.NET Reflection - how to get implemented interface method on class?

I'm trying to write custom attributes for .net that mimic the jpa converter in Java that allows me to decorate a class like so:

@Entity(name = "PersonTable")
public class Person {

    @Convert(converter = PersonNameConverter.class)
    private PersonName personName;
}

I want to restrict the type of converter the user can pass into the Converter attribute to be a particular interface that I defined.

public interface ICustomConverter<T1, T2>
{
    T1 ConvertToType1(T2 source);
    T2 ConvertToType2(T1 source);
}

I could not find a way to do this during compile time, so I'm testing out this during runtime with this code:

class BooleanConverter : ICustomConverter<bool, string>
{
    bool ICustomConverter<bool, string>.ConvertToType1(string source)
    {
        if ("y".Equals(source.ToLower()))
            return true;
        else
            return false;
    }

    string ICustomConverter<bool, string>.ConvertToType2(bool source)
    {
        if (source)
            return "Y";
        else
            return "N";
    }
}

[XmlMapper(ParentNodeName = "Loan")]
class Loan
{
    [CustomConverterAttribute(typeof(BooleanConverter))]
    public bool IsJointAccount { get; set; }
}

public CustomConverterAttribute(Type converter)
    {
        _converter = converter;

        _convertToType1 = converter
            .GetMethod("ConvertToType1", BindingFlags.Public | BindingFlags.Instance | BindingFlags.FlattenHierarchy);

        _convertToType2 = converter
            .GetMethod("ConvertToType2", BindingFlags.Public | BindingFlags.Instance | BindingFlags.FlattenHierarchy);

        _type1 = _convertToType1.ReturnType;
        _type2 = _convertToType2.ReturnType;

        _converterInstance = ReflectionUtils.CreateInstance(_converter);
    }

But both .GetMethods() and .GetMethod() do not return the implemented methods. I have tried various different BindingFlags such as (BindingFlags.DeclaredOnly) but none of the binding options actually return these methods.