protected TActionParameters Parameters { get; private set; }

    protected abstract bool ParametersAreValid();

    public void CommonMethod() { ... }
}

Only a concrete implementation of BaseBusinessAction knows how to validate that the parameters passed to it are valid, and therefore the ParametersAreValid is an abstract function. However, I want the base class constructor to enforce that the parameters passed are always valid, so I've added a call to ParametersAreValid to the constructor and I throw an exception when the function returns false . So far so good, right? Well, no. Code analysis is telling me to " not call overridable methods in constructors " which actually makes a lot of sense because when the base class's constructor is called the child class's constructor has not yet been called, and therefore the ParametersAreValid method may not have access to some critical member variable that the child class's constructor would set.

So the question is this: How do I improve this design?

Do I add a Func<bool, TActionParameters> parameter to the base class constructor? If I did:

public class MyAction<MyParameters>
{
    public MyAction(MyParameters actionParameters, bool something) : base(actionParameters, ValidateIt)
    {
        this.something = something;
    }

    private bool something;

    public static bool ValidateIt()
    {
       return something;
    }

}

This would work because ValidateIt is static, but I don't know... Is there a better way?

Comments are very welcome.

Answer 1

This is a common design challenge in an inheritance hierarchy - how to perform class-dependent behavior in the constructor. The reason code analysis tools flag this as a problem is that the constructor of the derived class has not yet had an opportunity to run at this point, and the call to the virtual method may depend on state that has not been initialized.

So you have a few choices here:

1. Ignore the problem. If you believe that implementers should be able to write a parameter validation method without relying on any runtime state of the class, then document that assumption and stick with your design.
2. Move validation logic into each derived class constructor, have the base class perform just the most basic, abstract kinds of validations it must (null checks, etc).
3. Duplicate the logic in each derived class. This kind of code duplication seems unsettling, and it opens the door for derived classes to forget to perform the necessary setup or validation logic.
4. Provide an Initialize() method of some kind that has to be called by the consumer (or factory for your type) that will ensure that this validation is performed after the type is fully constructed. This may not be desirable, since it requires that anyone who instantiates your class must remember to call the initialization method - which you would think a constructor could perform. Often, a Factory can help avoid this problem - it would be the only one allowed to instantiate your class, and would call the initialization logic before returning the type to the consumer.
5. If validation does not depend on state, then factor the validator into a separate type, which you could even make part of the generic class signature. You could then instantiate the validator in the constructor, pass the parameters to it. Each derived class could define a nested class with a default constructor, and place all parameter validation logic there. A code example of this pattern is provided below.

When possible, have each constructor perform the validation. But this isn't always desirable. In that case, I personally, prefer the factory pattern because it keeps the implementation straight forward, and it also provides an interception point where other behavior can be added later (logging, caching, etc). However, sometimes factories don't make sense, and in that case I would seriously consider the fourth option of creating a stand-along validator type.

Here's the code example:

public interface IParamValidator<TParams> 
    where TParams : IActionParameters
{
    bool ValidateParameters( TParams parameters );
}

public abstract class BaseBusinessAction<TActionParameters,TParamValidator> 
    where TActionParameters : IActionParameters
    where TParamValidator : IParamValidator<TActionParameters>, new()
{
    protected BaseBusinessAction(TActionParameters actionParameters)
    {
        if (actionParameters == null) 
            throw new ArgumentNullException("actionParameters"); 

        // delegate detailed validation to the supplied IParamValidator
        var paramValidator = new TParamValidator();
        // you may want to implement the throw inside the Validator 
        // so additional detail can be added...
        if( !paramValidator.ValidateParameters( actionParameters ) )
            throw new ArgumentException("Valid parameters must be supplied", "actionParameters");

        this.Parameters = actionParameters;
    }
 }

public class MyAction : BaseBusinessAction<MyActionParams,MyActionValidator>
{
    // nested validator class
    private class MyActionValidator : IParamValidator<MyActionParams>
    {
        public MyActionValidator() {} // default constructor 
        // implement appropriate validation logic
        public bool ValidateParameters( MyActionParams params ) { return true; /*...*/ }
    }
}

Answer 2

If you are deferring to the child class to validate the parameters anyway, why not simply do this in the child class constructor? I understand the principle you are striving for, namely, to enforce that any class that derives from your base class validates its parameters. But even then, users of your base class could simply implement a version of ParametersAreValid() that simply returns true, in which case, the class has abided by the letter of the contract, but not the spirit.

For me, I usually put this kind of validation at the beginning of whatever method is being called. For example,

public MyAction(MyParameters actionParameters, bool something)
    : base(actionParameters) 
{ 
    #region Pre-Conditions
        if (actionParameters == null) throw new ArgumentNullException();
        // Perform additional validation here...
    #endregion Pre-Conditions

    this.something = something; 
} 

I hope this helps.

Answer 3

I would recommend applying the Single Responsibility Principle to the problem. It seems that the Action class should be responsible for one thing; executing the action. Given that, the validation should be moved to a separate object which is responsible only for validation. You could possibly use some generic interface such as this to define the validator:

IParameterValidator<TActionParameters>
{
    Validate(TActionParameters parameters);
}

You can then add this to your base constructor, and call the validate method there:

protected BaseBusinessAction(IParameterValidator<TActionParameters> validator, TActionParameters actionParameters)
{
    if (actionParameters == null) 
        throw new ArgumentNullException("actionParameters"); 

    this.Parameters = actionParameters;

    if (!validator.Validate(actionParameters)) 
        throw new ArgumentException("Valid parameters must be supplied", "actionParameters");
}

There is a nice hidden benefit to this approach, which is it allows you to more easily re-use validation rules that are common across actions. If your using an IoC container, then you can also easily add binding conventions to automatically bind IParameterValidator implementations appropriately based on the type of TActionParameters

Answer 4

I had a very similar issue in the past and I ended up moving the logic to validate parameters to the appropriate ActionParameters class. This approach would work out of the box if your parameter classes are lined up with BusinessAction classes.

If this is not the case, it gets more painful. You have the following options (I would prefer the first one personally):

1. Wrap all the parameters in IValidatableParameters . The implementations will be lined up with business actions and will provide validation
2. Just suppress this warning
3. Move this check to parent classes, but then you end up with code duplication
4. Move this check to the method that actually uses the parameters (but then your code fails later)

Answer 5

Why not do something like this:

public abstract class BaseBusinessAction<TActionParameters> 
    : where TActionParameters : IActionParameters
{

    protected abstract TActionParameters Parameters { get; }

    protected abstract bool ParametersAreValid();

    public void CommonMethod() { ... }
}

Now the concrete class has to worry about the parameters and ensuring their validity. I would just enforce the CommonMethod calling the ParametersAreValid method prior to doing anything else.

Answer 6

Where are the parameters anticipated to be used: from within CommonMethod? It is not clear why the parameters must be valid at the time of instantiation instead of at the time of use and thus you might choose to leave it up to the derived class to validate the parameters before use.

EDIT - Given what I know the problem seems to be one of special work needed on construction of the class. That, to me, speaks of a Factory class used to build instances of BaseBusinessAction wherein it would call the virtual Validate() on the instance it builds when it builds it.

Answer 7

How about moving the validation to a more common location in the logic. Instead of running the validation in the constructor, run it on the first (and only the first) call to the method. That way, other developers could construct the object, then change or fix the parameters before executing the action.

You could do this by altering your getter/setter for the Parameters property, so anything that uses the paramters would validate them on the first use.