Interface Segregation Principle – Implementing in C#

cdesign-principlesinterfacesobject-orientedsolid

Does this IConvertible interface satisfy the Interface Segregation Principle (ie. the "I" in SOLID)?

Here is the definition:

public interface IConvertible
{
    TypeCode GetTypeCode(); 
    bool ToBoolean(IFormatProvider provider);
    byte ToByte(IFormatProvider provider);
    char ToChar(IFormatProvider provider);
    DateTime ToDateTime(IFormatProvider provider);
    decimal ToDecimal(IFormatProvider provider);
    short ToInt16(IFormatProvider provider);
    int ToInt32(IFormatProvider provider);
    long ToInt64(IFormatProvider provider);
    sbyte ToSByte(IFormatProvider provider);
    float ToSingle(IFormatProvider provider);
    string ToString(IFormatProvider provider);
    object ToType(Type conversionType, IFormatProvider provider);
    ushort ToUInt16(IFormatProvider provider);
    uint ToUInt32(IFormatProvider provider);
    ulong ToUInt64(IFormatProvider provider);
}

So if I would like to have a class which will implements this IConvertible interface the I have to implement all of those methods, right? Or if I don't implement all of them, then I have to at least make an empty method or throw an Exception, right?.

In my opinion, the better way is to make more interface with fewer methods, for example:

public interface IConvertibleInt
{
        short ToInt16(IFormatProvider provider);
        int ToInt32(IFormatProvider provider);
        long ToInt64(IFormatProvider provider);
}

Or even:

public interface IConvertibleInt16
{
        short ToInt16(IFormatProvider provider);
}

public interface IConvertibleInt32
{            
        int ToInt32(IFormatProvider provider);
}

public interface IConvertibleInt64
{
        long ToInt64(IFormatProvider provider);
}

Is my reasoning correct?

Best Answer

I like to interpret Interface Segregation Principle as

Interface should be closer related to the code that uses it than code that implement it. So the methods on the interface are defined by which methods client code needs than which methods class implements.

This implies that for each interface, there is always code that uses this interface. This is not the case of IConvertible. The interface itself doesn't make much sense. I also never saw property or function that was typed IConvertible, which empowers my claims. I even wonder how would code that only works on IConvertible looks and what would it do.

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