How to get all class-es that implement same interface?

I have design issue. I want have separate class for every validation. Also I want have one class that call all this validations. Idea is that if I need new validation, than I just add new validation class and everything works. I don't need to change anything else, just add new class. Something like this:

    public interface IValidate{
        bool Validate();
    }

    public class Validator1 : IValidate{    
        public bool Validate(){     
            //Do validation 1
        }
    }
    public class Validator2 : IValidate{    
        public bool Validate(){     
            //Do validation 2
        }
    }
    //...
    public class ValidatorN : IValidate{    
        public bool Validate(){     
            //Do validation N
        }
    }
    //...................................
    public interface IValAll{
        bool Validate_All();
    }
    public class ValidateAll : IValAll{
        public bool Validate_All(){     
             //Call all validators that implements IValidate interface
            //Do validation 1,2...N
            //If all validations return true, than this function return true. 
            //Else return false.
        }
    }

I don't know if this is best approach, but you get and idea what I want. Problem is that I don't know how to implement this Validate_All() method. How to get all class-es that implement same interface and do validation in one loop? Idea is to inject this IValAll interface and do all validation with one call. Thanks for help and advice.