public class AController : BaseController
{
    private bool validate()
    {
        //code for A validation

        return base.validate();
    }
}

public class BController : BaseController
{
    private bool validate()
    {
        //code for B validation

        return base.validate();
    }
}

public class BaseController : Controller
{
    protected bool validate()
    {
        //code for common validation
        return isValid;
    }
}

public class AController : BaseController
{
    protected override bool additionalValidate()
    {
        //code for A validation

        return isValid;
    }
}

public class BController : BaseController
{
    protected override bool additionalValidate()
    {
        //code for B validation

        return isValid;
    }
}

public class BaseController : Controller
{
    virtual bool additionalValidate() {
        return true;
    }
    protected bool validate()
    {
        //code for common validation
        return isValid && additionalValidate();
    }
}

public class AController : BaseController
{
    protected override bool abstractValidation()
    {
       //code for A validation
    }
}

public class BController : BaseController
{
    protected override bool abstractValidation()
    {
       //code for B validation
    }
}

public abstract class BaseController
{
    protected abstract bool abstractValidation();

    protected bool validate()
    {
        //code for common validation

        //then, call the implementations validation method
        var isValid = abstractValidation();

        return isValid;
    }
}